Chapter 6: Common Pruning Methods
You can easily get overwhelmed with the idea of pruning because overzealous pruning can harm the health of a plant. The best advice includes taking your pruning job slowly and remembering to continuously step back and observe your work. This will prevent over pruning and also give you a better perspective on what you hope to achieve. When in doubt, leave it alone — but do not be so afraid of pruning that you give up on the task. This chapter describes some basic pruning methods that anyone new to pruning can quickly learn and will give you confidence to move forward in your pruning.
Pinching
Pinching is an ongoing process for any gardener, and you perform it without tools: All you need is your thumb and forefinger. As mentioned in Chapter 2, auxin, the chemical compound concentrated in the end of every branch, tells the plant to produce growth from that area. The end bud always grows at the expense of the other buds growing on a branch or stem. In the case of a plant with opposite buds, the auxin level is the same in both end buds. If you want the plant to grow bushier and produce lower growth, you simply pinch off the actively growing end bud or buds. This forces the auxin level to rise in the lower buds, and they begin to grow. If the buds alternate with each other, pinching only one of the buds encourages the remaining bud to grow rather than generate growth of the lower buds.
You should know what type of plant you are working with before you start pinching. Plants, such as tulips, lilies, or other bulb plants where a single flowering stem constitutes the entire plant, should not be pinched. Other flowering plants, such as flowering annuals, produce more blooms and stronger plants when they are pinched back at 6 to 8 weeks of age. For example, snapdragons, marigolds, zinnias, and cosmos all generate more branches below the pinched area and produce more flowers. Pinching back flowering plants also helps the plants withstand adverse weather conditions. Although you pinch plants when they are young, pinching can be continuous process, such as when flowering plants are deadheaded, which is another name for removing old blooms. Occasionally, when you buy a plant, you will see instructions with it that tell you to give something a hard pinch, which takes off more than just the end bud and is done with a hand shear if the stem is too thick for a simple hand pinch.
Commercial cut flower growers use a different pinching process that can be expensive and cumbersome for the average gardener. They grow flowers for the florist trade using pinching to produce fewer and larger flowers per plant. They aim to produce the largest flower possible, even if this means the plant produces fewer flowers. To do this, the growers pinch lateral growth from some flowering plants like carnations or alstroemeria, removing all but a few of the best blooms. Commercial growers allow flowering plants like snapdragons and delphiniums, which produce only a single bloom, to grow tall without the competition from multiple blooms and extra foliage that would occur if they were not pinched at an early age. In this case, they use pinching to create large and outstanding blooms rather than stronger plants with numerous flowers. But, as a consequence of forcing the plants to make unnaturally sized blooms with extra long stems, field-grown plants grown by commercial growers are more prone to wind damage. To remedy this situation, most cut flower growers add support — using netting or stakes — to their plants.
You can pinch plants in the garden anytime during the growing season, meaning you can remove water sprouts and suckers as soon as you spot them. By doing this, you can save your tree from health problems and avoid a more complicated removal process. Also, you can pinch back tomato plants to three of four main stems to promote tomato growth. This helps eliminate wayward growth prone to aphid attacks.
Herbs that are commonly used for cooking — basil, oregano, marjoram, mint, lemon balm, coriander, dill, parsley, and thyme — benefit greatly from occasional pinching. Pinching forces the plants to grow more leaves and prevents the plants from blooming and going to seed. When these culinary herbs bloom, the taste of the essential oils change as the plants expend more energy producing blooms and seeds rather than more leaf growth. You can pinch back all culinary herbs further than just the tip end. Culinary herbs help attract beneficial insects, such as ladybugs and dragonflies. Beneficial insects prey on plant-damaging insects, such as aphids and caterpillars, and help pollinate your garden.
You can pinch all non-woody flowering perennials that die to the ground each winter early in the season to produce sturdier plants. A few examples of flowering perennials that respond well to pinching include Turk’s cap (Malvaviscus drummondii), Mexican mint marigold (Tagetes lucida), and the shrimp plant (Justicia brandegeana). Simply pinch out the top green growth when the plants are about 6 inches high. Keep in mind that a plant will grow sturdier and thicker after you pinch it but not as tall.
Pinching chrysanthemums
If you have ever wondered how professional growers create those bushel-sized chrysanthemums they sell every fall, you can do it yourself because the procedure is simple. Start the chrysanthemum plant indoors from a cutting or root division as early as possible in the spring. Once all danger of frost has passed, plant the chrysanthemum in the garden in full sunlight and rich garden soil. Make sure the plant is located away from artificial lighting, such as porch lights, because chrysanthemums are sensitive to light levels when it comes to setting flower buds. Fertilize with a full-strength, water-soluble fertilizer every two weeks throughout the growing season.
