Dying of thirst

COLUMBIA, Mo. – It might sound weird, but all the rain we’ve had in Missouri has primed plants, trees and shrubs for drought damage.

“Roots need oxygen to respire just like you and I need oxygen,” said David Trinklein, horticulture specialist for University of Missouri Extension.

“When it rains a lot, the pores in the soil fill with water and the roots become oxygen-deprived, at times to the point of death.”

For most woody plants, including trees and shrubs, the deepest roots will succumb first and only the shallow roots survive, because there’s more oxygen toward the surface of the soil, Trinklein said.

Plants established this spring have the same problem because they never grew deep roots in the first place. The top several inches of soil was “root heaven,” containing all the water and air they needed, he said.

Here’s the rub. Now that the rains have slowed and temperatures have soared, the water in the top layer of soil will disappear quickly. This can result in drought injury even though rainfall has been well above average this year.

“I recommend watering thoroughly but infrequently,” Trinklein said. “This will encourage roots to follow the water. This will help build deep root systems, which plants need to stay healthy.”

The amount of water depends on the species, age of the plant, soil type and weather conditions, Trinklein says. As a rule, plants need between 1 and 2 inches of water per week. As for frequency, a ballpark watering interval during summer conditions would be every seven to 10 days for woody plants—that’s trees and shrubs—and every five to 10 days for herbaceous plants, including flowers and vegetables.

There’s a long-term solution when conditions are too wet or too dry. Trinklein says. Working organic matter into the soil will build healthy soil structure that will let water easily percolate deeper into the ground. It also creates pockets that can fill with oxygen-laden air.

If you need tips to help your garden thrive, visit your local University of Missouri Extension center or go to http://extension.missouri.edu/LawnGarden.

Related article: “Wet Weather Woes,” http://ipm.missouri.edu/meg/?ID=343.