HEALTHY HERBS IN the garden or field begin with high-quality, disease-free plants and careful garden planning to suit specific cultivar requirements. These cultural needs include awareness of whether the plant grows best in full sun or in shade, dotes on dry or moist soil, or needs a high, low, or medium pH for optimum growth. Although large, healthy roots may be the most important part of the young transplant, the final stature of the plant is also important. Herbs may be small when you buy them at a nursery, but it is their ultimate height and width that determines the best site and spacing to grow them. Careful attention to all these factors early in the growing process is the key to healthy herbs.
Fertile soil is equally important for healthy, vigorous plant life, as is the variety’s ability to adapt to local climatic conditions. The identification tag that comes with the plant is the first place to look for cultural help. Many thoughtful herb specialists supply most of the basic information to grow healthy herbs on their plant tags or through free information available at their nurseries.
Successful gardeners have not only learned about plants’ unique requirements; they have also developed understanding of the visual language of plants. Plants communicate their internal condition visually, and the careful gardener learns to interpret the language accurately.
We have tried to outline much of the data specific to individual herbs later in the book. In this section, we want to concentrate on some simple, all-purpose gardening strategies to foster healthy herbs. Be mindful that no approach to plant health will succeed without frequent, even daily inspection of the herb garden to assess the plants’ condition. These visits to the garden will be more valuable when the gardener is familiar with the telltale signs that signal future problems.
It is difficult to define a weed, although every gardener recognizes what one is. It is, first of all, an unwanted plant that becomes undesirable vegetation in the garden. Herb gardeners are often sensitive to the word “weed” because so many cultivated herbs that are useful for food, flavoring, and medicine, are considered weeds somewhere in the world, sometimes in American backyards. The list of plants that reflect this duality is long; it includes such familiar flora as yarrow (Achillea millefolium), couch grass (Agropyron repens), lamb’s quarters (Chenopodium album), chicory (Cichorium intybus), chamomile (Matricaria recutita), sheep sorrel (Rumex acetosella), chickweed (Stellaria media), and dandelion (Taraxacum officinale). The problem with weeds is not that they present a universal evil, or an uncontrollable menace; they are just plants that sprout at the wrong time and place.
Small, slow-maturing herbs are no match for fast-growing weeds, harbingers of potential garden disasters. Such weeds cut off air and light as they crowd small plants. Weedy growth robs herbs of necessary moisture and nutrients and creates breeding areas for insects and fungal diseases. To the farmer, weeds cut profits because they lower yields and increase expenses for pest and disease control. To the home gardener, they present a never-ending chore that diminishes the pleasure of gardening.
While elimination of weeds appears to be a desirable goal, Stephen C. Weller, a weed scientist at Purdue University, notes that “eradication is seldom achieved.” Few herbicides that might do the job effectively are registered for use with herbs. “Even when herbicides are available,” Weller concedes, “their usefulness is limited due to the nature of the crop and the overall effectiveness of the herbicide available.” For the herb grower, he concludes, cultivation techniques that include “extensive hand hoeing” may be the most common way to control weeds.
Gardeners are greatly vexed by weed seeds that can stay dormant deep in the ground for many years. When they rise to the soil’s surface during soil preparation, they germinate and become problems. Anyone who has seen weeds growing in the cracks of city sidewalks must admire, even grudgingly, their tenacity and omnipresence.
Like other plants, weeds fall into three classifications: perennials, biennials, and annuals. Perennial weeds live for two or more years on winter-hardy roots. Many of these weeds are produced from seeds, but creeping varieties may also reproduce from spreading roots or root-like structures called rhizomes. Buckthorn plantain, pokeweed, and dandelion are examples of seed-grown perennials that overwinter on hardy roots. Ground ivy, bindweed, quackgrass, Johnson grass, and Canada thistle are common creeping weeds encountered by American growers. A few biennial weeds also trouble perennial herb growers. These plants produce seed after exposure to cold, usually in the second year of life, and then die. Wild carrot, common mullein, and bull thistle may take up residence in the field and garden.
Annual weeds germinate, grow, flower, produce seed, and die in a single year. They are divided by the time of year they grow. Some annuals come to life under cool fall conditions and overwinter; they flower and set seed in the spring or early summer before dying. Examples of these are chickweed, henbit, Virginia pepperweed, speedwell, and shepherd’s purse. Summer annuals begin growth in the spring and die with fall’s hard frosts. Purslane, ragweed, ivy-leaf morning glory, crabgrass, and yellow foxtail are examples of these.
