As we move into late July, we are already seeing quite a lot of yellow leaves, especially on flowering cherries, poplars, birches, dogwoods, sycamores, sweetgums and even pines. This leaf drop is normal and is the tree’s response to a lack of sufficient water whether it be from lack of rainfall or inadequate or incorrect irrigation or simply the summer heat. We see this every year and in some cases it is quite spectacular when a cherry, for instance, looses all of its leaves during the late summer.
The movement and loss of water through the plant is called evapotranspiration. Trees are very adept at “making hay while the sun shines”. The leaves are the food factories and the trees produce as many new leaves as they can each spring. As trees move into drought or temperature stress, they may be unable to bring in enough water to support this heavy leaf load. To reduce their water need they begin to reduce that load by dropping leaves.
As we move towards fall, the changes in light we encounter also affect leaf drop. This leaf drop occurs long before the first frost and may cause alarm if you are not prepared for it.
Another phenomenon we see in late summer is scorch. This occurs on leaves when temperatures are very high and the plants are not getting adequate rainfall or irrigation. Correct this with proper watering.
You may see leaf spots or insects appearing on the leaves late in the growing season which may cause you to be concerned. However, this late in the season, these insects and diseases are a minor threat to the plant because these trees are already in the process of abscising the leaves anyway.
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