Though you may think you have a sinister disease or insect pest attacking your trees, yellow leaves and leaf drop in the summer is simply due to lack of adequate water. This time of year we always see quite a lot of yellow leaves, especially on flowering cherries, poplars, birches, dogwoods, sycamores, sweetgums and even pines. This is normal in that it is a response to both the summer heat and, as summer progresses, the changes in light we encounter as we move towards fall. Every year along with the yellow leaves we also see a lot of scorch due to the high temperatures and no rain.
As the plants begin to discard these leaves, you begin to see the color change as the green chlorophyll pigment dissipates and the secondary yellow and red pigments, which are more persistent, are easily seen. Since the plants are discarding these leaves they turn off natural defenses in the leaves so the leaves become more susceptible to insect and disease attack. Consequently, you may see leaf spots appearing on the leaves or insects which may cause you to be concerned. However, these insects and diseases are not a threat to the plant. This leaf drop can initially begin as early as May but usually we see it in the summer.
It does serve as a reminder to check your irrigation on these trees and shrubs to be sure they are getting adequate water. If you find that individual plants are dry then augment with spot watering. When you see your trees looking dry and showing yellow leaves first ask yourself the question “am I watering correctly and adequately?”.
Trees use large amounts of water and this water must be replaced by rain or artificial irrigation. Give your trees at least two inches of water per week (about five gallons/inch of trunk diameter) during these periods of high temperatures.
Shrubs need about 1″ – 1 1/2″ per week. Additionally, frequent misting will help reduce the heat stress. However, do not substitute frequent misting for your deep watering which promotes the development of deep root systems. DO NOT apply the misting to your Japanese maples.
The key to water conservation and efficient and smart watering is diligent attention as to the needs of the specific plants.
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