The mystery object is a goldenrod gall, which is a ball-shaped growth on a goldenrod stem created by a goldenrod gall fly larva living inside. The gall fly lays its eggs on goldenrod, the larva then eats the stem's inside causing the gall to form, living within it for a year before emerging as an adult fly to mate and continue the lifecycle, with the gall providing shelter from predators and through winter.