How to Plant a Tree

Trees are a valuable part of any landscape. They provide shade, wind protection and visual appeal. They also reduce energy costs and serve as habitat for children and other wildlife. Given the value of trees, it is wise to put considerable thought into their selection, planting and maintenance. Regardless of where it is obtained, any tree suffers a certain amount of “transplanting stress” during the period between planting and establishment. It may take several years after planting for a tree to regenerate a root system that is in balance with the top. Once established, most trees require very little care. However, site preparation, planting technique and post-planting maintenance will all influence the rate at which a tree becomes established.

When planting a tree, the primary objective is to place a plant that is capable of growing roots into an environment where it can grow roots as rapidly as possible into the surrounding soil. Many factors influence a tree’s ability to grow roots.

Selecting a Quality Tree

Selection of a high quality plant that is adapted to conditions at the planting site is a necessary first step in the process of establishing a tree in the landscape. If planting a bare root plant (harvested with no soil around the roots), select plants with large, well-branched root systems that do not appear dry and shriveled. A tree harvested with a soil ball (balled and burlapped or B&B) should have a firm, symmetrical ball that is large enough to conform to accepted nursery standards. For example, a shade tree with a trunk diameter of 2 inches should have a ball about 24 inches in diameter. The trunk should not wobble in the ball. Container grown trees should also not wobble in the container. Roots should occupy the entire volume of the container, but should not circle the pot excessively. “Pot-bound” plants with a solid mass of circling roots will establish slowly and may continue root circling for many years after planting. This can lead to poor anchorage or even girdling (strangling) of the trunk.

Once a quality tree is purchased, is important to keep it in good condition until it is planted. Poor handling between harvest and planting can greatly reduce the vigor and root growth potential of a tree. It is critical that roots never be allowed to dry out. Bare root plants should have their roots soaked for a few (no more than 6) hours, and, if not planted immediately, should be “heeled in” with roots covered by mulch or soil. Container grown trees should be watered regularly and kept from freezing. The soil balls of balled and burlapped (B&B) trees should be covered with mulch or soil if not planted soon after purchase. Remember to water B&B trees on a regular basis until planted to keep the soil ball from drying

Time of year

If good horticultural practices are used, trees can be planted at any time of the year. Container grown trees suffer minimal root disturbance during transplanting, and, if carefully watered after planting, can be successfully planted even in midsummer. Bare root trees are generally not available for planting except in the dormant condition, during late winter and early spring. However, research at the University of Missouri has shown that bare root trees up to 2 inches in stem caliper can be planted with excellent survival in full leaf, in midsummer if preconditioned in a bed of irrigated pea gravel (called the Missouri Gravel Bed) for 10 to 12 weeks. If harvested and stored properly, B&B trees can also be planted at any time of the year. However, since conditions during midsummer are often stressful to plants, most nursery professionals recommend planting in spring or fall.

Planting Site Preparation

Since a tree may represent a considerable long-term investment, it is wise to prepare the planting site before the tree is purchased. Test the soil for pH and mineral content to determine whether amendments are required to make the site suitable for the tree species being planted. Check the water percolation rate by filling an 18-inch-deep hole twice with water. If the second filling takes more than 24 hours to drain, consider improving the drainage by creating raised beds or using drain tiles. If the soil in the proposed planting site is a dense clay subsoil, it is unrealistic to expect any tree to develop to its full potential if planted in that location without extensive soil modification. Light is an often overlooked microenvironmental factor. Determine whether pruning or removal of surrounding plants might be necessary to provide enough light for the new tree.

The “Planting Hole”

Based on recent research on tree root growth after transplanting, the term “planting hole” should be laid to rest. In most soils, 90% of the actively absorbing root tips are located within 12 inches of the soil surface. Thus, it important to create a soil environment surrounding a new tree in which roots can grow easily near the surface. Recent studies have shown that trees root much more slowly in high-density soil than in loose soil. Current recommendations, therefore, call for planting trees no deeper than they grew in the nursery, but into a soil that has been loosened 8-12 inches deep over an area equivalent to 2-3 times the diameter of the tree’s soil ball. Although peat moss and compost may help create a favorable root growth environment when mixed with the soil around a new tree, these amendments should be used in moderation (less than 20% of the soil volume).

