By John Jett, retired WVU Extension Service horticulture specialist
Properly installing trees and shrubs in the landscape involves much more than just digging holes and setting plants in them. The planter is responsible, as far as possible, for developing a satisfactory microclimate for optimum growth and development of the plant. First, a healthy and vigorous plant is required. Healthy plants need less maintenance in the years following establishment.
The planting hole is important because it makes up the environment of the plant root system. Suggestions vary about the size of the planting hole, but generally make the hole two to three times the diameter and the same depth as the soil ball.
A traditional way to prepare a planting hole for trees and shrubs is to incorporate organic matter into the backfill soil before returning it to the hole around the plants. Recent research has cast doubt on the value of this practice. In fact, it appears that energy could be better spent digging a slightly larger hole rather than working organic matter into the soil.
Apparently, adding organic matter to the backfill soil creates an interface between the amended soil and the undisturbed soil around the planting hole. This interface is detrimental to root growth and water movement between the two soils. In tests conducted at the University of Georgia, examination of the root systems of plants in holes with amended soil revealed that the majority of roots were confined to the original planting hole.
When planting bare-root trees, add 6 to 8 inches of backfill before placing the plant in the hole. Firm in place to reduce settling. This should bring the top of the root collar to a level slightly higher than the surrounding soil. After settling of the soil, the plant should be at exactly the same depth as when it was growing in the nursery. Probably more plants are lost because they were planted too deep than for any other reason (Figure 18-8).
In general, it is no longer recommended that balled and burlapped or container-grown plants be set on top of backfill because this causes excessive settling. A more practical approach is to excavate the hole no deeper than the depth of the soil ball. When planting in a poorly drained site, set the plant so that several inches of the root ball are above the soil level (Figure 18-9).
An alternate method of planting in poorly drained soil or on sites where soil is very shallow is to use a wide-spreading hole. This practice replaces the poor soil on the site with better material or elevates a major portion of the root system above the existing site.
On sites where soil is poor, stony, or heavy clay, a hole be prepared to the depth of the root ball, as mentioned earlier. However, the diameter of the hole should extend 6 to 8 feet beyond the trunk of the tree. This provides the opportunity to introduce better quality soil to accommodate root expansion as the trees becomes established. The method is labor-intensive but has proven successful on poorer sites that can accommodate this degree of excavation.
Another method of dealing with heavy, poorly drained soils or those with high water tables involves elevating the root system above the problem area. The technique involves some regrading of the site, and the final contour of the property will be different.
Planting container-grown plants (as well as B&B plants) is accomplished by placing the tree in a shallow depression or saucerlike hole about one-quarter the depth of the root ball. The exposed upper portion of the root ball is filled around and supported with good-quality topsoil. The key to success with this method is to use an adequate amount of soil to support the tree physically and provide plenty of room for future root expansion into the improved soil.
The soil depth should be level with the top of the planted root ball for a distance of 8 to 10 feet from the trunk on shade trees and 6 to 8 feet on small flowering trees. The remaining backfill should be spread gradually outward from the root ball to a distance of as much as 15 or more feet from the trunk of large-maturing trees, slightly less for smaller trees. The gradual taper or sloping off of the soil should provide an adequate root zone for the establishing tree. Over time, there will be some root extension deeper into the original soil at the site.
Carefully place the plant in the hole. Balled and burlapped material must be handled carefully. On most species if the soil ball is broken for any reason, many of the roots will be severed from the trunk and the plant may die. Always pick the plant up by the soil ball or container, never by the trunk or stem.
It is important to determine the type of covering on dug plants. In recent years, there has been an increase in the use of synthetic materials to support the root balls on dug plants. The synthetic material looks and feels like the original organic fiber product (burlap), but it does not decompose if planted into the soil with the tree or shrub. The root-restricting capacity of this material is such that plants generally will not survive more than a few years if the material is allowed to remain on the root system in the planting hole.
It is a simple matter to test the consistency of the root covering. Organic burlap will burn when exposed to an open flame, but the synthetic material will only melt. If the root covering melts in flame, it should be removed after the tree has been set in the hole. If you are unsure of the nature of the material, remove it. Cutting the material all the way around the bottom of the root ball should allow for easy removal of the covering and all supporting twine. Never allow the supporting twine to remain around the trunk of the tree either because it too may be synthetic and will not decompose.