Perennial weeds are the most difficult to control; those with long taproots seem to defy banishment. Even careful soil preparation to eliminate these weeds in the home garden—the best way to conquer them—is often marked by failure. A small morsel of root overlooked in the garden will spring to life and eventually stiffen the back of even a careful and forgiving gardener. Where many acres are cultivated, a program of plowing unplanted land to control annual weeds and applying herbicides on perennial species is often effective, according to Weller.
Herbicides are traditional remedies for weeds in agricultural crops. Two types of weed killers are used. A pre-emergent herbicide is applied before the weed seeds germinate; a post-emergent herbicide is applied to weeds in full growth. But Weller points out restrictions on using these herbicides: “Outside of spearmint and peppermint, few herbicides are presently registered for use in other herbs.” Little research has been performed to determine the effect of herbicides on herbs because of the high cost of registration. This high cost, coupled with the limited acreage in herbs, has deterred large agrichemical firms from registration testing.
In recent years, a federal government program called IR-4 has permitted researchers to perform much of the preregistration herbicide and pesticide analysis for minor crops such as herbs. Their work has led to the use of some herbicides and pesticides on a state-by-state emergency basis.
Hand weeding is not an unpleasant task in a small herb garden, but even there it is a chore that is easily postponed. A variety of mulches, inert materials that smother weeds as they germinate, may prove to be an alternative and lessen the need for hand cultivation. Straw, leaves, sand, small stones, wood and bark chips, and even newspaper have traditions of use as mulch. Plastic mulches met immediate acceptance with American farmers and many home gardeners; the material now covers hundreds of thousands of acres.
Plastic mulches are favored by many commercial growers because they are easy to apply. Two basic types of plastic sheet mulches are available. One is made from woven strands or from a solid plastic sheet with tiny holes punched in it. This type permits water to flow through to plant roots while suffocating weeds. A more traditional type is a solid black sheet of poly that does not permit water penetration; some system of drip irrigation to supply moisture is needed under this type of plastic mulch.
The weight of the plastic material and method of manufacture provide a guide to its lifespan in the garden or field. Two polyethylene mulches provide up to three years’ protection—VisPore Black Mulch, a 4-mil fabric, and Weathashade Weed Stop, weighing in at 7 mil. Weed-X Landscape Fabric, a 7-mil polyolsin material that is black on one side and white on the other, has a life of about five years.
Some vegetables respond to specific wavelengths of light reflected from plastic mulch, USDA researchers Michael J. Kasperbauer and Patrick G. Hunt found. A red mulch increased tomato yields by about 20 percent and improved the quality of the fruit. Tests on peppers showed yields increased about 20 percent with a white plastic mulch. Although they have not been tested thoroughly, herbs may also respond to reflected color. White sand has been shown in at least one case (with ‘Dutch’ lavandin) to provide increased yields, and white plastic may prove efficacious for herb growers. Mulching with light-colored sand, small stones, marble chips, and/or ground oyster shells also smothers weed growth, cools the soil, and radiates drying light and heat through plant interiors, lessening the spread of diseases; marble and oyster shells also supply extra calcium for the herbs from the Mediterranean. Dark organic matter used as a soil mulch around herbs has been associated with some soil and foliage diseases, partly because of its ability to retain moisture.
Allen Barker of the University of Massachusetts considers sand, gravel, and stones beneficial only in the short term. “Over time,” he says, “the weed growth in the sand or gravel and around stones destroys the effectiveness of these materials, making the planting unattractive and requiring control by hand or herbicides.” Barker favors 3 or 4 inches of straw or sawdust for weed control. “Composts are excellent for weed control and usually contain few weed seeds,” he says, but hay and manures often contain weed seeds. Aggressive weeds such as nutsedge, dock, perennial grasses, or Jerusalem artichoke can be controlled with plastic sheets or heavy paper. “A layer of newspaper about five to eight sheets thick placed underneath 1 or 2 inches of straw, compost, or manure will control most weeds,” he believes.
An innovative mulch, suggested by Barker, consists of vegetable garbage ground in a garden shredder and poured around plants. The concoction dries into an impervious mat.
Mulches of clear plastic can be used to warm soil 6 to 10 degrees Fahrenheit to a depth of 6 inches for an early spring start. “At a depth of 4 inches,” says Barker, “soils under leaves can be as much as 20 degrees Fahrenheit cooler.”
Along with the benefits of mulches, there are disadvantages. “Several cultural problems can counter any beneficial effects of mulching,” according to Barker. “Packed mulches will reduce plant stands, especially with perennial herbs where each year’s new growth must emerge through the mulch.” Another problem pointed out by Barker is that mulched crops have shallower root systems; as mulches decay, the root systems come closer to the surface, where they require more water and become vulnerable to physical damage.
