A lovely phenomenon, the willow rose, created when in early spring an itty bitty willow gall midge laid her egg on the willow growth tip bud.
The midge's egg hatched into a grub and burrowed in, feeding on the growth tip bud, stunting the willow's ability to expand normally in spring.... thus the leaves, rather than emerging singly up the growing stem, pushed themselves out in this compact whorled rosette.
Ironically, this rosette is also irresistible to a certain species of wasp, which comes along and also lays eggs on the already ravaged willow bud, it's grubs devouring the midge grub....
This fascinating tiny tête-à-tête happens almost exclusively on one species of willow, the Barclay willow, which seems to be the most obvious way to identify the species.
Ned Rozell, science writer for the University of Alaska Fairbanks, is my source for many fascinating natural wonders.