When preparing a planting “hole,” it is helpful to place excavated soil on a tarp next to the hole to facilitate placing it back around the root ball after the tree is in place. Dig the hole only as deep as the soil ball and place the ball on un-loosened soil. If the soil is heavy clay, it is advisable to plant slightly higher than the tree was grown in the nursery. Backfill so that the ball is about 1/2 exposed and is stable. Remove any twine or rope around the trunk. Many tree planters put water in the hole at this point to settle the soil. If the ball is being planted high and the top will project out of the ground, it is best to remove the burlap from the top of the ball to prevent wicking of water from the ball where burlap is exposed. If the ball comes in a wire basket, most tree professionals recommend removing the top row of squares to minimize chances of eventual root girdling or contact with mowers. After all of the unwanted hardware is removed from the trunk and ball, finish backfilling. If it is not possible to irrigate the new tree with some form of drip irrigation, it may be advisable to create a basin with backfill soil that will serve to hold water applied from a hose or bucket.

Finishing The Job

Wrapping the trunk of a new tree is good insurance against winter sun scald. Use a light colored wrap, such as the kraft wrap commonly used by landscapers and wrap from the soil line up to the lowest branches. If the tree is located in an area exposed to strong winds, it is best to stake it. However, secure the tree no higher than 1/3 of its height from the ground so that the top of the trunk can sway in the wind. Whether the tree is secured using stakes or guy wires, make certain that no rope or wire contacts the trunk directly. Use rubber hose or specially made fabric straps to attach the tree to the supports. Also, make certain to remove the supports one season after planting to prevent trunk girdling.

Roots of turfgrasses compete strongly with tree roots for water and minerals. Therefore, it is important to mulch a new tree as soon as possible after planting to restrict the growth of turf roots into the loosened soil. If trees could talk, they would tell us that there is no such thing as a too-wide mulch ring. Mulch should be no deeper than 2-3 inches but should be as wide as is acceptable to the tree owner.

Pruning of new trees should generally be done in moderation. Remove broken or crossing branches and those growing at a narrow angle with the trunk. Thin out some branches if they seem too close, but try to maintain the natural form of the plant. In many cases, the grower will have taken care of these problems before the tree left the nursery.

Be careful not to over-fertilize a new tree. If a soil test is done prior to planting, deficiencies in phosphorus and potassium can be corrected according to soil test recommendations by incorporating fertilizer containing these nutrients into the backfill soil. Excessive nitrogen fertilization at planting time can promote rank top growth at the expense of root growth, making the tree more susceptible to drought in the heat of summer. A light application of nitrogen fertilizer in midsummer may help to promote root growth over the fall and winter. Keep fertilizers containing broadleaf weed killers as far away from new trees as possible.

Don’t Forget to Water

The single most important factor influencing survival of newly planted trees is soil moisture during the first few months after planting. It is important to remember that until a tree grows roots out into the soil surrounding the original root ball it is entirely dependent on water contained in the soil ball. The ball of a container grown tree may become dangerously dry within a day or two after planting in midsummer. Similarly, B&B trees need to be frequently watered with relatively small amounts of water for the first season after planting. It is important, however, to keep in mind that overwatering new trees during hot conditions can kill them almost as quickly as lack of water. An effective way to control the moisture level in the original soil ball of a new tree is to use some form of drip irrigation. The simplest method is to drill 1/8-inch diameter holes in the sides of plastic 5 gallon buckets near the bottom and allow water to slowly trickle into the root ball. Repeating this operation 2 or 3 times per week will keep the root ball moist without drowning the tree.

Brad Fresenburg, UMC Extension/Research Associate, Turfgrass Research Center, (573) 443-4893