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Too much is just as bad as not enough when it comes to watering newly planted trees and shrubs during hot, dry conditions, especially in poor soils, a University of Missouri horticulturist said.
"It's been a stressful couple of weeks for plantings," said Chris Starbuck, MU extension woody ornamentals specialist, during a weekly extension horticulture teleconference. "I've received a number of calls about declining trees and shrubs, and in many cases, the problem is the plants are drowning in poorly drained, clay soil bathtubs."

Starbuck explained that until a tree or shrub begins to grow roots into the surrounding backfill soil, it must rely solely on moisture in its original root ball.

"Well-drained growing media have very limited water-holding capacity, and container-grown plants can deplete the available water from the root ball in less than a day during periods of high air temperature, wind and low humidity," he said. "Balled and burlapped plants have a somewhat larger moisture reserve, but that reserve can be quickly depleted, too."

To avoid stressing plants, water every few days during hot, dry conditions. Starbuck added that mulch can help conserve moisture in the root ball and surrounding soil by reducing evaporative loss from the soil surface.

However, when irrigating in poorly drained clay soils — as is common across much of Missouri — homeowners should be cautious not to over-water because excess moisture can be just as lethal. "The plants essentially sit with their root balls in clay bathtubs, and the challenge is to water enough to keep root balls moist without filling the bathtubs," he said. "Roots in saturated soils can be damaged within a few days to the point that they are no longer effective at taking up moisture."

Homeowners can over-water to the point that "plant are practically floating out of the holes," Starbuck said. And in some cases, mulching intended to help conserve moisture compounds the problem. Mulch also hides the problem so a homeowner is less likely to notice that plants are flooded, he said.

With their roots submerged during periods of high air temperature, flooding-sensitive species can die of drought stress within one week. "Flooding can also lead to accumulation of toxic levels of iron and manganese in plant roots," Starbuck noted.

The MU horticulturalist recommended drip irrigation as the most effective means of maintaining root ball moisture without flooding the plant. A five-gallon bucket with a few holes punched in the side, near the bottom, is a low-tech alternative.

To estimate proper watering, calculate the volume of the root zone in cubic feet and add about 2 gallons per cubic foot. For example, a newly planted tree bought in a 5-gallon container will have a volume of 1/2 cubic foot. It should receive about one gallon of water three times per week during drought. (Editors note---Greenscape Gardens recommends 2-5 gallons twice a week.)

Another method is to stick a metal rod into the soil ball, Starbuck said. "If you are unable to easily push the rod more than a few inches into the ball, you're not applying enough water. If the rod easily slips all the way through the soil ball, you may be applying too much."

He added it is also a good idea to dig a small hole in the soil alongside the root ball with a trowel or bulb planter to make certain there is no standing water in the planting hole.


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