Yikes! What is that on my Ash tree!? Ever travel down the road or walk out into your yard and notice some odd looking brown clusters on the branches of your Ash tree? If you have, you just found Ash Flower Gall.
Ash Flower Gall is an aesthetically displeasing result of mites feeding on the male flowers of an Ash tree. Ash trees are a dioecious species; either all male flowers are on the tree or all female flowers on the tree. The mite that causes this damage is a eriophyid mite, which only feeds on male flowers. The tree responds to this feeding by growing around the feeding insect and as a result the flowers enlarge. These galls can stay on the tree for up to two years. First they will appear a green color and then as the season goes on they will become brown. They are not an impact to the health of the tree, unless in rare cases they become overly excessive and begin to weigh heavy on the branches and potentially lead to them breaking from the weight.
Control measures are not commonly recommended, but as an unsightly problem, in some instances control may be necessary. Dormant oil applications in the late winter or early spring have shown effective control.
So, next time you’re out in your yard or going for a walk, check out the Ash trees and see if you can identify Ash Flower Gall! Good luck!
Paul Filary, Illinois Certified Arborist #5016A