Q: I would like to know what these potato-looking things are. They fell from an oak tree in my brother’s backyard in New Jersey. They look like a honeycomb inside and the tree is full of them.

A: It’s a gouty oak gall.

Most plant galls are caused by insects which lay eggs on leaves or twigs. Hormones in the eggs cause the plant tissue to swell in characteristic patterns.

Oak trees get apple gall, hedgehog gall, sower gall, felt gall, spindle gall and others.

Most galls cause little damage. Even though the branches fell from the oak tree, I don’t think the tree will be permanently harmed. Gall numbers will decrease and increase over the years, depending on how the environment affects the gall-making insects.

Nothing can be done to prevent the galls anyway…so enjoy knowing about a small aspect of how trees and insects interact.

See FAQ on Plant Galls