Table 9.1
Suggested Cover Crops
Orchard floor management lies at the heart of organic fruit production and is the key difference between conventional and organic growers. It includes all aspects of weed control, fertilization, pH management, management of ground- and soil-dwelling beneficial organisms, and companion plantings. Simply exchanging conventional pesticides for those approved for certified organic growers does not make an orchard organic and seldom gives great results.
In the not-distant past, this chapter would have been titled “Weed Management” and may have contained a few short sections on soil structure and erosion. Today the situation is far more complex and integrated.
Prior to the development of tractors and herbicides, orchards were complex ecosystems that included fruit trees, often a wide assortment of understory plants, and an even greater assortment of micro- and macroorganisms. Cultivation in the form of tillage was limited to horse-drawn implements and hand weeding. Trees were often planted in square grids with a tree at each corner of the square, allowing cultivation in all directions. Given reasonable care, soil compaction was minimal and biodiversity within the orchard high.
With the introduction of these two factors, the situation changed. As recently as the early 1980s, many commercial orchards were kept bare of all vegetation except trees by using a combination of mechanical cultivation and herbicides. While the scorched earth approach to orchard floors eliminated weed competition, serious problems began developing. In many orchards, soil compaction and erosion became severe. With the compaction and loss of soil structure, water infiltration decreased and root damage from diseases and physiological disorders became common. Root damage from improper mechanical cultivation was sometimes serious. Erosion became even more severe, and concentrations of soil organic matter decreased, as did populations of micro- and macroorganisms.
By the mid-1980s, much research on the use of alley crops was under way. The focus was initially on reducing soil compaction and erosion. With the introduction of size-controlling rootstocks for apples and pears, tree density began increasing — we went from about 100 trees per acre or fewer to around 200 or more trees per acre planted relatively closely together in rows separated by distinct alleys. The use of permanent alley crops, narrower alleys, and more closely spaced trees altered mechanical soil cultivation practices.
Most of the alley crops originally tested were monocultures or blends of sod-forming grasses. Tree rows were kept weed-free with preemergence and contact herbicides. Erosion and soil compaction were reduced, but some of the early results showed disappointing fruit yields and rates of tree growth. Some turf crops appeared to reduce tree growth rates, not only due to competition for water and nutrients, but also due to alleopathic effects of the grasses on the trees. Rodent problems also increased as alley crops were reintroduced to orchards. Providing for the nutritional needs of the tree fruits and alley crops became more complicated than it was for a scorched earth monoculture.
As tree density continued to increase with improved rootstocks and training methods, closely spaced fruit trees in well-defined rows became standard practices. In most cases, the alleys were covered with grasses, sometimes with clovers mixed in. Herbicides were used to create 2- to 4-foot-wide weed-free strips along the tree rows. These practices remain very common for nonorganic fruit growers. In arid regions with limited irrigation water, both alleys and tree rows (again, except for the fruit trees) may be left bare for at least part of the year.
In the late 1900s and early 2000s, the viability and importance of organic fruit production gained notice, and research on developing environmentally sound and economically productive orchard floor practices evolved. Much progress was made from about 2002 to 2010, but there is much we still do not understand about orchard floor management.
What we know is that everything we do in an orchard affects everything else. Imagine your orchard as a net or web. Pull on one small string and it pulls on others, which pull on others, which pull on yet others until the entire net moves. Planting an alley crop increases soil organic matter and helps create soil structure, which influences water infiltration and soil aeration, which affects nutrient uptake and micro- and macroorganism populations in the soil, which affects root health and habitat for beneficial organisms, which affects tree health and vigor, which affects tree root growth, which affects the soil structure, and so on.
Early biodynamic and organic farming supporters recognized the importance of soil organic matter (SOM), despite lacking the technology and ecological science backgrounds to fully understand why it is important. Greatly improved research equipment and practices are showing that soil organic matter is even more vital than its early advocates recognized.
Soil organic matter is made up of plant and animal remains that have been partially disintegrated and decomposed, as well as organic compounds that have been synthesized by soil microorganisms. About 60 to 80 percent of soil organic matter consists of humus (humic material), a complex mixture of organic compounds formed by microbial activities. Humus, which is quite resistant to further decomposition, is made up of fulvic acid, humic acid, and humin. Fulvic acid is the most susceptible to decay by microbes and breaks down over 15 to 50 years. Humic acid is more stable, lasting at least 100 years. Humin is the most stable and highly resistant to degradation.
Soil organic matter is important to organic fruit growers for a variety of reasons:
Improves CEC and nutrient uptake. Humus, one of the components of soil organic matter, increases the soil’s cation exchange capacity (CEC), which refers to the soil’s ability to bind and hold positively charged plant nutrients. Humus has a much greater cation exchange capacity than the mineral portions of the soil. As humus concentrations increase, so does the soil’s ability to store positively charged plant nutrients. Further, humus enhances mineral breakdown, making nutrients more available to plants as organometal complexes (specialized organic compounds that include metallic elements) that form with ions, such as Fe3+, Cu2+, Zn2+, and Mn2+. These organometal complexes are more available for plant uptake and use than the mineral form of the elements.
Increases soil aggregates. Soil organic matter is also critical in binding mineral soil particles into stable aggregates. These aggregates combine to form the macro- and micropores in the soil that are critical for water infiltration and drainage. The presence of aggregate-building organic matter improves the drainage of fine-textured soils, yet also improves the water-holding capacity of sandy soils. Aggregated soils are less susceptible to erosion and compaction than those low in organic matter. Aggregate formation is also important for gas exchange, root penetration, and providing suitable habitats for micro- and macroorganisms. The degree to which a soil is composed of aggregates is a measure of the soil’s tilth or friability and is a vital key to the soil’s productivity.
Adds nitrogen and sulfur. Soil organic matter is a source of nitrogen and sulfur. As organic matter is broken down by micro- and macroorganisms, nitrogen and sulfur are released into the soil and made available for plant uptake. By some estimates, decomposing soil organic matter releases about 10 to 20 pounds of nitrogen per acre each year for each 1 percent of soil organic matter present. The actual amount of nitrogen released, however, is quite variable and heavily influenced by many factors, including temperatures.
Increases biological activity. While helping to support plant growth, soil organic matter is also critically important for biological activity in the soil. It is, literally, the base of the food chain for soil-dwelling micro- and macroorganisms. Soil biological activity, in turn, is a key component of healthy and productive soils that are needed for successful organic fruit production.
Soil organic matter concentrations vary greatly across different soil types, from less than 1 percent in sandy soils to 6 percent or more under grasslands and up to 80 percent in some wetland soils. How much organic matter is needed to produce a high-quality, sustainable crop, however, is very hard to say. Commercial organic orchard crops are produced successfully in Washington State on sandy soils with as little as 1.5 percent organic matter. Unlike pH and nutrient concentrations, we have no recommendations for soil organic matter concentrations. We also lack a standard method of measuring soil organic matter, and testing methods vary across regions and between laboratories.
Rather than worrying about the soil organic matter concentration in your orchard, use tests every 3 to 5 years to monitor the effects of your soil-building cultural practices. Such practices might include the use of short-term green manure or permanent alley crops, in-row cover crops, and applications of composts or mulches. Because laboratory tests measure the decomposed portion of organic matter, and not pieces of roots or other visible plant materials, organic matter concentrations in the soil change slowly. Given effective orchard floor management practices, losses of organic matter should stop within a year or so of transitioning to organic or starting a new orchard, and levels should gradually increase over 10 to 15 years. On sandy soils in arid climates, 3 percent SOM is a good target, while 5 percent is reasonable on loamy soils. The goal is not to increase soil organic matter concentrations for the sake of increasing them but rather to develop and maintain a biologically diverse and active soil with good structure and adequate, but not excessive, plant nutrient concentrations. Monitoring soil organic matter, as well as tree health and productivity, will help you track your progress. Because testing procedures (and therefore the results) for soil organic matter differ between laboratories, find a good lab and stick with it. If you switch from one laboratory to another between years, you will not be able to accurately compare results for different years or track your progress.
