Ox-eyed daisy (Chrysanthemum leucanthemum)
Wow! Grow 'em by the acre. These come from supercheap Wildseed Farms seed, and these plants are hardy and strong. They start to regrow from the moment the snow melts back - you can see green leaves already photosynthesizing moments after being exposed to the spring sun for the first time. They require no care - there's a patch in a field next to the garden planted by the kids next door who have since moved. I am going to look at other daisies this summer and see how they do around here.
