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Care for the Landscape During Dry Spells

by Don Janssen, UNL Extension Educator

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When summer dry spells parch the soil, home gardeners are usually quick to pour the water to flower and vegetable gardens and lawns, but they sometimes overlook the larger plants in the landscape -- trees and shrubs.

Woody ornamentals need about an inch of water per week. A healthy tree growing in a good environment can tolerate dry weather better than a tree weakened by insects, disease, poor soil or other problems. Trees under stress from any of these factors and newly transplanted trees and shrubs, however, may need watering to survive.

Newly transplanted ornamentals need frequent watering because they usually have limited root systems. Often the tops of these plants, even after pruning, are proportionately larger than the root system, so the plants have a lot of leaf surface from which to lose moisture and not enough roots to take up water from the soil to replace it, especially during dry weather.

Frequent watering keeps the tree well supplied with the moisture it needs to manufacture food for growth. Be sure to take into consideration the soil the tree is growing in. Though it's essentially impossible to overwater trees in sandy soil, heavy or clay soils can become waterlogged. Roots need oxygen as well as water, and overwatering deprives them of it by driving oxygen out of the soil.

Other plants especially in need of supplemental watering in the summer include those plants in or near sidewalks or other paved areas, where roots may be restricted and the soil surface where water can soak in is limited; trees under stress from insects or diseases; and plants trying to recover from storm damage or other mechanical injury.

Lawn sprinklers and garden hoses are the only tools required for watering trees. Use a coffee can placed near the base of a tree to measure the water being applied by a sprinkler. Two inches of water applied every two weeks should wet the soil well down into the plant's root zone. If you use a hose, simply place it on the ground near the tree and let water trickle into the soil. If the water runs off rather than soaking in, cut back the flow.

A good way to reduce runoff and make watering more efficient is to apply several inches of shredded bark or wood chips around the base of a tree or shrub. Piling the material higher around the outer edges and lower in the center will make a basin to catch and hold water so that it soaks into the soil where the plant can use it. Mulches also slow the evaporation of water from the soil and so decrease the need for irrigation.

Mulching with fresh wood chips may also cause nitrogen deficiency in the plant. Soil bacteria take nitrogen out of the soil and use it in breaking down the wood. Use either aged or partially decomposed wood chips for mulch to prevent nitrogen deficiency.

Another benefit of mulching around landscape plants is that it eliminates the need to mow right up next to them. This reduces the potential for damage to bark by lawn care equipment.

To prevent weeds and grass from coming up through the mulch, you may be tempted to put down a sheet of black plastic or some other material first, but don't do it. Plastic and other such materials keep air and water from moving in and out of the soil and plant roots need both to stay healthy and do their job for the plant.

In the fall, increase the depth of mulches from 2 to 3 inches to 4 or 5 inches. A potential problem is that mulches lapping up against the main stems or trunk of a plant may harbor mice that will gnaw the bark and girdle plants. To prevent this, pull mulches back several inches from the stems of valuable landscape plants, especially fruit trees, young maples and other tender-barked species. You may even want to consider using rodent guards made of quarter-inch mesh hardware cloth or chemical repellents to protect especially valuable plants.

This resource was added August 2008 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement

PHOTO Credit: Don Janssen, UNL Extension in Lancaster County