I always leave the flower heads on until early spring (easter is a bit late) to provide some protection from the frost. When I prune, I only take the flowers heads off down to the next bud you can see. I remove all brown dead stems from the inside of the plant right down to the base. This lets new shoots come up. If it has overgrown too tall, I would take the stems off by about 1 foot just above a bud, but again I would not do this until early spring. I have 2 of these plants, one in the garden and one in a tub and they both flower prolifically every year. Good luck, grannie.