For Release August 10, 1999

Falling Leaves Don’t Mean Its Autumn

by Chuck Otte, Geary County Extension Agent

The hot weather the end of July certainly changed the looks of a lot of trees. Leaves started falling fast and furious, trees that had been a deep green suddenly had the hardened, off green look of late summer and some leaves turned yellowish brown or even transparent. None of this was a surprise and most of it is nothing to worry about.

You always need to relate what is happening now with a plant with what has happened during the earlier part of the growing season. May and June was a very lush growing time. High humidity, rainfall, cooler than normal temperatures. A lot of trees put on incredible amounts of growth. There were also a lot of leaves infected with minor leaf diseases. Then, when there is a sudden shift in the weather, the trees react to deal with that stress. Think of every leaf on that tree as a little spigot that lets water out of the system. Also think of every leaf as being a little factory worker producing food.

When the weather is good there isn’t much stress on the system. If a leaf is not producing as much food as it takes to keep it alive it isn’t very important. If there is plentiful moisture and low water use conditions then a lot of leaves on a tree aren’t an issue. Now we’re going to put some stress on that tree. We’re going to sharply reduce the amount of moisture available to the tree and greatly increase the water use rate. All of a sudden tree management takes a look at all of it’s employees and factory production and decides that some cutbacks are in order. Employees (leaves) that aren’t working at maximum production, or that are wasteful, are "let go" literally.

Any leaf that has very much insect or disease damage will be one of the first to fall off. Then the tree will just randomly start to shed leaves trying to bring the water use rate in line with the available water resources. Cottonwoods, elms, and maples are all classic examples of trees that will shed a lot of leaves in a hurry when the weather turns hot and dry in mid summer. The curious behavior is that even in August and early September these same trees will grow new leaves if cooler wetter weather arrives.

The good news is that the trees have already stored up most of their food reserves for winter and early spring. Heavy leaf drop at this time is not generally a problem for most shade trees. On the other hand, this is a critical time for fruit trees as they are busy producing the flower buds for next year. If we get into another hot and dry spell keep those fruit trees well watered to minimize stress.

You’ve probably also noticed quite a few oak and elm trees that are looking pretty sorry. A lot of this is insect feeding damage. Some of the feeding is occurring right now, some occurred some time ago and is just showing up now with the other stresses. In all cases I wouldn’t worry about this feeding damage (unless it’s from bagworms feeding on evergreens). As I said earlier, most of the root reserves are already stored up and this late season loss of leaf material is rather inconsequential so I wouldn’t waste money and time spraying.

Most trees had a pretty good growing season and put on a lot of growth. What looks like a late season set back isn’t really all that bad. Keep young trees watered during dry spells. Keep evergreens well watered all through the fall and winter and spring will soon be here again with new leaves and blossoms!


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