How we manage an orchard floor has great impact on organic matter. Cultivation and bare-ground treatments cause soil organic matter concentrations to decrease. Green manure crops, alley and in-row companion crops, and mulching increase soil organic matter.
Some organic practitioners object to the use of the word “weed,” believing that all vegetation in an orchard is desirable. For our purposes, we will consider any plant growing where we do not want it to be a weed. Regardless of semantics, vegetation management on the orchard floor is critically important while we are establishing our trees and remains important throughout the life of many orchards.
It is important to understand that any time we create a vegetation-free area, we have created a vacuum that nature will strive to fill. As with any ecosystem, highly aggressive, invasive, and competitive plant species have the advantage over our nonaggressive and relatively slow-growing fruit crops. On the positive side, the required amount of vegetation-free space around the fruit trees is smaller than most conventional herbicide strips. The need for aggressive weed control also diminishes during the course of the growing season and as the trees mature.
In orchards consisting of large, established trees that essentially form an unbroken canopy across the entire orchard floor, weed control is often a rather minor concern. The trees have extensive root systems capable of obtaining water and nutrients far beyond the reach of shallow-rooted understory vegetation. The dense shade produced by the trees also greatly reduces the establishment and growth of other vegetation in the orchard.
While weed problems are often minor in a mature orchard with large trees, it is important to manage vegetation throughout the life of high-density orchards if you want to produce high-quality fruit and high, sustainable yields. Orchard systems that confine the trees to narrow, trellised rows have a particular need for careful vegetation management for two reasons: First, the trees are kept small, often with weak trunks and relatively little foliage. These trees are designed and maintained to produce high yields of fruit, not structural wood and extensive root systems that provide abundant nutrient and carbohydrate reserves. Large trees are better able to compete with orchard floor vegetation for nutrients and water. Second, an abundant amount of sunlight penetrates the orchard floor, greatly increasing the amount, diversity, and growth of competing vegetation.
For young trees just getting started, in any type of orchard, weed control is a major concern. One of the greatest challenges in establishing an organic orchard is weed pressure. Perhaps 90 percent of the young trees’ root systems are in the top foot of soil, placing them in direct competition with “weeds.” Most young fruit trees and bush fruit plants are rather poor competitors.
In Washington State trials, trees in organic plots where weed pressure was high had far lower survival and growth rates than those in conventional plots with few weeds. The situation is worst in areas with ample summer rains, where vegetation is naturally abundant, varied, and vigorous. An old axiom in horticulture is that you can double the size of a tree during its first 3 years by keeping a vegetation-free zone of several feet around the tree.
The challenge is to minimize competition with the trees while maintaining biologically diverse and active orchard floors and soil environments, as well as minimal soil compaction and erosion. Unfortunately, there is no perfect strategy and no single strategy fits all situations. The vegetation management practices of a West Texas apple grower, a Louisiana mayhaw grower, a British Columbia cherry grower, and a Quebec plum grower are likely to be quite different. The strategies below will guide you in developing a system that works for your crops and growing region.
A key strategy in weed management is preventing the weeds from forming viable seed. Whatever weed suppression practices you employ, strive to prevent the weeds from setting seed. After a time, the weed pressure will diminish. Weed seeds can be very persistent, however. The seeds of common purslane can survive in the soil for 20 years, while black mustard seeds can remain viable in the soil seed bank for 40 years or more.
Thermal weeders use heat produced by propane-fired flames, infrared radiation from a red-hot metal or ceramic plate, or steam to kill or stunt unwanted plants. Thermal weeders work by disrupting the cells in the plants, causing the affected plant parts to wilt and die. Although not effective against established perennial grasses or many perennial broadleaf weeds, these weeders are especially effective against annual weeds and young perennial weed seedlings and offer a chemical-free way to manage orchard floor vegetation without damaging the soil. Thermal weeders can be adapted for maintaining the centers of tree rows in some orchard designs and in maintaining the narrow bare ground strip in the “sandwich” system (see page 317).
Using fire as a weed control strategy in crop production has been practiced for centuries, if not millennia. Although the thought of fire raging through your orchard can be unsettling, there are applications for flame weeders. Flame weeders are the simplest type of thermal weeder, usually consisting of a handheld, pushcart-mounted, or tractor-mounted nozzle attached by a flexible hose to a propane tank. Flame weeders were first used in North America for cotton in the 1930s. In orchard situations, they are most useful for managing vegetation along fencerows and for spot-spraying within tree rows on bare ground. They should not be used on or near combustible mulches such as bark, straw, or alleyway clippings.
Large, tractor-mounted flamers consist of metal beds up to about 8 feet wide with flame nozzles underneath the beds that create sheets of flames. Bed flamers are designed primarily for preparing planting beds for field crops or for burning off plant residues after harvest. In an orchard situation, bed flamers might be useful for preplant site preparation. Of the three types of thermal weeders discussed here, flame weeders create the greatest risk of damaging trees or of starting unintended fires. Use these with extreme care.
Infrared weeders prevent direct flames from contacting the plants and soil. Instead, the flame is directed onto a ceramic or metal plate that radiates heat at temperatures around 1,800 to 2,000°F (982–1,093°C). As with flame weeders, units range from small handheld to large tractor -mounted devices. Infrared weeders are safer to use than open-flame weeders and can be effective in some orchard situations when used carefully.
Infrared weeders are presently manufactured in Sweden and available in North America. They generally cost about three times more than comparable flame weeders. A commercially available unit presently being marketed in North America uses a handheld wand that resembles an open flamer and is primarily designed for spot treatments. The unit costs about $300 USD, not including the cart and propane tank.
Steam weeders can be useful in an orchard floor management program. Early steamer models were extremely expensive tractor-mounted units. Pushcart models with handheld wands are now available in North America for around $4,600 USD, compared with a comparable flame weeder for around $300 or less. Steam weeders have promise for fruit growers, but their high initial cost, slow speed of travel, and large propane requirements limit their usefulness for many growers at this time. Expect improvements in design that will lower equipment costs and make the devices more attractive to fruit growers.
Infrared and steam weeders are less likely to cause unintended fires than flame weeders, but they can still damage fruit trees if used incorrectly. Be sure that the equipment’s design fits your application.
While conventional growers enjoy a wide range of herbicides registered for fruit crops, organic fruit growers have very little to choose from. Worse, herbicides that are accepted by organic programs do not work very well for anything but young seedlings. Herbicides can be effective for spot treatments in an organic orchard. Five basic types of organic herbicides are available to organic fruit growers: acetic acid (vinegar), citrus products, clove oil, corn gluten, and herbicidal soaps. Except for corn gluten, all appear to be effective against young seedlings, although none control established perennial weeds well. Combining soap and citrus oil products may prove more effective than either product alone.
Acetic acid can be used as an organic herbicide. Acetic acid is the active component of vinegar, and it chemically burns delicate plant tissues. The vinegar sold at grocery stores contains about 5 percent acetic acid; several commercially available herbicides contain higher concentrations. In tests conducted by Cornell University, household vinegar had some short-term effectiveness in suppressing weeds. The commercial products marketed as organic herbicides performed better, and a solution containing 20 percent acetic acid provided the greatest and longest -lasting control. Young annual weeds were killed readily. Quack grass was suppressed, but it continued to regrow, and the 20 percent acetic acid had to be applied three times to effectively suppress the quack grass. In contrast, a single application of glyphosate (not allowed in organic production) provided effective control of even quack grass throughout the season.
When comparing products, the authors of the study concluded that organically acceptable acetic acid products cost around $40 per 1,000 square feet of area treated. If three applications are needed, the cost rises to around $120 per year. Conventional growers can achieve equivalent or better control by using a single application of glyphosate herbicide for about $13.
Even household vinegar (5 percent acetic acid) can irritate eyes and respiratory systems, so products containing 20 to 25 percent acetic acid must be used with great care. Like soap and citrus oil products, acetic acid herbicides kill succulent green tissues and are probably best used for spot treatments within an orchard.
Several citrus products are approved for use as herbicides in organic crops. These products, some of which are on OMRI’s approval list, are based on various forms of limonene (citrus oil). These materials are effective degreasers. They act by dissolving oils and waxes in the epidermal layers of plants, causing those plants to desiccate (dry out) and die. To be effective, the materials must be sprayed on succulent green tissues. They can quickly kill green tissues (within about two hours of application) and are most effective against young annual and perennial plant seedlings.
