Last summer, my dad and I discovered a small willow tree with a leaf deformity forming perfect rosettes at the end of some of the branches. Dad's research showed it to be willow rose gall. Galls are deformities in plants caused by a reaction to insect invasion.
The other evening, I found another willow with the same galls...leftover from last year. The leaves have browned and hardened, and formed lovely "wooden roses"