Living mulches composed of alfalfa and red clover in northern climates and subterranean clover in southern areas also provide weed control, prevent erosion and pests, and may add nitrogen to the soil. It is important to grow herbs in an 18-inch strip with the living mulch growing on either side. The mulch should be kept cut low.
It is easy to see why stem pruning becomes almost instinctual around herb plants. The foliage begs to be touched and release its aroma; it insists on being cut away and savored. This constant pruning is good for herbs because it shortens their stems, reducing their natural tendency to become leggy, tangled whips that provide ideal conditions for the spread of disease.
Where climates have high summer humidity, pruning is often necessary to keep plants disease-free or to restore fungus-plagued plants. Humid conditions and rapid new growth often coincide with many herbs’ most handsome period; it is also a time when summer turns on its furnace and trifling rain showers provide the perfect conditions for rapid fungus growth.
Fungus grows in a moist, shaded atmosphere and spreads on droplets of water. It thrives on dead leaves and other interior plant debris and goes unnoticed by the garden stroller. One day, often after a summer shower, the darkened, fungus-killed leaves suddenly show through their green canopy. Unfortunately, it is necessary, when faced by these conditions, to make gorgeous herbs look ugly.
A midsummer “buzz” haircut with sharp pruners helps fungus-susceptible herbs such as culinary sages, whether variegated, small or large-leaved; semi-upright thymes that make thick mounds; French tarragon that bends under its own weight; thickly branched santolina; tangled oregano; heavy stands of thick mints; floppy lavender; unruly rosemary; wildly spreading lambs’ ears; and others herbs that embrace the earth too closely. If the disease has not progressed too far, a quick cleanup and pruning—the horticultural equivalent of a shave and a haircut—does the trick. Once the dead foliage is removed and the plant is opened to drying air and sunlight, the need for other preventive treatment decreases.
This polite surgery, often undertaken in late spring or early summer, is definitely not a whack-whack affair, though. One third to one half of each stem, especially those nearest the ground, should be carefully removed. The remaining stems, usually covered with soggy dead foliage, are thoroughly cleaned, and debris on the ground removed. At the plant’s base, extremely low branches may need to be removed to permit air circulation under the plant. The purpose of this gentle beheading of herbs is not only to root out sources of evil; it also encourages new, vigorous growth that produces a bountiful late summer harvest.
Part of the need for this midsummer snipping session is the speedy growth of some established herbs. In addition, if spring pruning was skipped, summer disease problems are more likely. Spring is when careful herb gardeners create stick gardens to rejuvenate many woody-stemmed perennial herbs and to control their growth according to the space available in the garden. When one plant grows faster and larger, its neighbor may suffer from disease fostered by lack of air and light.
The spring pruning ritual begins when small nodules begin to swell along herb stems. These infantile branches will burst into growth after pruning. Thyme, sage, lavender, rosemary, and other woody perennials welcome these spring rites. It is useful to look among the sprouting wood carefully for winter damage and remove dead branches and stems. As with summer pruning, shorten the branches of these budding herbs by one third to one half their length. If plants have been wintered indoors, more of the stem length may need to be removed. Inside or outside, leave some greenery on the branches or make sure there are plenty of growth nodules remaining on bare stems.
The goal of this spring amputation is to energize the lower reaches of the plant with new growth. Branching often comes at the end of unpruned stems, and if they are not cut as spring growth commences, the lower part of the plant becomes woody and barren of foliage. Pruning herbs is not a simple indulgence that begins the process of adding flair to food; it also provides a practical means to grow healthy plants.
After a tough winter—almost every winter to those who garden where temperatures drop below 25°F (−4°C)—gardeners are flushed with both enthusiasm and anxiety. The anxiety is created by concern over the continued existence of their perennial herbs: Did they survive the winter?
The wait for the first signs of spring growth is sometimes agonizing, partly because replacement plants may not be available if the wait is too long. How long should you wait to write the death certificate? Spring comes at different times of the year throughout the United States, despite what the calendar may say. There are guidelines for some common culinary herbs, based on observable growth of other plants.
French tarragon, French sorrel, and chives are among the first herbaceous herbs to show new growth, which springs from energy stored in the roots. Look for this growth as the daffodils and forsythia bloom. Cut last year’s top growth from these plants shortly after the plants die back in the fall, or during the winter to decrease chances of disease after the new growth appears.
Thyme, lavender, and sage begin to show swelling growth nodes along their woody stems about the time the lilacs bloom. Mints and oregano begin to push fresh shoots through the spring soil around the same time. The evergreen rosemary will begin to show new growth at this time also.