Sometime during the first part of summer, you will see flower buds beginning to form. Pinch off the buds as they appear using your hand or a pair of hand shears. This process is called debudding and will cause your plant to produce fewer but larger blooms rather than many smaller blooms. Do not allow the buds to show color or open. You should end up with a nicely rounded specimen. If the shape is a little off, take a pair of manual shears, and make a few adjustments. Only cut the amount needed to remove all the flower buds and shape the plant. Continue to fertilize and water as required, and you will see the chrysanthemum grow into a nice bushel. In late summer, when you see the second round of blooms begin to form, stop fertilizing, but continue to keep the soil evenly moist, and enjoy the fall show once the blooms open. After the plant freezes, cut the dead foliage back to the ground. If desired, this is a good time for separating the roots for new plants to put out the following spring.
Shearing
Shearing is a pruning process used on hedges and evergreens where you redirect growth and remove unwanted leaves and branches. This stimulates dormant buds among the inner branches and nudges them to grow into a thick, bushy appearance. The best time to shear is a few days after plant growth starts; for most plants, this is in late spring. Shearing at this time allows you to effectively redirect new growth. This will curtail the growth of the end buds and form new buds where you make the cuts. Maintaining a hedge or shaped evergreen can be time consuming. If you shear your hedges to a formal style, it will require constant shearing to maintain their shape. If you have more informal, natural-looking hedges, you will still need to shear them a few times a year to stimulate flowering and to keep the foliage dense. If you live in a cold climate, stop any pruning six weeks before you expect fall frost because any new shoots need time to harden and gain strength to withstand winter weather. Properly maintained hedges can last for years.
Root Pruning
If you are a gardener, you know that the roots of a plant form the foundation of any plant and should never be bothered. You water them, mulch over them, and do whatever you need to do to keep them thriving. Root pruning involves cutting back the roots of a plant, much like you would when pruning parts of the plant above ground. In some situations, roots, just like the other portions of a plant, need pruning or reduction in size. Examples of these instances include preparing a plant for transplanting, slowing the growth of the plant, forcing blooms and fruit production, and keeping a container plant healthy.
Moving large plants
The procedure a plant nursery uses for removing trees grown in the ground for later sale is the same one you need to follow if you are going to move a large plant or shrub. It might be impossible to transport a 6-foot-tall or larger tree with all the roots intact. The roots may extend 5 feet or more in every direction. Furthermore, the amount of dirt needed to keep the roots covered would be heavy. Root pruning keeps the plant healthy if moved. The root system of a plant has feeder roots that form toward the outer ends of the root system, and if you suddenly cut all the roots of a tree and dig it up, the tree has no way to take up moisture and nutrients when planted in the new location.
You should perform root pruning to prepare a large bush for transplanting during the winter or late fall while the plant is dormant. This takes careful planning. If you move a bush without properly preparing it, the plant will suffer and may die. One year before digging the plant from the ground, you should use a shovel with a long, sharp blade or a mechanical blade to slice half the roots off at a certain distance from the tree. The distance depends on the size of the tree and the method of transportation, but for the 6-foot-tree example, you would work in a circle 10 inches from the trunk of the tree. Large machinery can dig and remove a large tree with the entire root ball intact, but if you are moving by hand or using lighter machinery, you will have to work closer to the tree. The theory behind root pruning is that once you cut the roots, numerous feeder roots will form at the ends of where you pruned. When the tree is dug the following winter, the chance of transplant shock is lessened considerably because there are established roots in place.
In the year before you move the bush, prune the roots by cutting all of the roots in a half circle around the plants, or prune by marking the circle around the plant into four sections and cut one-quarter of the roots on opposite sides of the circle. Another method is to use the blade to make a cut, and then skip a space the size of the blade, and make another cut. You should give extra moisture to the tree throughout the growing season because you reduced the root ball, but be careful not to add too much water because it could drown or rot. You make the rest of the cuts the next dormant season when the tree or shrub is pulled from the ground to sell or transplant. Since the new feeder roots form at the end of the pruned roots, you must dig the plant up at least 6 inches away from where you pruned the roots.
When properly pruned, a transplanted shrub or tree’s roots will quickly heal and send out hair roots, which are smaller roots that grow from established plant roots. They increase the amount of surface area the plant can reach for nutrients.
Slowing a plant’s growth
You can root prune established trees to slow down the growth of the tree. You might think you can simply prune back a shrub or tree’s branches. But because pruning branches causes shrubs and trees to grow new branches, this can cause the plant to grow even quicker.
