after almost an hour of intensive internet browsing, i was just about to give up my search to identify this growth when i threw a hail mary and google caught it in the back of the end zone. this is a cottonwood gall.  a deformity caused by a tiny aphid.  the feeding aphids in spring transform the emerging leaves into these twisted galls. the galls are green in summer, but turn brown in winter.  although there may be many galls on a tree, they cause little harm to the tree. the aphids return to the same galls they left in the spring, so the same tree tends to be infected year after year, while nearby trees remain uninfested.

cotttonwood tree galls caused by poplar vagabond gall aphid

rice creek regional trail, saint paul, minnesota