These benchmarks are general, for some variations occur among cultivated herb varieties. Severe cold or a late hard freeze sometimes stuns unprepared plants and cautions them to be slow starters. Sadly, new growth on a winter-damaged plant is often feeble and weak, and while it is alive, it may not be worth saving; most severely damaged plants should be replaced.
Nearly everybody recognizes that a droopy plant needs water; wilting is a plant’s visual messages about its health. If you’re an attentive gardener, it’s as if your plants talk to you.
Misinterpreting a plant’s subtle visual messages is common because people and plants don’t speak the same language. For instance, when rosemary leaves turn black or brown and drop, it is commonly believed to signal an insufficient humidity in the air. This prompts the often-heard advice to mist the plants.
What usually makes rosemary leaves turn color and drop has to do with moisture, but not around the leaves. Too much water around the roots makes them rot and discolors the leaves at the other end of the plant.
As even a wallflower at a high school dance knows, one glance does not a conversation make. It takes more than a wink and a hello to create meaningful dialog with your herbs. Just as our own language is tangled with ambiguity, an herb’s messages may have several translations. A thorough assay of all the possibilities is useful when testing possible ways to rescue sick plants, but it is essential to have some basic information about the cultivar under study. What are the herb’s optimum cultural requirements: light, temperature, fertility, water needs, and soil pH? What insects and diseases typically attack this cultivar?
It is not easy to find a good grammar explaining plant language, either. Many herb books give pests and diseases a quick brush-off, as if problems didn’t exist or didn’t matter much. Unfortunately, herbs have as many of these problems as other plants.
Here’s an example of how a concerned gardener might develop a good bedside manner. Let’s have a conversation with four basils, beginning with a general examination that leads to questions that probe the vital signs. From ten yards away, it appears that the basils have been pitifully neglected. The plants are stunted. The leaves are limp and yellowish. Days of hot sun have caked and split the soil. It is easy to see that the plants are too dry and need water. But what caused the lack of growth and the anemic leaf color?
To quiz plants, a basic vocabulary of plant sign language is necessary. The yellowed leaves are saying something. Now, basil is a big nitrogen gobbler, and yellowed leaves and failure to grow might be a way of showing a lack of water-soluble nutrients. To test this hypothesis, mix and apply 20-10-20 liquid fertilizer (expressed as 20 percent nitrogen, 10 percent phosphorous pentoxide, and 20 percent potassium oxide) in the amount of water recommended by the manufacturer (fish emulsion may be substituted). This remedy combines prompt irrigation and essential nutrients for growth. If the plants are no longer wilted in a matter of hours, part of the cure is clearly working. If the color has returned to the basils’ leaves in a week and the plants begin to grow again, we know the procedure has succeeded; the interpretation was correct.
When plants yellow, even with optimum nitrogen levels, pH may be the culprit. When the pH is too high or too low for the specific herb, the plant cannot take up nutrients and break them down into a useable form. This single problem can be devilishly difficult to diagnose. A pH test of both irrigation water and soil should be taken. Home soil test kits are available or soil samples may be sent through a U.S. Department of Agriculture extension agent for analysis.
Later in the season, the same basils that perked up quickly with a dose of fertilizer may appear wilted again, and even lose some leaves, although the soil does not appear dry and most leaves look quite normal. On careful inspection, however, brown lesions can been seen on the stems just below the wilted area. This is a telltale sign of a moisture-loving basil killer called Erwinia that performs its dirty work by interrupting the flow of water and nutrients up the stem. Some research has indicated bark mulch around the base of basils may effectively combat Erwinia’s debilitating actions, but certainly the plant’s diseased stems should be removed, and a general cleanup of dead foliage is in order.
Even something as simple as an overabundance of nitrogen may become a problem. While there is little evidence that an herb’s flavor is affected by such a condition, herbs grown for their flowers respond to a nitrogen glut with few blooms, although they appear super healthy and have large, dark-green leaves. Too much shade may produce similar symptoms, but plants grown in poor light tend to appear thin and lack the bushiness of those grown in full sun.
Problems occur with more obscure nutrients from time to time, in even the best-kept garden. Accordingly, here are the symptoms of a variety of nutrient deficiencies.
Too little phosphorus in the soil may bring about unusually small plants with thin stems and smaller-than-typical leaves. Foliage and stems may take on a purplish tint followed by early defoliation. On the other hand, too much phosphorus, although rare, may cause conditions resembling nitrogen, potassium, or zinc deficiency.
Uncommonly weak growth and green leaves with abnormal browning along their margins and sometimes leaves that are unnaturally curled may herald a potassium deficiency. An excess of potassium mimics magnesium deficiency.
Leaf yellowing followed by browning of leaf tissue is symptomatic of deficient magnesium, iron, or boron.
