Isn’t it a funny thing? The more you know, the more you see. Before I knew much about galls, I never saw any. Once I started to learn a bit about them, however, they suddenly seemed to be everywhere! For example, when I was driving down a rural road last weekend, I noticed this tree with suspicious blobs on the lower branches. I stopped the car and took a closer look. At first, I thought they must be some sort of seed cone, but no, they’re galls. Ash flower galls, to be exact.
The galls are caused by mites (Eriophyes fraxiniflora), who attack male flowers during blossom development in the spring. The mites feed in the flower clusters, causing them to become irregular masses. Female mites then lay eggs in the developing galls. The galls are green early in the season, but gradually turn brown or black. The galls can remain on the tree for as long as a couple of years. The tiny mites can develop from egg to adult in under two weeks and many generations can occur over the growing season. Females overwinter under bud scales or bark. The galls do not harm the tree and the galls add visual interest in the winter. However, most ash trees are dioecious – a single tree will contain only male (staminate) flowers or female (pistillate) flowers. As the mites attack only male flowers, gardeners who don’t like the galls can avoid them by planting only female trees.
Willow House Chronicles 