Our unusually hot, dry summer is causing much angst for gardeners on both sides of the Cascades. Many people are confused about when and how to water thirsty plants. Here's a snippet from a recent question: "My neighbor told me never to use a sprinkler on the plants; only to water by hand with the hose very close to the ground. He said it otherwise will damage the leaves and petals. I understand that if I water in the daylight, the sunlight on the droplets is bad."

I wrote a myth column about this topic seven years ago but it's worth reviewing. Briefly, water drops do not magnify sunlight and scorch the underlying leaf tissue. Hundreds of scientific publications have investigated leaf scorch on all kinds of crop and ornamental plants, and not one has linked the problem to water droplets on the leaves.

What does cause leaf scorch? There are many environmental causes, and nearly every one is related to leaf water content: in other words, when leaves are not fully hydrated, they can scorch. Leaf hydration can be reduced by wind, heat, freezing temperatures, high light intensity, high salt levels in the soil or irrigation water (including gray water), and of course lack of soil water. Plants without healthy, extensive root systems will also suffer leaf scorch when demands of the foliage outstrip the ability of roots to supply water. This is a common occurrence in trees and shrubs that have been planted too deeply or have not had their roots properly prepared before planting. Obviously, it can be difficult to pinpoint the ultimate cause of leaf scorch.

So keep a very close eye on your plants during these hot summer days. When you first see signs of wilt, irrigate immediately. Don't wait. Once leaf tissues have passed the terminal wilt stage, no amount of water will save them and you will soon see scorch on these tissues. To reduce evaporative loss, do keep the water source close to the ground. Soaker hoses are great, as are the cast iron stationary sprinklers (they cost less than $10). Sprinklers that put a fine mist high into the air waste a lot of water and don't deliver much to the root zone.

leafscorch.pdf

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