Yellow, young terminal growth with the older, lower leaves remaining green is a sign of a lack of calcium. As the process continues, growth becomes stunted and stem-tips die; new shoots exhibit abnormal growth and then die. An examination underground will reveal stubby roots with dark spots. Ammonium-based fertilizers used on container plants often cause calcium problems. Using fertilizers from calcium nitrate are recommended to keep calcium levels stable. Ground limestone may also be added. Too much calcium may cause leaves to yellow, a condition termed chlorosis.
Diseases and insects also leave telltale signs that speak to the educated gardener. In some cases, the disease attack may be caused by environmental conditions over which the gardener has little control, such as an extremely wet summer or winter. Diseases also take advantage of plants’ weaknesses created by lack of nutrients; the disease may enter through dying foliage or soft growth created by an overabundance of nitrogen in the soil.
The following conversations with plants offers some solutions, as well as a methodology of communicating with herbs and other garden plants. As with any conversation with a plant, a translation of the original message is necessary before a response can be formulated.
Message: Brown leaf tips.
Translation: Over- or underwatering may be the culprit, but too much fluoride, copper, or boron in the water can produce the same appearance; so can excessive fertilizer.
Response: If changed watering and fertilizing techniques fail to alleviate conditions on new leaves, check water quality with a comprehensive test.
Message: Foliage is covered with a white powdery substance.
Translation: Suspect powdery mildew.
Response: Plants are most susceptible to powdery mildew when humidity is high, temperatures are below 80°F, and there’s poor air flow. Leaf dampness and low light conditions also favor mildew growth, which is why plants such as rosemary become infected when they are brought into the house for winter. Powdery mildew is usually not a problem where plants are out in the open, especially when afternoon temperatures are above 86°F (30°C) and air is continually circulating through the foliage.
Combat this infection with sprays of a solution of 2 tablespoons of baking soda in one quart of water. If this fails, try sprays of garden sulfur. Use caution, however: high temperatures combined with sulfur sprays are sometimes toxic to plants.
Message: Leaves have an oily, greasy appearance or have water-soaked spots; plant stems wilt and blacken.
Translation: Probably a bacterial disease.
Response: Destroy infected plants; do not reuse their potting soil. Space healthy plants so that air can circulate around and through plants. Water so that foliage receives little moisture. Keep plants in as much sunshine as they can stand to create strong leaf tissue.
Message: Plant roots are brown to black instead of their usual firm consistency and light or white color.
Translation: Could be root rot.
Response: Roots need air. When they are irrigated, water fills the small holes in the growing media that contain air. If plants are watered too often, or given too much water, air cannot reach the roots, and they begin to die. It is sometimes helpful to test how long a plant can go without water before it shows signs of stress and then adjust the quantity and frequency of irrigation.
Message: Leaves and stems of new and old tissue are covered with a gray or brown mold.
Translation: Likely culprit: Botrytis.
Response: Botrytis is encouraged by cool, damp weather and conditions that cause stress to plants. Provide plants with plenty of sun or longer hours of plant lights. Be careful not to overwater; instead, toughen plant tissue by withholding water. Keep plant leaves from touching so that air can circulate through and around them. Indoors, use a fan to gently push air through the plants.
Message: Dead leaves hanging on branches low to the soil, while upper leaves are healthy.
Translation: Suspect fungus disease.
Response: The disease is spread by moisture and reduced air flow. Remove infected, dead, yellowing, and damaged foliage. Increase air circulation within and around the plant by judicious pruning and increased spacing. Add a pea-gravel mulch or white sand to reflect heat into the plant and help keep leaves dry.
Message: Lower leaves wilt, yellow, and die. Process proceeds up the plant. Plants may suddenly wilt and die.
Translation: A good chance that the soil is infected with Pseudomonas, Verticillium, or Fusarium.
Response: Plant resistant varieties or set new, disease-free plants in uncontaminated soil.
Message: Sudden collapse or wilting of the plant when adequate irrigation is available.
Translation: Suspect Phytophthora, Fusarium, Verticillium, and/or Pythium, soil diseases that attack many herbs native to the Mediterranean basin. These diseases clog the plants’ vascular tissue and reduce the transmission of water.
Response: Phytophthora, Fusarium, Verticillium, and/or Pythium are molds and cause damage to a wide variety of cultivars. These diseases are often accelerated by over-watering, poor drainage, and inadequate air circulation. Copper foliar sprays are sometimes effective on infected plants. Changing the plant’s location may be helpful, but a fresh start with new plants is desirable. Incorporate abundant sand and gravel into the soil, enough to raise the planting bed’s level above the rest of the garden. Consider a light-colored mulch of sand, gravel, marble chips, or ground oyster shells. Mycorrhizal (beneficial) fungal dips may be helpful in some instances.