Citrus products are not effective against established perennial weeds, such as Canada thistle or quack grass, because the oils do not kill the roots, rhizomes, or other underground organs that produce new shoots when the tops are killed back. These products can damage young trunks and stems, foliage, flowers, and developing fruit on fruit trees and bushes, but they should be safe to use in an orchard, given careful application.
Their greatest value would appear to be as spot treatments within tree and “sandwich” rows. At least one citrus oil product now approved for organic use also contains a soap or surfactant component similar to the herbicidal soaps described in this section. Surfactants help herbicides and other pesticides cover and stick to plant surfaces and can make some contact herbicides more effective.
Clove oil (eugenol) is found in several commercially available products, at least one of which is registered with OMRI. In some cases, the clove oil is blended with vinegar and other ingredients to increase the herbicidal activity. While these products show promise for ornamental landscape and garden use, more testing is needed to determine how well they work in commercial orchards.
Corn gluten meal is a by-product of the wet milling process for corn. It was originally patented and introduced by Iowa State University as a natural herbicide in 1991. According to the developers, corn gluten works when applied before weed seeds germinate, when it can inhibit the root formation of germinating plants. Unfortunately, after 2 years of trials in Oregon, researchers concluded that corn gluten meal did not control any of the weeds under any circumstances. It would appear that, for organic fruit growers, corn gluten might best be used as a fertilizer rather than an herbicide. It has a fertilizer analysis of 9-0-0. OMRI presently lists two corn gluten products, one of which is advertised as a fertilizer and soil amendment.
Several herbicidal soap products are sold for use in North America. The materials are quite similar to insecticidal soaps that have been used for many years, and they are marketed primarily for use in vegetable and flower gardens, landscapes, lawns, and areas such as driveways. Chemically, herbicidal soap products are salts of long-chain fatty acids. They act by damaging the outer layers of green, succulent plant tissues, causing the affected plant parts to wilt and die. They are not effective against woody stems or trunks and do not damage roots or other underground organs. The following discussion of one particular herbicidal soap, ammonium nonanoate, provides a good description of this type of product.
The following excerpts on the use of ammonium nonanoate are taken from the Ammonium Nonanoate (031802) Fact Sheet Related Information, issued by the U.S. Environmental Protection Agency (EPA) in November of 2006. This fact sheet applies to a particular herbicidal soap product presently approved by the Organic Materials Review Institute (OMRI).
Ammonium nonanoate is closely related to other salts of fatty acids known as soap salts. The active ingredient is a C9 saturated-chain fatty acid soap salt. It . . . is a clear, colorless to pale yellow liquid with a slight fatty acid odor. Ammonium nonanoate is a non-systemic, broad-spectrum contact herbicide that has no soil activity. . . . Ammonium nonanoate is to be used for the suppression and control of weeds including: grasses, vines, underbrush, annual/perennial plants, including moss, saplings, and tree suckers.
Ammonium nonanoate can be applied using all standard methods of liquid herbicide application, including hand-held, boom, pressure, and hose-end sprayers. For use, the concentrate is diluted with water to a specified concentration.[ . . .] For the active ingredient to be effective, the leaves of undesirable vegetation must be uniformly sprayed and thoroughly wetted. Application can be repeated as often as necessary to obtain the desired control.
The Agency concludes that no risks to human health will be expected from the use of ammonium salts of higher fatty acids (C8–C18 saturated and C18 unsaturated), based on their low toxicity and the fact that residues from pesticide use are not likely to exceed the levels of naturally occurring or intentionally added fatty acids in commonly eaten foods. Since ammonium nonanoate has the potential for eye, skin, and mucosal irritation [mucous membranes], the Agency is requiring stringent precautionary labeling. Exposure and attendant risks are expected to be negligible for applicators when they follow the directions for use by wearing the appropriate personal protective equipment.
Ammonium nonanoate is expected to degrade rapidly, primarily via microbial action, with a half-life of perhaps less than one day. Although ammonium nonanoate is slightly toxic to both warm water and cold water fish species and highly toxic to aquatic invertebrates, the agency believes that ammonium nonanoate, when used as directed, will not persist in the environment. If ammonium nonanoate is used according to the directions on the label, it should not seriously impact aquatic invertebrates because it is not applied directly to water and it undergoes very rapid microbial degradation in the soil.
Application rates of herbicidal soaps used for organic production are high. One product recommends using 26 fluid ounces of their product per 1,000 square feet of area covered. The same amount of glyphosate herbicide, which is very popular in conventional horticulture, would cover an area nearly 24 times as large and kill both annual and many perennial weeds.
While herbicidal soaps may have a place in organic orchards, they appear to be best suited for spot treatments of young perennial and annual seedlings. They are not effective in controlling established perennial weeds.
Chickens and weeder geese are used to control weeds in gardens and orchards. You will need to use pens to confine the birds to the areas that you want weeded. Young geese are more effective at weeding than older geese, and some people prefer Chinese strains over other breeds.
Unfortunately, organic certification standards greatly limit the use of livestock in orchards. The U.S. National Organic Program requires that livestock, including birds, be removed from a tree fruit orchard 90 days before harvest. For bush fruits, the livestock must be removed at least 120 days before harvest. For most organic orchards in North America, these rules effectively prevent the use of livestock for preharvest vegetation management. The animals can be returned to the orchard after harvest. (Chickens can also serve a valuable pest control role in orchards by devouring apple maggot pupae, as discussed in chapter 11.)
Insects are also used in biological weed control, although their usefulness in orchards is still very limited. Beetles, for example, are used to manage yellow starthistle and knapweeds in western North America, but neither starthistle nor knapweeds are typically problems in orchards.
Non-fruit plants in and around orchards provide many benefits. Clover and other legumes provide habitat for beneficial organisms and add nitrogen to the soil. Perennial grasses and alfalfa in the alleys reduce soil compaction and erosion while providing easy access in wet weather and reducing dust in dry weather. Alley and in-row crops also help prevent nutrients from leaching out of the root zone and release them slowly to the fruit crops. Cover crops add organic matter, play key roles in the formation of soil aggregates, and help create biologically diverse and active soil. Insectary crops provide habitat for beneficial insects and mites.
Several types of non-fruit crops can be used in orchards, including alley crops, in-row cover crops, and insectary crops. Depending on where and how the non-fruit plants are grown in the orchard, benefits include reduced soil compaction and erosion, improved soil structure, increased biological diversity and activity, increased nutrient cycling, and habitat for beneficial organisms.
Including an annual or perennial alley crop in your orchard is highly recommended when climate and soil moisture allow. Suggested annual and perennial cover crops are listed in table 9.1. These crops work well in most fruit-growing areas of the United States and Canada. Check with Cooperative Extension and provincial fruit specialists in your area for additional recommendations. In-row cover crops, also called living mulches, have great potential for organic orchards as well.
In general, alley crops are highly recommended for many orchard situations. Alley crops are critically important in reducing soil compaction caused by tractor and vehicle wheels, reducing wind and water erosion, maintaining or increasing soil organic matter, providing improved habitat for soil micro- and macroorganisms, reducing dust contamination on fruit, and improving access during wet weather. Depending on the alley crop, they can provide an attractive appearance, and grass cover crops provide a pleasant surface to walk on. These latter two benefits are especially important in home and U-pick orchards.
In choosing understory crops, try to select those that:
There are also disadvantages to alley crops, however, and you need to take precautions when using them. The two greatest concerns with alley crops are 1) competition with the trees for nutrients and moisture and 2) the crops can provide habitat for rodents that may damage fruit crops by girdling trunks and stems.
There are other potential drawbacks associated with alley crops. Besides rodents, they can harbor other pests and diseases that attack fruit crops. They increase the humidity low in the tree canopy, which can increase disease problems, particularly in low-growing crops. In arid climates, lack of precipitation and limited or prohibitively expensive irrigation water may make alley crops less attractive. Invasive weeds can also be troublesome in cover crops: In the arid Southwest, sandbur, johnsongrass, and Bermuda grass can all be invasive problems; Canada thistles, quack grass, and hawkweeds can make life difficult for fruit growers farther north. For home and U-pick orchards, cereal crop and alfalfa stubble are less attractive and less pleasant to walk on than grass. These cover crops may be better suited to commercial grower-pick orchards.
