Answer: Yellowing leaves on birches this early in the year usually indicates stress of some sort. Excessively hot weather can cause the problem, as can too much or too little water, or over-fertilization. All birches require a regular supply of moisture and nutrients. When you apply water, be sure to apply slowly so it has a chance to trickle down and wet the entire root mass. The frequency of water application depends upon weather and your soil type - sandy soil drains rapidly and needs to be watered more often than clayey soil. As a rule of thumb, we recommend one deep soaking once each week to help newly transplanted trees and shrubs become established. Aphid feeding can also cause yellowing leaves. Aphids usually leave behind a sticky substance called honeydew. If you can't inspect the leaves on the tree, collect a few of the fallen leaves and check to see if they are sticky or if you see shed insect skins on the leaves. If not, the problem is most likely water stress. At this point I'd continue to water regularly. The trees will probably replace the leaves they've already dropped. If not, they should leaf out well for you next spring.
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