Q: I saw these weird things hanging off a tree sapling while walking in Mason Mill park in early May. They don’t seem to belong to the tree. Can you tell me what they are?

A: They are oak apple galls. The galls are caused by a tiny wasp who lays her eggs on a developing leaf bud. Hormones in the developing eggs cause the tree tissue to form a spongy mass with a thin papery shell.

The galls don’t hurt the tree. If you had one in your yard you could pick them off if they are objectionable.

Read more here.

with more pictures here.