Now this is an odd thing, isn’t it? I spotted these growths on a rose bush last year and here they are again—but now I know what they are. It’s Diplolepis rosae, otherwise known as the rose bedeguar or Robin’s pincushion gall, among other things. It’s “caused by the parthenogenetic hymenopteran gall wasp,” and the details are fascinating.
I was interested to see Wikipedia’s comment that it “has more folklore attached to it than most”—one can certainly see why.