Each plant will grow at it's own rate, and if that's rapid, you may need to thin a bit after 2 years although that's quite rare. I have one bed that is 3 years old this summer, and most of them will need thinning this summer with a few exceptions. A few of them had to be thinned last summer, and some are really slow growers or suffered some loss to rot and will be OK for a while yet. You just want to keep them from over crowding or they don't bloom well. You will also want to keep the different cultivars from growing into one another. Hope that helps.