Message: Your chive shoots in early spring are covered with black insects.
Translation: Aphid infestation.
Response: Aphids are among the most common garden pests, with more than 4,400 species. They feed by sucking plant sap, and a heavy infestation can stunt and deform their hosts. In addition, about 60 percent of plant viruses are transplanted by these little critters. Aphids are often seasonal in the garden, especially during periods of cool nights.
Light aphid infestations in the garden can be handled with a strong spray of water that knocks the insects from the plants. If the garden or greenhouse is overrun, spray a combination of insecticidal soap and rotenone. Biological controls include lady-bugs, lacewings, parasitic wasps, and insect-pathogenic fungi.
Message: Bay tree leaves feel sticky and are covered with a soot-like substance.
Translation: Most likely a scale infestation, especially if the bay tree was summered outdoors and brought in for winter.
Response: Scale, a tiny sucking insect, is commonly found on bay plants. Coating the plant with a fine horticultural oil mixed with water and applied either by spraying or dipping (if the plants are small) suffocates the insects and gives the plant a pleasant, healthy shine.
Message: Tiny flies float around potted plants.
Translation: Fungus gnats, probably.
Response: In greenhouses, stores, and homes where light levels are low and soil moisture is high, fungus gnats are likely inhabitants. The adults are merely unsightly, but their offspring burrow into roots and stems and often transmit diseases such as pythium, fusarium, phoma, and verticillium. Bacillus thuringiensis (BT) is a spore-forming bacterium that destroy the larvae of fungus gnats; use it to drench the soil.
Message: Green or yellowing leaves show pinhead discolorations
Translation: Thrips or spider mites. Thrips leave an irregular, discolored pattern that looks like the pigment has been sucked from small areas of the leaves. Spider mites leave many tiny, round marks on leaves from their feeding; the color has been drawn from these areas. Minute webbing with tiny insects scooting over it may be visible during major infestations of spider mites.
Response: Sprays of insecticidal soap and rotenone every three days for two weeks can break the insect life cycle and control the pests, if not totally eradicate them.
Message: Wiggly discoloration through leaf surfaces, especially sorrel.
Translation: Probably leaf miners, the larva of the flying adult. Little clusters of eggs are often clearly visible on the undersides of leaves.
Response: Create a barrier to prevent the adults from laying eggs by covering foliage with spun-bonded poly, especially in spring and fall.
Message: Ragged holes in leaves or leaf-margins, particularly basil and sorrel.
Translation: Probably night-foraging slugs who spend their days under layers of mulch, large stones, and sidewalks, wherever there is darkness and moisture. Also look for caterpillars; they normally feed during daylight hours.
Response: Seek out and destroy these pests’ resting places. Trap the pests by placing flat pieces of wood or inverted, scooped-out melon shells on the soil—a veritable slug hotel—on the soil. During the day, destroy the guests. Four-inch-high copper flashing pushed into the soil on its edge makes an impervious slug fence. Some mail-order garden-supply firms sell copper material to block slug attacks, and copper flashing is available at roofing supply firms. Recent tests of beer as a bait and trap for slugs showed the brew was not effective at significantly reducing slug predation of strawberries, and the beer-baited traps are effective for only a few feet.
Message: Poor growth, yellowing leaves, dying growing tips.
Translation: After ruling out pH imbalance, poor drainage, and lack of nutrients, consider microscopic soil insects called nematodes. These pests burrow into roots, causing raised nodes, interfering with nutrient uptake, and slowly killing the crop. Nematode-infested roots can spread the critters to previously uncontaminated areas, particularly when field-grown plants are dug and sold or new plants are created from divisions. Nematodes also promote the spread of soil-borne fungi.
Response: Soil enriched with grass clippings destroys most nematodes. If infestation is severe, turn to container gardening using a growing medium containing equal parts of peat moss and perlite.
In rare occasions, the coveted plant is simply the wrong one for the site or the climate. This was the case with lavenders planted on the edge of nineteenth-century London in the famous fields of Mitcham. The lavenders grown in Mitcham were gorgeous and profitable, but they were ravaged by shab (Phoma lavandulae), a disease prevalent in Great Britain. The disease causes young spring growth, which carries lavender’s expressive flowers, to yellow and wilt. After the shoots die, the disease spreads into the plant and eventually kills it. Selection of lavender varieties resistant to shab, a disease unknown in the United States, eventually overcame the problem.
