Irrigation & Mulch

Survive, or Thrive?

Water and mulch may make the difference between plants that survive, and plants that thrive.

Trees, shrubs, and other plants in the landscape are at a disadvantage. In the natural environments to provide shade and wind protection. The soils are not disturbed or compacted. Microorganisms that provide benefits such as aeration, nutrients, and organic matter abound. In the planted landscape found around most residential and commercial properties the conditions have been modified. For these landscape plants, the rules have changed. For these landscape plants, the rules have changed.

Influencing the Environment

To have plants growing and thriving in the unnatural environments created in the urban landscape, we have to make some adjustments. This means restoring those factors of the site as close to natural conditions as we can.

Whenever possible, it is best to use plants that will tolerate the conditions on the iste. But we can't always start at the beginning. Sometimes the plants are already in place, and we must work with what is now in the landscape. In this case, it is necessary to modify the landscape to meet the plants' needs. We must influence the environment to provide conditions the plants will tolerate. Among the simplest and most basic factors is supplying water.


Irrigation is simply supplying water in addition to the natural rainfall. Many methods can be used to get water to the plants, but the important thing to remember is that it has to be where the plant can use it, when it needs it. A general rule-of-thumb is one inch of water per week, for most landscapes in the middle-Atlantic and Northeastern United States. When rainfall does not meet this level, irrigation should be considered.

The use of irrigation depends also on the quality of the soil and the type and age of the plants. Deep, well-drained soil with sufficient organic matter will hold large amounts of water, available to the plants for long periods, and may not dry out as quickly as a dense shallow soil. Bare soil dries much more quickly than soil that is mulched or has ground cover growing over it. These factors should be considered when deciding whether irrigation is needed.

Inexpensive soil moisture meters can be useful aids in the decision making process. When used consistently they offer information on the "drying habits" of the soil. Occasional use, or one-time readings don't provide enough information to be really helpful, however.

The method of irrigation is important, too. Overhead sprinklers that throw the water over large areas can actually be harmful to the plants. It is also wasteful of water, since half or more of the water may evaporate before reaching the soil. Wetting the foliage of some plants encourages disease infection, and can lead to other related problems. Constantly wetting the bark of some trees can result in canker diseases that disfigure the tree, and can cause decay problems in later years. Timing of the water applications is also very important, to allow quick drying of the parts that are wetted. Early morning application is usually the best time. Using directed sprinkler heads, or systems such as soaker hoses that place the water right on the soil surface, should be used wherever practical.


Placing mulch beneath trees and shrubs is beneficial to the plants in many ways. Aesthetic appeal, moisture conservation, and supplying organic matter are the more commonly cited reasons to use mulch. Many types of mulch are available, and the kind you choose will probably depend on availability, cost and the desired results. The mulch should be compaction resistant, wind and water erosion resistant, slow to decompose, and able to restrict weed growth. Mulch recreates the natural environment to some extent. The litter that accumulates on the ground in nature is not removed, but breaks down to return nutrients and organic matter to the soil. The mulch we use provides many of the same benefits. It reduces soil temperatures and lessens evaporation. It encourages microfauna, the insects and other tiny creatures that live in the soil. It helps to buffer the effects of soil chemistry, moderating the availability of nutrients. And over long periods of time, it helps to loosen compaction in soil through several processes.

Mulch can be harmful, too, if misused. There are two common and costly mistakes seen in the application of mulch: 1. Applying it too deep, and 2. Piling it against the bark of stems and trunks. The mulch should be no more than two to three inches deep. More than this will restrict the exchange of air between the atmosphere and the soil. It will also reduce the amount of water that penetrates to the soil during brief summer showers. It can even cause toxic acids to build up near the soil surface as the mulch decays under anaerobic conditions, if it is very deep. Piling the mulch against the bark interferes with oxygen exchange through the bark, and holds moisture that encourages disease infections. The result is reduced growth and eventual decline of the tree or shrub. Don't reduce the amount of mulch used, but rather spread it out over a greater area. More of the root system will then receive the benefits.

Survive, or Thrive?

Mulch and irrigation are tools for the landscape manager, just as the saw, the mower, and the sprayer. They help us accomplish our goals of healthy, beautiful and functional plants in our landscape. Like any tool, they can cause much damage, or even plant death, if misused. Properly applied, with an understanding of how and why it should be done, these treatments may mean the difference between plants that merely survive, and plants that grow and thrive under our care.

© Copyright 1996, Russell E. Carlson

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This page was last updated on June 11, 2003.