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Shrubs: The New Shrub-Planting Gospel
by J.C. Raulston
During the past decade, extensive research by plant scientists has resulted in the revision of many of the traditional "gospel truths" about planting practices, particularly shrub planting practices. For example, gardeners were—and often still are—routinely advised to amend the soil in their garden—that is, to change its physical properties (which is very different from fertilization, which changes its chemical properties) to make it more suitable to the new shrub. Yet amending the soil often has no value, or in some cases, detrimental effects. For instance, adding sand to "increase soil aeration and drainage" rarely does. In most cases, and particularly in clay soils, it actually decreases aeration and drainage and is not recommended.
Adding organic matter is also questionable in many cases. Organic matter is the result of the breakdown of living tissue as organisms die and decay. Breakdown is the key word—the organic matter ultimately disintegrates into water and carbon dioxide. The levels of organic matter in soils are controlled by the temperature, aeration and moisture levels of a given climate and soil type, and cannot be changed on a permanent basis in a garden. In other words, adding peat or other organic matter has a temporary effect that will disappear with time, although the speed at which it will disappear is quite variable. In a cool northern location with low aeration and moisture it may remain for many years; whereas in warmer, wetter Florida, it will completely disappear within months.
Unlike the annual vegetable or flower beds where amendments can be added easily each growing season, shrub plantings are relatively permanent. It makes far more sense to choose a shrub on the basis of its adaptability to the site than to try to make the site suitable to the shrub. So the first axiom for proper planting is to choose the right shrub for your site.
When you're ready to plant, carefully remove the growing medium from the outside of the roots. Then work the roots into the new soil with your fingers.
The Right Plant for the Site
Hardiness—the ability to tolerate cold winters—is the major criterion used to determine a plant's adaptability to a particular location. The U.S. Department of Agriculture and others have divided the country into "hardiness zones," and plants are assigned a range of zones in which they have proven reliably hardy. However, low winter temperature is only one of many climatic factors to which a shrub must be adapted, including out of season frosts, heat and rainfall.
What's more, there are two major—and very different—environments to consider: not only the above-ground world of climate but also the underground root zone. Half of a plant is underground, and conditions in this subterranean environment must also match the needs of the shrub you choose.
Your soil's physical properties are the most critical to consider when choosing a shrub, as they are difficult to change. The size of particles (ranging from extremely fine in clays to coarse in sands and gravels) and their relative proportions are used to classify soils into various categories: sandy loams, silty clays and so on. As soils become coarser in texture, the proportion of open pore space between the solid particles increases. This open pore space contains the air (oxygen) and water essential to good plant growth.
Your soil's chemical properties revolve around the relative availability of a group of 14 to 18 "essential elements" required by all plants, including the familiar nitrogen, phosphorus and potassium. The relative acidity or alkalinity (the pH) of your soil is also important because it determines which essential elements are available to your plants, and to what degree. Neutral to slightly acidic soils (pH 5.5 to 7.0) are usually best. Because deficient elements can be added, and pH can be adjusted up or down with commercial fertilizers, the initial chemical properties of your soil are less critical than its physical characteristics in appropriate plant selection.
It is possible to find excellent shrubs for all areas of the country that will grow well in virtually any kind of existing soil without difficult and expensive modifications—a far easier and more ecological approach than trying to "fit the soil to the plant."
There are a few instances in which amending the soil may be advisable. For example, shrubs in the Ericaceae or heath family (Arctostaphylos, azaleas, Enkianthus, Gaultheria, Kalmia, Leiophyllum, Leucothoe, Oxydendrum, Pieris, Rhododendron, Vaccinium and Zenobia, among others) have relatively specific soil requirements. They generally require fairly acidic soils with excellent aeration, constant moisture and high levels of organic matter, and so you may want to create a raised planting bed of mostly organic material. Or, no matter what shrub you're planting, if the "soil" in your garden is highly artificial—the contractor has bulldozed any existing topsoil, say, and left mostly subsoil and builder rubble in its place—you should consider buying a good quality topsoil and creating a new planting bed.
Left: Burlap or other wrapping materials should beremoved before planting. Center and right: Pass up plants that are rootbound or have girdling roots.
Bare Root, Balled and Burlapped or Container Grown?
