Hi, Kevin,
In most cases, it is good for the plant to deadhead or cut back spent flower stems after flowering - it makes the plant use energy for stronger roots and stronger vegetation. As far as how low to go and when, you will get conflicting advice. What you do also depends on how cold the winters are where you live. I live in an area with a mild winter. When I am sure it has finished flowering, I cut back the spent flower stems to the clump of basal foliage. Then, in late autumn, I cut back low to the ground. This produces a nice, bushy basal clump in spring. If you live in colder area, I would still cut back spent flower stems after flowering. Then, by mid- to late autumn when it is getting colder, add a dry mulch. Leave the cut stalks to protect the plant from frost damage. Then, in early spring, cut back the tatty old foliage to make way for fresh new growth.
All the best,
Kathy C
Replied: Monday 16th of April 2012 06:42 PM