To grow healthy, vigorous herb plants, gardeners are wise to practice the art of observation. Reading herb leaves may at first seem as tricky as reading a crystal ball, but this plant language can be learned by observation, knowledge, and repeated close encounters with failure. For most gardeners, failure is the second-best teacher; the first is having a wise, old gardener for a neighbor. When your own talent as a reader of herb leaves is unequal to the task, the local agricultural extension agent may be able to help or can refer you to a specialist at the state university, whose experience is probably wide enough to interpret your plant’s vital signs.
Many herbs, as well as other plants, have gained reputations as nurses who help restore vigor to ailing flora, ward off insect infestations, and by their mere presence make nearby plants grow better. Such ideas have existed for centuries in folklore and were widely spread in the United States beginning in the 1940s, when interest in French intensive gardening methods grew. The idea that was eventually called companion planting was popularized by Rudolf Steiner’s bio-dynamic movement and takes its common name from a 1943 pamphlet called Companion Plants, written by Richard B. Gregg.
The movement got a large boost by the publication in 1966 of Companion Plants and How to Use Them by Gregg and one of the method’s leading champions, Helen Philbrick. Rodale Press’ Organic Gardening Magazine, the largest home-gardening publication in the United States at the time, also promoted the method by publishing numerous articles. Increasing environmental awareness and the desire to produce vegetables and herbs without using pesticides buoyed the movement’s popularity. Companion planting has taken on a life of its own, separate from its narrow beginning in a small cultish group, and now has a ready acceptance.
From the beginning, the idea of placing differing plants close to each other to provide improved growth or keep insects at bay was more than a simple gardening technique. “Among the plants, symbiotic relationships usually depend upon immediate touch or some other close connection,” Philbrick wrote in the 1969 Herbarist, a publication of The Herb Society of America. “They may be caused by root secretions between two species of plants. They may be caused by leaf secretions. Finally, they may even be caused by scent between two plants—or as the books elegantly term it, ‘exhaled aromatic substances.’”
Philbrick identified seven ways that plants influenced each other:
• Some plants directly aid other plants
• Some enrich the soil, indirectly providing assistance to others
• Some oppose or harm each other
• Some provide positive influence for some of their brethren, helping them grow, and at the same time hinder or harm other plants
• Some repel harmful insects
• Some attract useful insects
• Some even repel animals
Philbrick believed in symbiosis among numerous plants. Late cabbage and early potatoes are good together, she said. Carrots go well in the garden with peas, lettuce, and chives. Asparagus was aided in growth by tomatoes and parsley. Garlic benefits nearby roses.
Herbs play a large part in the companion-planting scheme. While basil is “generally compatible and good for the whole garden,” she says, it does not “get along” with rue. In “small quantities,” chamomile increases the essential oil of peppermint and aids cabbage. She recommends chamomile blossoms soaked in cold water for two days to control seedling damping-off diseases. Coriander aids germination, growth, and seed formation of anise, but hinders seed formation of fennel. Watch out for wandering dill; it greatly reduces carrot yield, according to Philbrick. Hyssop increases grape yields, but is “not good close to radishes,” she warned. Pennyroyal repels ants and mosquitoes, while peppermint reduces flea beetles and white cabbage butterflies. Spearmint comes in handy to repel rodents, ants, and aphids.
Rosemary and sage are said to stimulate each other. Summer savory, when used as a border, “helps” onions and beans. Southernwood repels moths on fruit trees and cabbages, while tansy keeps flies and ants away. Thyme is another barrier to the ever-present cabbage moth. Yarrow, said Philbrick, increases the “aromatic quality of all herbs and their general health.”
“What we see in the garden is real,” Philbrick wrote of companion planting. “It is now provable by scientific methods in a biochemical research laboratory where everyone can see and measure it.” Chromatography, a scientific method that breaks a plant’s essential oils into color bands, was the tool she believed proved her point. “Having once discovered that one plant extract always produces the same picture,” Philbrick wrote, “laboratory technicians have now experimented with combinations of plant extracts. Plants known to be mutually helpful produced harmonious pictures when extracts were combined. Plants having a harmful effect on each other produced pictures showing confusion or disharmony, or a canceling out of the finer forms of the individual pictures.” For Philbrick, these pictures drawn by chemical mixes became a truthful pseudo-scientific validation of her garden observations, when they could just as easily have been viewed as fanciful interpretations without scientific validity.
Companion planting was a palatable doctrine, delivered with evangelistic fervor, that went unchallenged for many years. Its appeal was almost mystical: Plants work together to enhance their lives for the benefit of humans. Scientists and experienced gardeners who might have been expected to question some of the claims made for companion planting may have had their doubts, but they also saw the possibilities of using companion planting to reduce the use of expensive and often harmful pesticides in gardens and farms.
