I have a little planter box around 2 sides of my porch. I tried all sorts of pretty little bedding plants but the dogs would go out the door, down the steps and use the box as a launch point. This was fun to watch, but turned all my lovely plants into trampled mush. So a couple of years ago I decided to plant rosemary. I figure it's a good bit tuff-er and if the dogs brush through it, they'll smell good. Well it worked. No more plant crushing. Actually, the dogs don't seem to like the smell, they have stopped jumping off the planter boxes and now take the steps like the rest of the family. Here's the thing. The plants are turning into shrubs. I mean BIG! One of them is so big that it's starting to spread open from the middle and sort of lay over a little. I'm concerned that it's goign to break or split. Can I 'prune' it like a hedge? If so, when? And how often?