Woofie, I have YET in all my years of gardening to have success growing Euphorbia marginata!
One of the houses nearby has an amazing mass of it growing near the mailbox at the end of their driveway and I admire it every year. Like any of the perennial Euphorbias, they need to be pruned/pinched back in the
early spring. Once they start getting too tall I'm thinking since "Snow on the mountain" has such a short season, maybe it's almost too much of a shock if it's pruned later in the season? I'll have pay closer attention to those growing around that mailbox and see if I can catch the homeowner some time and ask them how they keep them so beautiful!