Each growing region faces its own particular challenges in orchard floor cover crop management. We will discuss strategies to overcome some of these potential problems.
For a newly planted orchard where the tree root systems are small, tilling under the alley crop to make it a green manure crop can provide some soil-building benefits. This practice, however, makes access to the orchard difficult during the growing season when the trees need care. A better strategy is to grow your green manure crops during the site preparation phase, before planting your trees.
Annual crops are often cereal grains, such as wheat, barley, oats, or annual rye. Austrian winter peas and other legumes are sometimes used as annual cover crops, and grains and legumes can be blended together in a single crop. Some annual cover mixes might include 80 pounds of wheat, barley, or oats per acre; 30 pounds of common buckwheat mixed with 4 pounds of clover; or 100 pounds of winter or spring peas per acre.
Buckwheat is well adapted to cool, moist climates, short growing seasons, and acid soils, but it is not winter-hardy and is killed by mild freezing temperatures. Buckwheat is particularly effective at taking up otherwise insoluble phosphorus and making it available to the fruit crop. Buckwheat can become a troublesome weed, so be sure to till it very shallowly into the soil before the seeds ripen. Mowing is not an effective way to prevent reseeding because some buckwheat plants are short enough to escape the mower blades, leaving some seed production.
When to plant. The best time to plant an annual cover crop depends on your climate and the availability of water. Where spring rainfall is abundant or water is readily and economically available for sprinkler irrigation, you can plant early in the growing season. This strategy is good for spring wheat, spring barley, oats, peas, and buckwheat.
In areas where precipitation during the growing season and irrigation water are limited, another strategy is to plant late in the season, after harvest in bearing orchards. This practice works well when using winter wheat, winter rye, or winter barley as an alley crop. No further care of the cover crop should be needed until the following summer. As with spring-planted alley crops, you may need to mow from one to several times. Leave the stubble in place until it is time to plant the annual cover crop again. In Washington State University trials, winter rye proved particularly effective in managing grassy weeds in orchard alleyways.
How to plant. In a new orchard, when tree roots are still confined to the tree rows, there are few problems with plowing in preparation for planting a cover crop. As the tree root systems expand, however, cultivating the alleys can damage the tree root systems. Remember that many of the tree feeder roots are located within the top six inches of soil.
Many years ago, some authorities recommended cultivating deeply around fruit crops to drive the roots deeper into the soil. Apparently they thought the fruit crops would be more drought-tolerant and have better access to soil nutrients if their roots were deeper. Deep cultivation, however, does not drive the root systems deeper into the soil. It simply cuts off the vital shallow roots that lie in the most nutrient-rich and biologically active portion of the soil. If you must cultivate in your orchard, keep the cultivation as shallow as possible.
When planting an annual cover crop through the stubble of the preceding cover crop, use a no-till planter, if possible. Set it at the shallowest planting depth that will serve for your cover crops. Regardless of the planter used, keep tillage as shallow as possible to reduce damage to tree roots. Suggested annual cover crops are listed in table 9.1.
How to manage. Mowing annual alley crops once to several times during the growing season prevents them from becoming overgrown and producing seed. Cereal grain crops provide excellent rodent habitat. Reduce this habitat by mowing the crops before they set seed. Mow by early- to mid-fall to leave a short stubble going into winter, when most tree girdling by rodents occurs.
Blowing the clippings into the tree rows during mowing helps reduce weed problems and adds organic matter to the tree rows. In areas with heavy rainfall or where the soil moisture levels are already naturally high, keeping the soil in the crop rows covered with mulch can increase root rot problems in some crops. Mulching has aggravated root rot problems in some New York apple orchards, for example.
Despite their advantages, especially for young orchards, annual alley crops have some drawbacks. Most importantly, the stubble provides little weed control and leaves the orchard floor open for invasion during much of the growing season. Wheat and barley stubble provide reasonable access for equipment, but they are not overly pleasant to walk on and are less desirable for U-pick and home orchards than smooth, turf-covered alleys. Before using annual alley crops, be sure that you have serious weeds under control, particularly established perennial weeds.
Perennial alley crops have many of the advantages of annual cover crops, as well as a few more. The greatest advantage is not having to replant each year. Permanent (or at least long-term) alley crops eliminate or reduce the need to cultivate the alleys, protecting the shallow tree roots.
Permanent cover crops should be very durable and resistant to wear from machinery and foot traffic. Particularly in organic orchards, where there are few approved herbicides, the cover crops must be noninvasive or they can move into crop rows and become weed problems themselves.
Permanent alley crops can be sod-forming or bunch-forming grasses, alfalfa, clovers, or mixes of these and other crops. You need to match the alley crops to your climate and irrigation practices.
In northern Idaho trials, I had success with a variety of sod-forming and bunch-forming grasses as orchard alley crops. A combination of sheep fescue, hard fescue, and white clover worked well as a low-maintenance alley crop, although this combination was slow to establish and benefited from sprinkler irrigation during the planting season. The advantage of this type of blend is that the fescues are naturally low-growing and become dormant during the dry summers, minimizing competition with the trees for moisture and reducing the need to mow the alleys. Various other bunch-forming grasses are available that are adapted to different climates.
A problem with low-growing, bunch-forming grasses is that they are not particularly good at competing with weeds. Especially troublesome are deep-rooted perennial weeds, such as Canada thistle, and spreading rhizomatous weeds such as quack grass. Make sure you have perennial weeds well under control before planting slow-to-establish or otherwise poorly competitive cover crops. Another drawback to the low-growing covers is that they produce very small amounts of clippings that can be blown into the tree rows to serve as mulch.
In areas with cold winters and particular types of soil, bunch-forming alley crops can suffer severe damage due to frost heaving. This damage makes weed invasion likely and occurred during my Idaho trials. Despite that, the fescues persisted and continued to provide a reasonably good working surface for 20 years. On soils that experience moderate to severe frost heaving, sod-forming grasses are probably better suited as perennial alley crops if sufficient soil moisture or irrigation is available for their establishment and maintenance.
White clovers can make good orchard cover crops. They establish quickly, provide good weed suppression, and add nitrogen to the soil. White clover stands are often fairly short-lived, however, and typically need to be reseeded every few years. In my Idaho trials, white clover thinned out to a few scattered clumps within 3 years, leaving mostly the sheep and hard fescue grasses. Depending on your location, certain other clovers can also be used as orchard cover crops and are longer-lived. In practice, blends of several different clovers are more effective than monocultures of a single species. Table 9.1 suggests clovers that can be used as orchard cover crops. Avoid using white or yellow sweet clover (Melilotus sp.), which are highly aggressive, invasive, and hard to control in orchards. These crops are best used for forage production and are classified as invasive species in some areas of North America.
One drawback with all clovers is that their roots, rhizomes, and seeds are attractive to rodents. Should you choose to include clovers in your orchard understory, also include an aggressive rodent control program (see page 372).
These are used successfully as perennial alley crops in home and commercial orchards. They require more frequent mowing than low-growing fescues, but they are more aggressive at competing with weeds and tolerate frost heaving in the soil much better. Orchard grass and tall fescues produce large amounts of clippings that can be blown into the fruit crop rows as mulch. A commercially available blend of perennial ryegrass and creeping red fescue, known as Companion grass, has proven popular in many different kinds of fruit plantings. This crop is relatively low-growing. While it does not provide much in the way of clippings to mulch the rows, it forms a dense sod and requires less mowing than taller-growing crops.
In arid climates, drought-tolerant perennial grasses can provide advantages as alley crops, although some are problematic. Bermuda grass is often planted as lawn turf in warm, arid regions. Native to Africa and Asia, Bermuda grass gained its name because it became a serious invasive weed there, and it can also become an invasive weed in orchards. Texas fruit specialists suggest managing Bermuda grass on the orchard floor if the grass is already present, but not to introduce it into an orchard.
