JoAnne Skelly: Dealing with a September heat wave

JoAnne Skelly

JoAnne Skelly

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I have never experienced 100-degree temperatures in Northern Nevada in September.
When I started as a horticulturist in 1992, we told people to slowly reduce the frequency of irrigating trees and shrubs in late August, early September. To water deeply, but less often. To harden trees off for the cooler weather soon to arrive. The logic of this was supported by at least one freeze in August every year in cooler locations. Back then, the first frost date was Sept. 15. This is no longer the case.
With this heat (and of course, wind), plants, especially trees, need a lot of water. In an ideal world, trees would be watered deeply, rather than as a side product of watering a lawn. They would have their own drip irrigation or bubbler system that watered out to the outer edge of the trees canopy to a depth of 15 to 18 inches.
When the top four inches or so dried out, they would be watered again. All too often trees are watered along with flowers and shrubs, with one, or maybe two, one to two-gallon per hour emitters. The line is then run for an hour, two or three times per week.
This gives the tree perhaps two to three gallons of water per week with a one-gallon emitter or three to six gallons per week with a two-gallon emitter. The average watering can holds approximately two gallons. That’s not enough to wet the soil deeply out to the edge of the canopy. It may only water the top few inches of soil, but not where the roots are. Think of the depth of a root ball in a container. That is the bare minimum of area that needs to be soaked and that’s for a new tree. An established tree needs much more water.
I have had people say to me “Well, I thought the roots would be in the water table after a couple of years.” I doubt the water table is at root depth since roots rarely grow deeper than 18 inches. Instead, roots spread outward from a tree four to five times the height of the tree.
That’s the area to be watered. If your trees are 50 feet in height, you may not be able to water that far away from the trunk, but you can water out to the perimeter of the leaf canopy.
The point is, take care of your trees with deep watering during this heat wave.
JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email


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