Mulching May Aggravate Problems Associated with Planting in Poorly Drained Soil

In many landscape situations, trees and shrubs are planted in very poorly drained soils. This makes establishment difficult because maintaining the optimal level of soil moisture in the root zones of new plants is nearly impossible. Cultural practices such as irrigation and mulching are critical to the survival of new trees and shrubs planted in heavy soils. However, if used indiscriminately, these practices can lead to disaster

Until a tree or shrub begins to grow roots from the original root ball into the surrounding soil, it must obtain all of its moisture from the root ball itself. Container grown plants are grown in well drained growing media that have very limited water holding capacity. During a period of high air temperature, a newly planted container grown tree or shrub may deplete the available water from the root ball in less than one day. While the soil ball of a balled and burlapped plant generally has a somewhat larger moisture reserve, it can also be quickly depleted during hot weather. Wind and low humidity can intensify the problem, often leading to leaf browning within a few days. Thus, trees and shrubs must be watered every few days during hot, dry conditions until they have begun to grow roots into the backfill soil. Mulching can help to conserve moisture in the root ball and surrounding soil by reducing evaporative loss from the soil surface

While drought stress can kill a newly planted tree or shrub in short order, excess soil moisture during periods of high air temperature can be just as lethal. Roots in saturated soil can be damaged within a few days to the point that they are no longer effective at taking up moisture. Plants of flooding-sensitive species can die of drought stress within one week when their roots are submerged during a period of high air temperature. Flooding can also lead to accumulation of toxic levels of iron and manganese in plant roots

While irrigation of newly planted trees and shrubs during hot, dry conditions is always tricky, it is particularly challenging on sites where soil drainage is poor. In many cases, the plants essentially sit with their root balls in clay bathtubs. Excess water from rainfall or irrigation collects in the bathtubs by running into loose soil in the planting hole and into the porous soil balls of container grown plants. This can occur even on steep hillsides, but is generally most problematic at the bottoms of slopes or in low spots. The challenge is to water enough to keep root balls moist without filling the bathtubs. In this situation, mulching may compound the problem. Not only does mulch prevent evaporation of water from the soil surface and the bathtubs, it hides the problem so it is less likely that someone will notice that plants are flooded. If mulch is applied to trees and shrubs on a poorly drained site, it is critical that the moisture level of soil balls of new plants be checked frequently and that irrigation be adjusted to maintain root ball moisture without flooding. Use of drip irrigation with pressure compensating emitters, buried under the mulch is the most effective way to achieve this balance because it allows delivery of a fairly precise amount of water to each root ball

Christopher Starbuck
Horticulture
573-882-9630