In their usual quiet way, plant scientists began testing the claims made for companion planting. After more than half a century of independent investigation of companion planting, what can be said for certain about the symbiotic relationships among plants? In his 1986 book Designing and Maintaining Your Edible Landscape Naturally, Robert Kourik sought to summarize research on the subject. “For eight years, I have looked for ‘hard data’ to double-check and substantiate the traditional companion planting guidelines,” he wrote. “I found a mixed bag of results: less than half of the studies confirmed the recommendations; the majority seemed to indicate that faith, not fact, may have been an important aspect of specific recommendations.”
Following are some of the positive study results that concern companion planting, according to Kourik’s review of the scientific literature.
While you can forget about marigolds to keep insects away, some herbs have shown promise as companion plants. For instance, catnip and tansy both reduced Colorado potato-beetle infestations on potatoes by more than 70 percent. Catnip reduced by 91 percent green aphid populations on peppers. Squash had fewer squash bugs when interplanted with tansy and catnip. Most studies found some reduced yields due to interplanting with herbs, which could have been due to overcrowding.
Another part of the garden environment has been elevated due to the scientific interest in companion planting. For some plants, particularly the brassicas, simple weeds are excellent protectors. The weeds fill in the space between the plants, thus confusing flying predators, while those that stand out provide a bare, earth-highlighted target. Unfortunately, weeds may also compete with crop plants for nutrients, and the end result may be a reduction in pests as well as lower yields. Weedy groundcovers also reduced common pests on apples, beans, cabbage, cauliflower, collards, corn, grapes, mung beans, peaches, and walnuts.
While companion planting may be unreliable, herbs can be used in different forms to solve garden problems. Sprays of essential oils or aqueous suspensions of ground herbs may repel insects. J. Hough-Goldstein at the University of Delaware found that aqueous sprays of tansy were an effective antifeedant of cabbageworm, while sprays of tansy, sage, basil, catnip, dill, or rue deterred feeding of adult and larval Colorado potato beetle. Catherine Regnault-Roer and associates in France found that the essential oil of marjoram and mother-of-thyme were toxic against a bruchid insect that attacks kidney-bean plants. A review of the use of essential oils as potential repellents and insecticides was published by Dan Palevitch and Lyle E. Craker in 1993; they noted some effective uses of the oils of anise, basil, catnip, chamomile, coriander, dill, eucalyptus, fennel, feverfew, garlic, hyssop, pennyroyal, ponderosa pine, rosemary, rue, sage, sour orange, tansy, and wormwood. In the greenhouse, I. Tunç and S. Sahinkaya of Turkey found that the essential oils of cumin, anise, oregano, and eucalyptus were effective against the carmine spider mite and the cotton aphid.
Nematodes are particularly noxious in the soil despite their near-invisibility. Jerry T. Walker at the University of Georgia found that many herbs may resist or tolerate nematode infections. Yet, such nematode infestations are often followed by soil-borne, sudden-wilt fungi, such as Phytophthora, that ultimately kill the herb. Synthetic nematicides may be effective, but they are one of the chief, persistent pollutants of ground water. One of the best natural nematicides is released by the roots of marigolds (Tagetes spp.), but in the field it shows rather weak nematicidal activity; fresh grass clippings are often just as effective. Since nematodes are composed of chitin, mulches of ground crab shells, also composed primarily of chitin, have been thought to foster fungi that degrade chitin, but the field results are very mixed.
Good news for weed lovers also comes from Mexico. It was there that farmers discovered a common weed that enhanced corn yields if it was managed in a way that kept it from becoming tall and competing with the corn for nutrients. Francisco J. Rosado-May, a scientist at the University of California, Santa Cruz, wanted to find out why. In 1990, he and his university colleagues discovered the roots of the weed (Bidens pilosa) secreted compounds that killed corn-destroying fungi and nematodes. The farmer’s practice of keeping the weed mowed until harvest let the weed control soil pests without stealing nutrients from the corn, the scientists concluded.
The difficulty of making sweeping generalizations about the benefits of companion planting, or about gardening in general, is that everybody’s garden is not the same; soils differ, as do climates. The environment is such an important determinant of growing conditions (and pest populations), and local fluctuations so numerous, that it is often difficult to reach precise and universal advice for growing a single plant.
As most experienced horticulturists know, there are few universal truths to be found in the cultivation arts except that farming and gardening are hard, often satisfying work. The subtleties in local environments and microclimates make it almost impossible to universalize personal experience. A method of companion planting that is successful in an East Coast location may not work in the mountains of West Virginia or in Berkeley, California. When it comes to having healthy herbs, nothing can replace the continual care and attention of a knowledgeable gardener.