Buffalo grass is less competitive than Bermuda grass and tends to be expensive to establish in orchards in arid regions. Buffalo grass has not proven hardy in colder areas. King Ranch (K.R.) Bluestem is a clump-forming grass that shows good promise as an orchard crop for warm, arid regions. Kleingrass, another clump-forming grass, makes a good alley crop for the Southwest, but it is more difficult to establish than K.R. Bluestem. Fescues work well in northern and southeastern U.S. plantings, but they tend to die out in Texas orchards.
In Washington State apple orchard trials, alfalfa proved to be an effective perennial alley crop. In that climate, alfalfa crops normally persist for about 5 years before they need to be replanted. In these particular trials, the alfalfa created an effective alley crop and the large amount of top growth that was produced provided good weed control when blown into the tree rows as mulch. In related trials, however, alleyways maintained in alfalfa became heavily weed-infested.
For both annual and perennial alley crops, a good practice is to mow adjacent rows alternately, several weeks apart, if possible. Alternate mowing leaves the predators and other beneficial organisms in every other row undisturbed for a time, allowing them to repopulate the mowed rows.
Alfalfa, clover, peas, and other legumes on the orchard floor will fix nitrogen, provided they are inoculated with the proper Rhizobium before planting. During establishment of the legumes, they compete with the fruit crop and other cover crops for nitrogen. When they mature or die, the legumes begin adding nitrogen to the soil. When alfalfa and clover are cut and the clippings blown into the fruit crop rows, the clippings add nitrogen to the soil there, even though the legumes in the alley remain alive.
Be cautious when using legumes in your cover crops. While nitrogen is necessary for plant growth, too much creates excessively lush, disease-susceptible foliage, reduced flowering, and poor fruit quality. This problem was observed in Washington State University orchard trials with alfalfa alley crops. Carefully monitor foliage nutrient concentrations and fruit quality. Be prepared to change the types of plants in your alley crop to adjust the nitrogen availability, if necessary.
Sometimes called nectary crops, the primary purpose of these plantings is to provide habitat for beneficial organisms, such as green lacewings, that are predators of orchard pests. Many of these beneficial insects depend on nectar, pollen, and other plant materials during certain stages of their life cycles. Insectary crops provide the habitat needed to complete their life cycles, and they also provide habitat for sufficient prey to support the predator populations. The goal in organic pest management is seldom to eradicate the pests. Doing so eliminates the food for beneficial predators that then leave or die, leaving our orchards open to rapid reinvasion by new pests. Our goal is to maintain enough pests to support the predators but not enough to do serious damage to our fruit crops.
Insectary crops can be annual, perennial, or a combination of both. They include such plants as dill, chamomile, hairy vetch, spearmint, Queen Anne’s lace, buckwheat, yarrow, white clover, cowpea, and cosmos. Other plants also serve as effective nectary crops, and a blend of different plants is better than a monoculture.
If you adopt some variation of in-row living mulches (see page 316), include insectary crops within the planting. Another option is to include insectary plants in the alleyway cover crop, although it can be difficult to manage an insectary crop in alleys due to such activities as mowing, and they may not be as effective as separate plantings. For example, if you maintain a bare strip within the tree fruit rows, blowing the seed heads of insectary plants into the rows as a mulch could create weed problems. Alley crops of all kinds provide some habitat for beneficial organisms. If you choose to include insectary crops in your alleys, you will have to actively manage them.
In a desert orchard, roadways and alleys are often dry and firm most of the year. In areas that receive abundant rain and/or snow, access during wet soil conditions can be challenging. Pruning, mulching, fertilizing, mowing, spraying, and harvesting can become difficult or impossible. The worst situation involves bare alleyways.
In general, sod-forming grasses for roads and alleyways provide the best year-round access. They may be supplemented with clovers or other nitrogen-fixing plants. For alleys, well-established alfalfa also provides reasonably good access. In arid locations where perennial sod-forming grasses are not well adapted, bunch-forming fescues, buffalo grass, and other arid-adapted crops can be used. Even stubble from an annual alley crop of wheat, rye, or barley can reduce soil problems while improving access.
A third option is to plant rows of insectary crops within the orchard, spaced about 50 feet apart. From a commercial standpoint, this reduces yields per acre and can reduce profits. It might be possible for commercial fruit growers to recoup a return on the investment by planting herbal or ornamental cash crops within the insectary crop rows. Regardless of where you plant the insectary crops, try to keep them within 50 feet of your fruit crops and preferably closer.
While intuitively it seems beneficial to increase the diversity and abundance of flowering plants in an orchard, either alone or as parts of alley and in-row cover crops, research has seldom shown measurable improvements in pest management with such an approach. Washington State University researchers report they are having better success targeting specific pests and their predators or parasites. Alfalfa, for example, has proven to be a promising insectary crop in controlling leaf rollers. Unfortunately, we are just at the beginning of this type of research. For now, strive for a biologically diverse understory and carefully monitor the understory and fruit canopy for pests and beneficials. Insectary crops provide excellent habitat for rodents, so employ aggressive rodent management practices (see page 372). Keep careful records of the understory crops that are present, your management practices (such as mowing), the types and abundance of pests and beneficials, and the fruit yields and quality.
In chapter 8, we discussed crop nutrition and fertilization in detail. Here we will integrate nutritional goals with orchard floor management practices.
Integrating tree fruit nutrition practices with vegetation management in the alleys and under the trees is far more complicated than simply adding fertilizer to the soil or foliage. Some alley crops add nitrogen to the system during one phase of their growth and take it away during another phase. Other alley crops provide no nitrogen to the soil and are always competing with the trees for nutrients, but even then they are of benefit to the fruit crops by preventing the nutrients from leaching out of the root zone. Alley and companion crops also add organic matter to the soil, increasing nutrient reserves for the fruit crops. In-row companion crops may compete for nutrients or add nitrogen, but they also alter when the nitrogen is available for the trees. Adding compost to the orchard floor provides nutrients and organic matter but can interfere with mowing, tillage, and other practices.
Although the situation is complicated, the approach is quite simple:
The way the orchard floor is managed has a great impact on soil structure and erosion. Part of the soil building program required by organic certifying organizations involves protecting and enhancing soil structure and reducing erosion. Alley crops are very helpful in preventing erosion, as discussed above. Even in the most arid climates, try to maintain at least an annual cover crop in the alleys, or a perennial crop of locally adapted plants.
If possible, maintain roads in permanent sod to reduce soil compaction and erosion, provide access during wet weather, and reduce dust contamination on the fruit. These areas are somewhat removed from the fruit crops, and competition for water and nutrients is less. You have flexibility in cover crop selection and management.
Mulches, whether inorganic, organic, or living, help prevent erosion within the crop rows and some add organic matter to the soil. Bare soil is highly susceptible to compaction and erosion. Even raindrops or droplets of irrigation water striking bare soil can damage structure near the surface and lead to sheet or rill erosion. To the greatest extent possible, reduce the amount of bare soil in your orchard.
For many years, tree rows have primarily been maintained free of other vegetation — a strategy that eliminates much of the competition with the fruit crops and reduces rodent problems. For newly planted orchards, vegetation-free strips in the tree rows remain the method of choice. During their first few years in the orchard, fruit trees lack the size and vigor to compete with other vegetation. Even if they survive the competition, they will likely remain stunted and unproductive.
Many strategies are available to prevent competing vegetation from establishing within crop rows. For very small plantings, hand weeding can produce acceptable results. One of the best implements for this task is a “speed hoe” equipped with a looped U-shaped blade. This implement cuts weeds off at or just below the soil surface and does not damage the crop’s roots. As the orchard grows beyond a small home planting, however, more efficient methods are needed.
Mulches have long been used to manage weeds in fruit plantings. These materials come in many different forms, but most act to prevent light from reaching the soil surface or reduce the amount of light. Some mulches also create physical barriers that seedlings and shoots cannot penetrate.
Inorganic mulches include plastic films and porous weed barrier fabrics, as well as gravel and crushed stone. The latter are really only useful for small landscape plantings.
