Do you mean lop the top off or trim the sides of it. When we had the darn things we did it any old time they got too bushy or tall. They get to be so huge as time goes by, especially after loads of rain, we wished we never planted them. Strangely they are still there and they are MASSIVE and no one ever touches them ...amazing.

Don't cut back beyond the green growth as they will not regenerate if cut back too hard!