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 ash tree, in response to the insect feeding, grows tissue around the insect. As a result, the flowers enlarge and stay on the tree for up to two years, instead of dropping off the plant during the first spring. The galls are green at first, but become brown later in the growing season. Research has shown that the galls do not harm tree vitality, though they can be considered unsightly. In extremely rare situations, the weight of galls can cause branches to be strained.