Plastic films act primarily by forming a physical barrier that prevents weeds from growing. Dark-colored plastics also reduce the amount of light that reaches the weed seeds; germinating seedlings; and shoots from rhizomes, bulbs, and other underground organs. Clear plastic films are not good choices for weed barriers. In mild and cool climates, they serve as greenhouses and actually increase weed growth under the plastic. Even in mild climates, clear plastic heats up the soil, sometimes enough to damage or kill fruit tree roots. In short, avoid clear plastic film.
Black and other dark-colored plastic films can effectively eliminate most weed problems in crop rows. They do have many drawbacks, however. First, for all but the smallest orchard, they require large amounts of plastic made from a nonrenewable resource (petroleum). (So far, biodegradable, plant-based plastic films have not worked well as weed barriers.) As the plastic deteriorates, it creates significant amounts of waste that usually cannot be recycled. From an organic perspective, these characteristics make the benefits of plastic film questionable.
Impermeable plastic films also create challenges for irrigation and fertilization: You can run drip irrigation lines under the plastic, but spotting plugged emitters becomes impossible, and the films limit you to soluble fertilizers applied through the drip lines under the plastic. Plastic films also interfere with the movement of oxygen into the soil and the movement of carbon dioxide out of the soil. While impermeable plastic films have been valuable in some annual and short-lived perennial crops, their value in orchards is debatable. In warmer climates, the use of black plastic film has been problematic because of increased soil temperatures under the plastic. Reportedly, the problem has been particularly serious with highly dwarfing rootstocks. More vigorous rootstocks and larger trees have proven more tolerant of the increased soil temperatures.
Woven and spunbonded porous landscape fabrics (also called geotextile mulches) have proven more useful than impermeable films for orchard crops. High-quality fabrics effectively control weeds but allow water and oxygen to penetrate into the soil. Landscape fabrics allow less water to infiltrate than do organic mulches, and they can interfere with irrigation in some situations. The fabrics do prevent some pests from penetrating the soil, but they also prevent beneficial organisms from doing so. Some fertilizers can be applied over the top of weed barrier fabrics, but compost and alley row clippings will have little effect as fertilizers.
High-quality fabrics should last 5 to 10 years. By covering them with bark or other organic mulch, you can reduce deterioration due to sunlight and extend the life of the fabric. The downside here is that rodents that are repelled by the mulch tunnel under the fabric and attack the roots and collars of the trees. This has, reportedly, been a problem when wood chips were applied over fabric mulches. Although not all organic fruit growers find these weed barrier fabrics acceptable, they do have a place in organic production. Be sure your certification organization allows their use.
Rather than covering an entire row with weed barrier fabric, consider making squares, perhaps 2 to 3 feet per side. Center a square around each tree or bush at the time of planting, as we discussed in chapter 7. Leave a hole in the center of the square just large enough to allow the trunk or stems to pass through (figure 7.5). The fabric squares will greatly reduce weed growth near the young crop plants for at least 5 years if properly installed. They will not, however, interfere with irrigation, fertilization, or gas exchange. They also reduce the likelihood of root rot due to excessively moist soil under a large section of fabric.
For larger commercial orchards, plastic films and landscape fabrics are most useful in young, nonbearing fields. Harvesting operations and equipment can damage the films and fabrics in mature orchards.
Mulching crop rows with organic materials after planting can help manage weeds and is very popular for organic orchards. As with dark plastic films and fabrics, organic mulches reduce the amount of light that reaches the soil surface, thereby reducing weed populations. Seeds of many weed species must be exposed to light in order to germinate.
Types. The types of mulches vary by region, historically representing waste products from agriculture or forestry. In the northwestern United States, southeastern United States, and western Canada, bark and sawdust from lumber yards are often available locally. In orchard trials in British Columbia, researchers found that locally available wood wastes provided season-long weed control when used as mulches. In other trials across North America, wood chip and bark mulches usually provided good weed suppression for about 3 years before they needed replacing. Once free for the taking, however, these materials now have value as fuel for industry and municipalities and can be expensive to purchase, transport, and apply on a large scale.
In grain-growing regions, straw is often readily available and can be used for mulching. Straw mulches, however, provide excellent habitat for rodents and are generally not good choices for organic orchards. Many communities across North America now recycle yard wastes, either chipping them for use as mulches or composting the wastes. Be sure the straw or recycled yard wastes are free of pesticides before applying them to your orchard.
Coarse-textured products, such as bark and wood chips, generally help control weeds with fewer problems than fine-textured materials such as sawdust. In windy areas, it can be difficult to keep sawdust and fine bark from blowing away. Coarse bark proved very effective in orchard establishment trials and with young orchards in Washington State.
Shredded paper also provides good weed suppression but can be difficult to apply and must be replaced annually. Spray-on mulches made from slurries of shredded paper pulp suspended in water have provided very good weed control in orchards, but they require expensive mixing and spraying equipment and tend to be challenging to apply. When using paper mulches, be sure that the paper does not have glossy or colored inks. Spot treatments using hand cultivation, steam weeders (not flame!), and organic herbicides are typically needed to control weeds, particularly at the edges of the mulches.
Advantages. In some situations, organic mulches can be useful to orchardists by lowering soil temperatures, conserving soil moisture, and reducing the need for irrigation. Organic mulches can also add nutrients and organic matter to the soil, and some provide favorable environments for beneficial insects and microorganisms. In some cases, organic mulches have been found to increase populations of beneficial nematodes while reducing populations of plant parasitic nematodes.
Disadvantages. Organic mulches do not provide complete weed control and, when contaminated with weed seeds, insects, or diseases, can actually increase problems. Grass hays and alfalfa hay harvested after flowering, for instance, often contain large amounts of seeds and are not generally recommended for mulches. Perennial weeds, such as quack grass and Canada thistle, are especially troublesome in mulched fields because of their rhizomes. Rhizomes can spread quickly in some organic mulches, particularly sawdust or compost. Organic mulches, especially straw, can also harbor mice and voles, increasing the risk of girdling damage to orchard trees. When using agricultural and municipal wastes as mulches, contamination with pesticides can be a problem. Before using any materials, make sure they are pesticide-free.
In some regions, it’s a good idea to keep orchard soils cool and moist. If your soil is already naturally cool and moist, however, mulching can slow tree growth in the spring, increase problems with nutrient uptake, and greatly increase root diseases. In New York, researchers found that mulching greatly increased Phytophthora root rot and the death rate of young apple trees. This does not mean mulches cannot be used in your orchard; simply use them wisely. On soils that are slow to warm in the spring and that may already have ample moisture, delay applying mulches until early summer and rake them out of the tree rows in early spring.
Excessively deep mulches encourage collar rot and inhibit the movement of oxygen into the soil. Keep your mulches no more than about 4 inches deep, and keep them several inches away from trunks and canes to reduce disease problems.
Although often touted as adding nutrients and organic matter to the soil, mulches do not always achieve these goals. In Swedish trials conducted over a 4-year period, bark chip mulch did not add organic matter or biomass to the soil and caused nitrogen deficiency in black currants, despite manure applications that provided 178 pounds of nitrogen per acre.
Although compost is a good source of plant nutrients, it also provides excellent conditions for weed seed germination and growth. Compost also does not provide control against emerging perennial weeds. Consider compost to be a soil amendment, rather than a weed-controlling mulch.
Organic mulches for large plantings can be very expensive to purchase, transport, and apply. For example, applying 4 inches of bark to 3-feet-wide strips with the tree rows on 10-foot centers requires 177 cubic yards of bark weighing 35.9 tons per acre. Be creative and look for inexpensive, locally available mulch materials that will be effective and meet organic standards.
Regardless of the mulch that you use, take great care to eliminate perennial weeds (particularly rhizomatous weeds) before applying the mulch. Once the mulches are in place, your weed control options become more limited as mechanical cultivation may no longer be practical. Mulches can also make hand cultivation more difficult. Consider mulches to be a part of your orchard floor management program, and integrate them with your other practices.
