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Dry, hot weather makes watering newly planted trees, shrubs
tricky; too much or too little moisture can prove fatal
COLUMBIA, Mo. – Below normal precipitation and hot weather across much of Missouri can make watering newly planted trees and shrubs tricky, said a University of Missouri horticulturist.
“Most trees and shrubs cannot be considered completely established for two growing seasons after they are planted,” said Chris Starbuck.
Roots from balled and burlapped or container-grown plants may not grow out of the soil ball and into surrounding backfill soil for several months after planting, he said.
Until they do, trees and shrubs must depend on moisture in the original ball. Because water will generally not move from the backfill into the soil ball, the moisture content of the surrounding backfill is immaterial.
Small amounts of water must be supplied frequently to the soil ball. Too much or too little moisture can be equally fatal, Starbuck said.
On a windy 95-degree August day, leaves of a 2-inch-wide red maple will transpire two to three gallons of water. Given the limited root growth into the backfill outside the root ball, the tree may be suffering severe drought stress in just a few days.
The challenge is how to replenish the moisture in the soil ball without leaving it saturated for an extended period of time.
The best approach to watering a tree for the first year after planting is some sort of drip irrigation that will wet the soil ball itself. Using the red maple example, moist requires two or three gallons of water every four or five days.
Poke a few nail holes in the sides of a five-gallon bucket near the bottom and place the bucket next to the trunk, filling it with water every few days, he said.
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.
Such a device can be powered by 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. Do not operate a drip irrigation system without a timer. You can not 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, he said.
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.
For more information about planting and maintaining trees, go to http://extension.missouri.edu/explore/