Because a tree depends on a robust root system for healthy growth, pruning the roots slows down the tree’s growth and can lead to dwarfing of the tree. If you drastically prune the roots, then you need to prune the top of the tree to compensate. This is the theory behind the art of bonsai. Although bonsai is an extreme example, trees and bushes can be dwarfed or have their growth slowed down at any stage of growth through careful root and crown pruning. To learn more about bonsai, see Chapter 8.
Forcing blooms and fruit production
Root pruning offers an effective means of controlling the growth of fruit and flowering trees and bushes. A fruit tree can grow for many years without producing any fruit. By pruning a plant’s roots, you can force it to bear fruit or blooms. Also, root pruning is often done to encourage blooms on old perennial vines, such as wisteria or bougainvillea. The theory behind this method is the plant uses too much energy for root production at the expense of fruit or bloom production — in other words, the plant is too happy and needs a little stress to initiate blooms. After root pruning occurs, the plant takes up less nitrogen, which encourages flowering, not leaf growth. When it is not stressed by getting too much fertilizer or water, it only produces lush, green growth.
In this case, half to three-quarters of the roots are pruned in a circle 2 to 3 feet out from the base of the tree, vine, or shrub between the late fall and early spring. This is only a theory, and in some cases it does not work, but it should not do long-term damage to the plant if done correctly.
Root pruning container plants
Container plants, whether trees and shrubs you are preparing for transplant or plants grown in containers for the patio, often need root pruning to remain healthy. Also, you might notice a plant growing too large for its pot, which requires either having its growth limited or placing it in a new container.
If you choose to prune the roots of your container plant, you will take the plant from the container and see roots wrapped around each other, sometimes growing entirely around the root ball. These roots continue to grow in a circle, causing problems. Eventually, they become so thick, they surround the base of the tree, strangling it so moisture and nutrients cannot work their way up or down the tree through the normal transpiration process.
When a plant begins to grow too large for its pot, you can amend the situation in three ways. You can prune the roots and put the plant in the same pot; you can transplant the plant to a new container; or you can plant the shrub or tree in the ground. If you want to keep your plant in the same container, simply cut back all of the outside roots, and replant it in the container with new soil. This will save you from continually potting your plant in new containers. You need to do this once a year or every few years, depending on how quickly your plant grows. Once you put your tree or shrub back in its container, you will also need to prune the branches to keep the top part of the tree in proportion to the size of the pot.
If you are transplanting potted plants to a larger pot, you should follow the same method of cutting off encircling roots. If you are moving a plant from a larger to a smaller container, prune off enough roots so that the root ball is 1 inch from the sides of the new container. You should also prune off one-third of the top growth to compensate for the decrease in root mass.
When transplanting a tree or shrub for a pot into the ground, slide the plant out of the pot, and then slice the root ball with a knife in several places. Next, take your knife or a stick, and gently pull on the roots at the surface of the root ball. This encourages the roots to grow outward. If you cannot untangle some of the more tangled roots, make vertical slits in the root ball with your knife. This will encourage the plant to grow new feeder roots closer to the tree’s drip line. Feeder roots are dense networks of roots spread close to the soil’s surface that absorb most of the nutrients for the tree. A drip line is the area located directly under the outermost leaves of a tree. Encouraging roots to grow outward will help your plant stay healthy after transplantation.
When to avoid root pruning
People love their trees, but sometimes trees can cause problems — shallow tree roots buckle sidewalks, invade septic systems, or threaten building foundations. When this happens, the first reaction is to cut the offending root, but this can lead to the rapid decline of a mature tree. Because every situation is unique, you should consult a certified arborist for advice. The best way to avoid ever having to think about root pruning a mature tree is never planting shallow-rooted trees, such as cottonwoods, sycamores, or willows, close to a place they can cause trouble in the future. Most fast-growing trees and trees native to swampy or wet areas have a shallow root structure.
Sometimes, established trees and bushes will have roots that grow above the ground. These roots become exposed and make it difficult to mow or walk around the shrub. In addition to affecting the health of your tree, exposed roots can also affect the health of the grass and any ground covers around it. Some gardeners cover these roots by planting low bushes, like hostas, around or near trees to add visual interest to a garden. You may feel tempted to cover these roots with dirt or cut them out, but this can be disastrous to the tree and result in death to the top of the tree on the side where the roots were covered or damaged. Cutting the roots can also result in rot at the base of the tree. In addition, covering the roots with soil will reduce oxygen that roots need to survive. Once this happens, leaves may become discolored and fall prematurely, or you may begin to see dead branches on the tree. A better solution in dealing with exposed roots is to simply leave them alone. You can cut a bed around the tree, and then cover the exposed roots with mulch. It may not be your ideal solution, but it will keep your tree’s root system healthy.