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Missouri's drought

Water new trees and shrubs carefully

Chris Starbuck
Department of Horticulture

Drip irrigation with Gator Bags
Drip irrigation with Gator Bags. The zippered bags have a water reservoir with holes on the bottom that allow the water to slowly dispense into the root ball of transplanted trees.

Watering tidbits

Considering the below normal precipitation over most of Missouri this spring, it is important to remember that our newly planted landscape plants may need our help. Most trees and shrubs can not be considered completely established for two growing seasons after they are planted. Until they are, watering is a tricky problem. Roots from balled and burlapped or container grown plants may not grow out of the soil ball and into the surrounding backfill soil for several months after planting. Until they do, the plant must depend on moisture in the original soil ball to sustain it. Since water will generally not move from the backfill into the soil ball, the moisture content of the surrounding backfill soil is immaterial. The soil ball must be supplied frequently with small amounts of water. Too much or too little moisture can be equally fatal.

Let's apply some logic to the problem of watering a newly planted tree. According to the standards established by the American Nursery and Landscape Association, a 2-inch caliper red maple tree should have a soil ball about 24 inches in diameter with a depth of about 16 inches. The total volume of such a soil ball would be equivalent to the volume of about 20 gallons of water. Assuming that the soil in the ball is a clay loam, the volume of water that could be held by the soil ball is about 25 percent of the total soil volume or 5 gallons. On a single windy, 95-degree day in July, the leaves on a reasonably healthy, transplanted red maple tree the size of yours may transpire 2 or 3 gallons of water. Given the limited root growth into the backfill soil outside the root ball, the tree may be suffering severe drought stress in just a few days. The challenge then becomes how to replenish the moisture in the soil ball without leaving it saturated for an extended period.

There are several common approaches to watering newly planted trees. One is to water by hand, applying water to the planting hole with a hose. Another is to allow the hose to trickle at the base of the tree for some period of time. There are two problems with these methods:

In some cases, not nearly enough water has been added. Much of the water applied may have run off or gone down a snake hole. In other cases, the planting hole fills with water and the soil ball stays saturated for several days. Saturated soil during a period of high air temperature can cause as much stress as drought.

The best approach to watering a tree for the first year after planting is to use some form of drip irrigation that will wet the soil ball itself. To keep the ball of our red maple moist but not saturated during weather like we had in July requires 2 or 3 gallons every 4 or 5 days. The simplest way to meet these needs is poke a few nail holes in the sides of a 5-gallon bucket near the bottom, set the bucket next to the trunk and fill it with water every few days. A slightly more sophisticated method is to insert two or three one-gallon-per-hour drip emitters into a short length of plastic tubing that can be attached to the end of a hose with a hose fitting. This can be attached to an inexpensive electronic, battery operated timer programmed to turn the water off after one or two hours of operation. Mechanical timers that run by water pressure are not designed for use with drip systems. Many nurseries, garden stores or mail order houses sell drip irrigation kits that can be adapted to this purpose. These usually include a hose fitting and a simple pressure regulator that is required to make the system function properly. Do not operate a drip irrigation system without a timer. It is not possible to gauge the moisture content of the soil ball by observing the wetting of the soil surface. If the soil ball gets too wet, the tree will be under severe stress during hot weather.

Besides the appearance of your plant, another way to gauge whether you are supplying the right amount of water is to probe the soil ball with a metal rod. If you are unable to easily push the rod more than a few inches into the ball, you are not applying enough water. If the rod slides easily all the way through the ball, you are overwatering. Drilling an 18-inch deep hole next to the soil ball with a soil auger will allow you to check for standing water and pump it out with a small, drill-powered siphon pump. With a little care, you can gradually wean your tree from its dependence on the original soil ball. Once it has sent some roots out into the surrounding soil, it will gradually recover from the shock of having 95 percent of its roots cut off when it was dug from its previous location.

Updated 5/21/08

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