Living mulches provide all of the advantages of the organic mulches described above, and they eliminate some of the disadvantages. Perhaps the greatest advantage of living mulches is that you produce them on-site in the alleyways between your trees or grow them as in-row cover crops. As part of routine alley mowing operations, the clippings can be blown into the fruit crop rows, where they serve as mulches. You avoid the expenses of purchasing and transporting the materials, and you greatly reduce labor costs associated with applying the mulches. You also eliminate concerns about pesticide residues and avoid problems with your organic certification organization.
By tailoring the alley crop, you can fine-tune how much nitrogen is available to your orchard crops. If nitrogen is too abundant, shift to grasses and other non-nitrogen-fixing alley crops. If more nitrogen is needed, increase the amounts of alfalfa, clovers, and other legumes in the alley crop mix. Both annual and perennial alley crops can be used effectively as living mulches.
Much work has been conducted on living mulches recently, and they show great promise for organic fruit growers. In Washington State trials, alfalfa proved especially valuable as an alley cover crop and source of mulch, providing good weed control in the fruit crop rows and adding nitrogen to both the alleyways and crop rows. In one orchard, an innovative fruit grower installed a front-mounted mower on his tractor and had a mechanical cultivator and brush rake behind. This arrangement allowed the grower to mow the alfalfa crop and move the clippings into and out of the tree rows, as well as to cultivate in the tree rows.
Living mulches are not without their disadvantages. You must still grow and maintain the cover crop, which must grow tall and dense enough to provide adequate amounts of mulch. You will need a mower capable of cutting the tall alley crops and blowing the clippings into the crop rows. The clippings in the tree rows provide habitat for mice and voles, and you will need an aggressive rodent control program (see page 372). Depending on the crop and time of year, the increased reflectance of light from alfalfa clipping–mulched rows can increase fruit sunburn in some climates or help color fruits where light intensity is lower.
In an innovative trial, British Columbia fruit researchers found that Equisetum arvense (horsetail or scouring rush) “greatly reduced the annual weed populations in the tree row without significant competition to established apples in a high-density planting.” Horsetail is a common and aggressive “weed” on moist soils and is very difficult to kill. In this case, the weed itself became an effective weed management practice. Foliar testing of the apple trees showed that the Equisetum caused little, if any, competition with the trees for nitrogen.
In the same trials, four perennial herbs were tested as possible living mulches: Arabis alpina, Cerastium tomentosum, Saponaria ocymoides, and Thymus serphyllum. The Saponaria ocymoides filled in the fastest and provided good early weed suppression. The Arabis and Thymus also produced good coverage and showed promise as living mulches. The Cerastium gave the poorest results.
A narrow strip of bare soil is maintained between the alley crop and in-row living mulch using a combination of thermal weeding, cultivation, and organic herbicides.
Swiss sandwich design. In this orchard floor management system, the trees are planted in rows, as usual, and either an annual or perennial alley crop is planted. Within the tree row, in what would be the weed-free herbicide strip in a conventional orchard, either legume or non-legume perennial crops are planted in a strip 3 to 4 feet wide. At the edges of the rows where the alley and in-row cover crops meet, a narrow strip of bare soil is maintained (see figure 9.1). If you use the sandwich system, wait until at least the third spring after planting your trees before establishing the in-row living mulches.
In extensive orchard floor management trials, Washington State University scientists examined the sandwich system in a newly planted organic apple orchard. The sandwich system was compared with clean-cultivated, wood-chip-mulched, and other living mulched tree rows. Results showed that there was less competition between weeds and young trees in the sandwich system than in a living mulch system that completely covered the orchard floor.
Although weeds were reported as being abundant in the rows mulched with wood chips, those trees produced good growth during their first 2 years in the field. The in-row clean cultivation treatment produced the lowest weed competition and also produced good tree growth, but it disrupted the tree root systems and caused the trees to lean. In the trials, both the wood chip and the clean cultivation treatments produced greater tree growth than the sandwich system.
Although the in-row living mulches (a.k.a. in-row cover crops) showed promise for controlling weeds and enhancing soil fertility and overall quality, they competed with the fruit trees, reducing tree growth. Rodents were a problem in the living mulches and represent the greatest challenge to the use of in-row living mulches. In the Washington trials, sweet woodruff (Galium odoratum) had significantly lower meadow vole infestation than the other crops tested, possibly due to the coumarin found in sweet woodruff. Japanese pachysandra (Pachysandra terminalis) has also proven somewhat repellent to rodents due to the presence of steroidal alkaloids.
How to use. We are clearly at the beginning of developing and understanding in-row living mulch cover crops. While they show great promise, as does the sandwich system, use them with caution. A leading tree fruit researcher highly experienced with living mulches advised me that rodent problems and the stunting of young trees during establishment were serious obstacles for which we do not yet have all the answers.
During the first 3 years after planting, maintain a vegetation-free area at least 4 to 9 square feet around each tree using weed barrier fabric squares, mulching, spot treatment of weeds with thermal weeders or herbicides, hand weeding, or a combination of these practices. Using blow-in mulch from a vigorously growing alley crop such as alfalfa can be beneficial. If possible, avoid in-row cultivation using tractor-mounted or other powered equipment during the establishment years.
Once the trees are well established, consider testing in-row cover crops to manage weeds and provide habitat for beneficial organisms. Protect the trunks against rodent damage using hardware cloth screen cylinders (figure 11.2), and maintain an aggressive rodent control program. If you choose to use living mulches, you might consider using slightly less dwarfing rootstocks than you would employ if there were no vegetation under the trees.
What to use. Your choice of living mulch depends on your climate and available soil moisture. Alfalfa, grasses, or grass-clover mixes can make good alley crops. In New Zealand trials, researchers tested red clover, rye grass, and an herbal crop. Which crop was used made little difference, but its management did. Blowing the mulch into the tree rows instead of leaving it in the alleys increased the nitrogen, calcium, phosphorus, and potassium in the tree row soil while substantially increasing soil organic matter.
For in-row living mulch cover crops, look for shallow-rooted herbaceous perennials that are adapted to your area, establish quickly, and form dense stands. Both legume and non-legume species can be used effectively. Legumes can fix nitrogen within the tree rows, but they are especially attractive to rodents. Annual clovers can be included with non-legume species to provide early weed suppression while the non-legume covers establish. Remember to inoculate legume seeds with Rhizobium to promote nitrogen-fixing nodes on the roots. Some clovers require special inoculants. Some organic growers allow grass sod to grow in the alleyways and up to the tree trunks, and manage the in-row grass by mowing before fruit thinning and harvest. Suggested in-row crops are listed in table 9.2.
These species may also serve as insectary crops and can be blended with grasses for alleyway cover crops.
Alpine rockcress (Arabis alpina) A perennial member of the Brassica (mustard) family with 16-inch flower stalks and a basal rosette of leaves. Adapted to gravelly and alkaline soils and grows best in full sun. Rated to USDA Zones 4–7.
Creeping thyme (Thymus praecox ‘Minus’) Produces a low-growing cover with good weed suppression and little cover for rodents. Can be slow to establish and requires supplemental weed control during establishment. May benefit from a nurse crop (annual rye or oats). Drought-tolerant in cooler climates. Does not perform as well in hot, dry areas. Rated to USDA Zones 2–9.
Mother-of-thyme (Thymus serpyllum) A low-growing, drought-tolerant, evergreen perennial that seldom exceeds 3 inches in height. Moderate growth rate. Tolerates sun to light shade. Rated to USDA Zones 4–9. Produces purple flowers. Grows best on well-drained soils.
Irish moss (Sagina subulata) Produces a low-growing cover with good weed suppression and little cover for rodents. Can be slow to establish and requires supplemental weed control during establishment. May benefit from a nurse crop (annual rye or oats). About as traffic tolerant as grass and rated to USDA Zones 4–9. Not drought-tolerant.
Rock soapwort (Saponaria ocymoides) A fast-growing, drought-tolerant perennial that grows 6 to 8 inches tall. Produces abundant pink blossoms. Prefers full sun. Rated to USDA Zones 2–9.
Snow-in-summer (Cerastium tomentosum) A hardy perennial member of the carnation family. Grows 6 to 8 inches tall and is adapted to dry, sunny sites and poor soils. Can be invasive. Tolerates mowing to about 2 inches tall. Flowers heavily. Prefers full sun. Rated to USDA Zones 1–9.
