If you have ash trees in your yard, there is a chance you have encountered some unsightly green or brown clusters that resemble cockleburs hanging from your trees. If you have, your tree has ash flower gall. While the symptoms may appear alarming, the galls do not harm the health of the tree.
Galls are abnormal plant growths that can be caused by insects, mites or plant diseases. In the case of ash flower gall, the gall is caused by a tiny eriophyid mite (Eriophyes fraxinivorus). The galls are formed by the ash trees in response to the mites feeding on the flower tissues.
When it comes to ash flower galls, not all ash trees will be affected. This is because most ash trees are dioecious – meaning, the tree has either all male flowers or all female flowers. Since the mites only attack the male flowers only male trees are affected. Early in spring, the mites will begin to feed on the male ash flowers. Their feeding causes the male flowers to grow into a round greenish, 1∕2 to 1 inch diameter tumor-like structure. This time of year (late summer) the galls will begin to turn dark brown and woody. The galls can remain on trees for more than one growing season.
Ash flower gall mites are too small to be seen with the unaided eye as they are around 2/100 of an inch long. They are worm-shaped and spend the winter under the flower buds. When the weather begins to warm in the spring they will begin feeding on the male flowers and laying eggs. This feeding will initiate gall growth. The mites will go through several generations during the spring and summer. Finally, in the fall, fertilized females will move back into bark crevices and beneath bud scales to overwinter.
The galls are usually an aesthetic problem and chemical control is not necessary and rarely works. If you don’t like the appearance of the galls they can be pruned out of trees.