Many homeowners become alarmed when they see green or brown clusters hanging from branches on their ash trees. The clusters are ash flower galls and do not harm the tree. Most ash trees are dioecious – that is, the tree has either all male flowers (staminate) or all female (pistillate) flowers. Early in spring, a tiny eriophyid mite, Eriophyes fraxinivorus, feeds on the tiny male ash flowers. Ash flower gall mites are too small to be seen without the aid of a magnifying glass.
The tiny mites (about 2/100 of an inch long) spend the winter under flower buds and begin feeding and laying eggs in the developing flowers in spring. The feeding stimulates plant tissues to grow around the insects, creating the gall. Once the gall starts growing, it is too late to treat the plant. There are several generations during the spring and summer. In the fall, fertilized females move to bark crevices and beneath bud scales to overwinter.
Look for round, green, ½ to 1 inch tumor-like growths on the flowers. Sometimes leaf distortion is seen. Late in the summer, galls turn dark brown and become woody. They may remain on the tree for up to two seasons. The galls are more noticeable when the leaves fall off the tree in autumn.
The galls are usually just an aesthetic problem, so control is usually not necessary. One solution is to plant only female ashes. For already established trees, dormant oil sprays applied in late winter or early spring seem to be an effective control.