Organic burlap can be buried or planted with the tree because this material quickly decomposes in damp soil. However, it is very important that all knotted or tied portions of the burlap and support twine be removed from around the base of the trunk at the top of the root ball. If allowed to remain, it may cut into the base of the plant when the trunk grows. Either cut the top section of burlap from the root ball or roll it down beneath the soil surface and cover it with backfill soil. If burlap is left exposed to the air, it will dry out and may draw water from the root ball.
Remove all plastic or metal containers before placing the plant in the hole. Small containers with tapered sides can be removed by turning the plant upside down and giving the top edge of the container a sharp rap. Catch the soil ball in the hands as it slips from the container. Do not break the soil ball apart. Larger containers of 5 gallons or more can be cut away with special cutters. If plants have become overgrown in the container and the root mass is growing in a tight, compact circle around the soil ball, cut the outer roots with a sharp knife in two or four places around the soil ball. Make the cut from the top to the bottom of the soil ball. Decomposable containers should be removed from the soil ball. An “X” should also be cut across the bottom of the root ball.
Remove packing materials and all damaged or dead roots of bare-root plants. If possible, soak the roots in water for at least one hour but not longer than 24 hours before planting. Do not allow roots to be exposed to sunlight or dry out before planting. It is best to keep bare roots covered with moist burlap or some reasonable substitute until planting time.
After a B&B or container-grown plant has been placed in the hole, fill in around it with backfill until the hole is two-thirds full. For bare-root plants, the soil should be worked gently in and around the roots while the plant is being supported. The most satisfactory way of firming the soil and removing air pockets is to fill the hole with water. If it is not practical to use this procedure, firm the soil by hand around the plant ball or roots. However, be sure not to use excessive force, since soil compaction should be avoided.
Before finishing the filling process, make certain the plant is straight and at the proper depth; then complete the filling process with backfill. If the specimen is an individual, construct a ring of earth 2 to 3 inches high at the edge of the outside diameter of the hole to form a water basin. Plants in beds probably will not require a water basin. Water the plant thoroughly as soon as the water basin is constructed. After the water has soaked away, fill the basin with a mulch material.
Organic mulches, such as pine needles, bark, and wood chips, are satisfactory to use. Make certain all mulch is kept several inches away from the plant stem. Any organic mulch in direct contact with the stems of woody plants can encourage decay by keeping the bark moist. Mulch in contact with the trunk can serve as cover for mice that feed on bark; heat generated from mulch decomposition can injure the trunk and prevent the trunk from hardening naturally.
Note that no fertilizer is added to the backfill mixture. Newly developing roots can be damaged by too much fertilizer. If it is apparent from knowledge of the soil condition that fertilizer is needed, add a water-soluble material at the recommended rate during the final watering phase. Large areas should already have an established fertility level based on recommendations from soil test results. A fertility program should begin late in the fall of the first growing season.
It is common for a large tree to undergo a prolonged period of slow growth after being transplanted. This period of stagnancy often lasts several years.
On the other hand, smaller trees transplanted at the same time often experience a shorter period of reduced vigor and, in fact, may outgrow the larger tree before the larger tree has fully recovered its normal growth rate. The reason for this appears to involve the natural balance between portions of the tree above- and below ground.
When a tree is dug for transplanting, as much as 98 percent of its root system is left behind. This is true for both large and small trees, but large trees lose much greater mass and lateral spread of roots. Both large and small trees must restore the original balance between roots and above ground portions of the tree before vigorous growth can resume.
Because roots of large and small trees grow at the same rate – roughly 18 inches a year – it takes a larger tree several years longer to regain the size of its original root system. Thus, large trees often will experience a long period of slow top growth after transplanting. This extended period of reduced vigor often causes concern about the tree’s chances for survival. This period of slow growth should be expected because the plant is being supported by such a limited root system.
Until natural root-shoot balance is restored, a tree will continue to experience some degree of “transplanting shock.” During this time, the reduced root system is unable to supply the quantity of nutrients and water the upper portion of the tree needs for normal growth. This is one reason adequate watering and other maintenance is critical after replanting. Research has shown that a 4-inch caliper tree requires about five years to regain the size of its original root system (18 feet in diameter). A 10-inch caliper tree with a 45-foot diameter root system needs 13 years to regain its original root system size.
The root system of a smaller tree can become nearly as large as that of a 10-inch tree after 13 years. Because the small tree has had several years of vigorous growth while the larger tree was under stress, the smaller tree may overtake the large tree in growth by the time the larger tree restores its root-shoot balance.