Shrubs are generally available in three different forms: bare root, balled and burlapped and container grown. In years past, many shrubs were sold as dormant, bare-root stock—that is, the plants were field-grown in soil and dug bare root in winter when dormant. To prevent drying, they were packaged with the roots in various paper or plastic packages filled with a loose, moist medium.
If purchased while the plant is dormant and the roots are still moist and healthy, bare-root shrubs are likely to do as well as any. The roots are directly in contact with the new soil and grow quickly throughout a uniform environment. In recent years with the advent of container-grown plants, bare-root plants have declined in market share but are still an excellent way to buy plants.
Balled and burlapped (B&B) shrubs are also field-grown in soil. But they are dug with a quantity of the soil around the intact roots, and then tightly wrapped in burlap or other material for transport and handling. For the same size specimen, B&B plants are often more expensive than bare-root plants, as the digging process involves more expensive hand labor, and shipping and handling the heavy soil adds to costs as well. Today, with the shift to container-grown plants, which eliminates the digging costs, fewer B&B shrubs are available. Larger specimen plants, and certain slow-growing shrubs such as boxwood and yew, are still commonly sold B&B. They generally establish well.
However, if the soil in the root ball and the soil in the planting bed are very different, it may help to partially "bare root" the outside of the root ball to get some of the roots in direct contact with the new soil. Remove the burlap or other wrapping material just before you are ready to plant.
These days, the vast majority of shrubs are container grown. The new soilless growing mediums offer excellent drainage and aeration, which allows the plants to be grown much faster. What's more, because the expense of hand digging is eliminated, these plants are often relatively inexpensive.
It might seem at first that container-grown specimens would have great advantages, as no roots are removed or disturbed by digging or packaging. In general, this may be true, but there are circumstances in which they are difficult to establish. Plants that are rootbound—that is, their roots have completely filled the container—can be particularly problematic. This is because the new, coarse-textured soilless mixes often used in container production have been engineered for maximum pore space to provide optimum aeration for the fastest possible plant growth. On the down side, this also means that such mixes hold very little water, and an overgrown plant can completely remove all the water from the container medium in a matter of hours on a sunny or windy day. In a nursery with automated irrigation systems, containers are watered several times a day. But if a rootbound specimen is planted as-is in a garden, there will be no "connection" between the roots and the soil in the new bed; under such circumstances, plants sensitive to desiccation can dry out and die quickly.
"Butterflying" of container-grown plants is often recommended to get roots in contact with the soil. This involves making a vertical cut through the root system from the bottom of the container to about two-thirds the height of the roots, followed by a second cut at a right angle to the first. The four root segments are then spread out and soil is worked into the roots. Although this practice at first glance appears sensible and puts roots in contact with soil, it also severs most of the roots into many small spaghetti-like pieces no longer attached to the plant!
Gardeners often create their own problems by buying the biggest plant possible. The best container plants are those that have not become overgrown and rootbound—often smaller is better.
Planting Tips
If it isn't advisable to amend the soil, then what do you do when it's time to get down to the actual planting? Spade or till the existing soil well to the depth of the root ball or container. The width of the planting hole is especially important: two to three times wider than the root ball is a good rule of thumb.
Add fertilizer to provide the optimum pH and essential elements as indicated by soil tests or general recommendations for your area.
During planting, the most important goal is to get the new plant's roots into the new soil environment as quickly as possible so the plant will suffer as little stress from water loss as possible.
Remove as much of the growing medium from the outside of the roots as you can. Then proceed by working the roots into the new soil with your fingers to get them directly in contact with "real" soil.
Always plant the shrub with the roots at the same height as they were in the container or at the nursery, or slightly higher in heavy soils. Water immediately. Shield the plant from full sun and wind for a week or two with a translucent cloth or paper cover and continue to water every few days until it no longer wilts.
If any branches have been broken or damaged during transportation and planting, they should be pruned back to healthy wood. Because shrubs are normally shorter at planting time than trees, they rarely need staking.
Finally, it's a good idea to mulch the planting bed to conserve moisture, but don't overdo it. During the last ten years a deep-mulching fad has taken the gardening world by storm; mulch is often applied as much as a foot deep around trees and shrubs. This can damage trunk bark and prevent healthy oxygen exchange with the soil. Normally, two to four inches of mulch is quite adequate.
The late J.C. Raulston (1940-1996), was professor of horticultural science at North Carolina State University in Raleigh and director of the NCSU Arboretum, renamed the J. C. Raulston Arboretum in his memory.