Sweet alyssum (Lobularia maritima L.) Establishes quickly and provides good weed control during the establishment season. May not survive cold winters but has the potential to reseed itself. A popular ornamental plant that is usually grown as an annual because it can become scraggly by its second year. Rated to USDA Zones 4–9.
Sweet woodruff (Galium odoratum) This perennial grows 12 to 20 inches long, but usually lays over and reaches heights of 6 to 12 inches. Establishes quickly and provides good weed control during the establishment season. Produces coumarin, which may have some potential for repelling rodents. Tolerates shade well and is not drought-tolerant. Rated to USDA Zones 4–8.
White clover (Trifolium repens) A number of white clover varieties are available. ‘White Dutch’ and ‘New Zealand’ have been used in orchard floor systems. They emerge quickly and form dense stands 4 to 10 inches tall with good weed suppression. White clovers provided the greatest percentage of ground coverage of the legumes tested in recent university trials. Not long-lived and may need to be reseeded every few years. White clovers are best adapted to cool, moist conditions. Rated to USDA Zones 4–9.
Strawberry clover (Trifolium fragiferum) Resembles white clovers in growth habit but produces stands that are less dense. More heat tolerant than white clovers but less tolerant of shade. Establishes quickly. Adapted to wet saline and alkaline soils but also reported to be drought-tolerant. Not long-lived and may need to be reseeded every few years. In California orchards, often mixed with white clovers, bird’s-foot trefoil, and creeping red fescue in cover crops.
Kura or honey clover (Trifolium ambiguum) Slow to establish and can benefit from high seeding rates, fall planting, and a nurse crop (annual rye or oats). Very persistent in grass stands when established but does not tolerate weed competition during establishment. Longer-lived than white clovers and produce less aboveground biomass that can attract rodents. Produces an exceptionally large amount of root biomass. Requires a special Rhizobium inoculant. Best adapted to cool and cold climates from latitude 40 – 50°N.
Subterranean clovers (Trifolium subterraneum, T. yanninicum, and T. brachycalycinum) Many varieties are available of these cool-season annual legumes that grow 6 to 15 inches tall. ‘Koala’ and ‘Clare’ are tall varieties that have performed well in California orchards. Can be slow to establish and may be best when fall-planted. Low-growing and produce less biomass than white clovers, possibly attracting fewer rodents. Limited cold hardiness (5°F [–15°C]) and best grown in milder climates. May best be sown in blends with medics to provide season-long weed suppression. Although annuals, subterranean clovers aggressively reseed themselves. Survive best in hardiness Zones 7–9.
Bird’s-foot trefoil (Lotus corniculatus) Bird’s-foot trefoil is a long-lived perennial legume that produces dense stands and grows well in mixed grass and trefoil cover crops. It establishes slowly and does not tolerate competition well during establishment. Fall plantings may be best to reduce competition during establishment. When it establishes, it provides good cover and weed suppression. Adapted to USDA Zones 2–9, but can be a bit difficult to grow in parts of Florida and other parts of the extreme southern United States.
Mechanically cultivating alleys is usually straightforward and easily accomplished with various handheld or tractor-mounted rototillers, as well as tractor-mounted disk implements and harrows. Mechanically cultivating within the crop rows is much more of a challenge from an engineering perspective.
There are several tractor-mounted cultivators designed to keep a vegetation-free strip within tree fruit rows, but they are generally not useful for bush fruit crops. Some of these devices use vertical blades that rotate horizontally, essentially stirring the soil with a series of metal fingers. The cultivators can be mounted on hinged bars that swing the cultivators into and out of the tree rows. A sensor bar detects the tree trunks, automatically swinging the cultivator head away from the trees in time to avoid damaging the trunks.
These devices have proven useful in larger organic orchards, although they do have several drawbacks. First, the stirring action can damage shallow tree roots. In Washington State trials, trees in test plots where the in-row cultivators were used leaned significantly more than in other plots, due to poor anchorage caused by root damage. Adjusting the cultivator for very shallow cultivation will minimize damage to the roots. Placing metal or wooden stakes next to the trees helps protect the trees from being struck by the sensor bar or cultivator. These horizontal rotovators have proven rather slow and difficult to use in orchard systems.
A different type of in-row cultivator is presently manufactured in Washington State and appears well suited for organic orchards. This device, called a Wonder Weeder (Harris Manufacturing; see Resources), uses a series of angled, rotating cultivator tines and a scraper bar to remove vegetation within the tree rows. This device mounts to the front of a tractor and can be used at the same time as a rear mower to manage both the alley crops and tree rows with a single pass.
In the past, when orchards were slow to come into fruit production and all operations in the orchards were performed by hand or with animals, intercropping vegetables and low-growing fruit plants between the trees was common. Even today, you can find this practice in some parts of the world.
Intercropping is almost nonexistent today in North America, certainly in commercial orchards where trees quickly come into bearing and tractors and other equipment are used in the alleys. Trying to manage vegetable or other cash crops in the alleyways will prove very difficult for most growers and will probably not be worth the effort. Orchardists are generally better served managing fruit crops on the orchard floor and planting vegetables and other cash or subsistence crops elsewhere.
There are applications for intercropping in orchards, however. Some herbal crops can be grown as part of companion plantings within the tree rows. Likewise, flowering crops suitable for honey production might be included in the companion plantings, provided they are not allowed to flower during the fruit crop bloom.
The best choices for companion plants are perennials or annuals that reseed themselves without being so aggressive that they compete excessively with the trees for nutrients and moisture. Tilling to create planting beds in or alongside tree rows could damage the tree roots.
While intercropping is certainly not for every fruit grower, there are cases where it can be used. Should you choose to intercrop, select plants that fit well with your orchard floor management program. Even better, choose plants that also support beneficial organisms.
By this point, you should know that I am not in favor of maintaining much bare ground in a mature orchard. Bare ground strategies, however, do have important roles to play, especially in newly planted and some high-density orchards.
When it comes to adding soil amendments and fertilizers, keeping the planting rows and alleys bare of any vegetation besides the fruit crop provides the greatest flexibility. This strategy also eliminates competition with weeds, companion plants, and alley crops for nutrients and moisture. Orchards with bare ground also have far fewer rodent problems than those with cover crops and mulches.
For newly established orchards, I strongly recommend maintaining a vegetation-free zone for a radius of 2 to 3 feet around each fruit tree or bush. This vegetation-free zone should be maintained for at least 3 years after planting. For high-density plantings, permanently maintaining a vegetation-free crop row can be a good strategy. The vegetation-free zone does not necessarily have to be bare ground. Various organic and inorganic mulches can be used to reduce weed pressure while protecting the soil.
Maintaining bare ground requires more labor and machine hours for mechanical and hand cultivation, raises fuel consumption if tractors are needed for tillage, increases soil erosion and compaction, and reduces soil organic matter. Bare ground can also be lacking in biological diversity and activity.
Using mechanical cultivation to maintain bare alleys does not work in all orchards. On even gentle slopes, erosion can be severe. In any orchard, tilling more than 1 or 2 inches deep can damage vital feeder roots of the fruit crops. Mechanical cultivation works best for annual weeds. It is less effective and can even increase problems with perennial weeds such as nutsedge, field bindweed, Canada thistle, quack grass, johnsongrass, and Bermuda grass. When faced with aggressive perennial weeds, you will need a combination of weed suppression practices.
In unirrigated orchards in regions that have little precipitation during the growing season, some fruit growers practice what is called dust mulching. Frequent, very shallow cultivation creates a layer of dust on the soil that helps reduce moisture loss from the underlying soil and reduces weed problems in the alley. Dust-mulched alleys are highly susceptible to wind and water erosion, and depending on the method of tilling, you can also create a shallow hardpan. Use this strategy with great care.
According to some orchard experts, the far-reaching aspect we call soil quality is strongly influenced by cover and soil management practices, as shown in figure 9.2. The take-home message is to use mechanical cultivation as little as possible and optimize the use of cover crops in established orchards.
Relative effects of orchard floor management practices on soil quality
Adapted from publications by Washington University