I have Vinca growing in my cedar forest and they love it! I have clay soil but spread a layer of topsoil as I couldn't be bothered to dig up all the rocks which invariably live in the clay. They do spread out though, no doubt about it - thats' what makes them such a successful ground cover. So if you don't have the time to trim them away from areas where you don't want them, don't use them. I have them in my large wooded backyard where I simply want something other then weeds. I live in Zone 5B, anything warmer will get them growing even faster I would imagine.