Think Like a Root to Improve Planting Success with Landscape Plants

Given that trees and shrubs can represent a considerable investment, surprisingly little thought is sometimes devoted to maximizing the potential for their establishment in the landscape. In most urban and suburban landscape sites, the soil bears little resemblance to that which would be found in the native habitats of the trees and shrubs being installed. However, if one thinks like a root, he or she may be able to use some fairly simple site modi. cations and planting techniques to increase the chances of success.

When planting a tree or shrub, the objective should be to create a soil environment in which roots can grow as quickly as possible from the root mass being planted into the surrounding soil. For the first 4-6 weeks, the new plant will be dependent on its caretaker to apply frequent, light irrigations to prevent the original root mass from drying out. Ideally, the transplanted plant will grow sufficient roots in a single season so that it will require little or no irrigation, even during a mild drought in the second summer. However, achieving this level of success may require considerable thought and effort. A good way to start the thinking process is to visualize a soil environment that is conducive to root growth.

The basic environmental factors that limit root growth in soil are porosity, bulk density and availability of water and minerals. An ideal landscape soil might have 45 percent solids, 5 percent organic matter and 50 percent total pore space, by volume. Porosity is important for several reasons. First, roots need to breathe. Not only do they need oxygen, but they need to get rid of CO2 produced by respiration. Root-available water is stored in capillary (small diameter) soil pores and excess water drains away from the root zone through large (macro) pores, which then allow gas diffusion to and from the roots. In an ideal soil, about 50 percent of the pore volume should be capillary-sized pores and the other 50 percent large diameter macropores.

In a dense compacted soil with minimal macropore volume, root growth is often limited by diffusion of oxygen and carbon dioxide to and from the root, respectively. Much of the pore volume in such a soil is commonly occupied by water, and there is little opportunity for excess water to drain away from the root zone. Then, as the soil dries and becomes hard, it can physically restrict root growth due to the force required for root penetration. Unfortunately, the soil in most urban and suburban landscapes is highly disturbed with a high clay content, low organic matter and minimal macropore volume. Amending this type of soil or bringing in topsoil to achieve reasonable macropore volume over a signi. cant part of the total area is often cost prohibitive. Although it is human nature to ignore the problem (bury one’s head in the clay) and hope for the best, planting a $500 tree in such a soil is equivalent to investing $500 at a gambling casino. The probability of a good return on investment is not good.

Fortunately, there are some measures that can be taken to increase the chances of success when planting in a heavy soil. The first line of attack is to grade the site so that there are no areas where water stands for more than a few hours after a rain event. Use French drains and sub-surface drain tiles to carry as much excess water as possible away from planting areas. Avoid using plants that are known to be intolerant of poorly drained soil. For example, rather than cherry, sugar maple and azalea, consider crabapple, red maple and chokeberry. When planting in a heavy soil, it is particularly important that plants be set in the planting hole no deeper than the depth of the root ball. Many nurseries recommend planting trees with the root ball set several inches above grade and then bringing in soil to cover the ball. If planting trees and shrubs that require excellent drainage, the only practical approach is to create raised beds or berms with soil amended to create a good root growth environment. Planting such plants at grade, in bathtubs amended with peat moss is an exercise in futility. Also, adding a layer of gravel in the bottom of the planting hole will only make the drainage problem worse.

When amending landscape soil for improved drainage, keep in mind that some amendments do more harm than good. Incorporating 20 to 60 percent fine sand into a clay soil will actually reduce macropore space. Sand with large, blocky granules of fairly uniform size can improve drainage if added at more than 60 percent by volume. Coarse, inorganic aggregate materials such as haydite (expanded shale) and calcined clay (heated to create stable ceramic particles) can be effective. Some professionals have had good success with raised beds formulated with equal parts compost, coarse aggregate material and topsoil. For rhododendrons and azaleas, consider using ground pine bark as the organic amendment. When possible, mix such a material off site to improve homogeneity.

See http://www.bachmans.com/tipsheets/Soils/WorkingWithClaySoil.cfm for a good discussion dealing with clay soils.

See March 2003 MEG and MU Guide G6850 (http://muextension.missouri.edu/explore/agguides/hort/g06850.htm ) for suggestions on maximizing tree planting success.

Christopher Starbuck Horticulture
573-882-9630