I assume you are talking Mexican Heather. If so you can let it go or cut
back to 1/2 to 2/3 of the height you would like it to be in the fall.
Should be seeing some green on the mums but at any rate take all of last
years growth off. If you prune them quite severely on or about mid Aug you
will see more blooms in the fall.

"dc" wrote in message
m...
Last year I planted a few perennials - heather and mum. They bloomed
nicely
all summer and fall. I did nothing to then over winter. So far this year,
I've seen some green leave develop on the heather, but nothing yet on the
mums. Should I trim back the old growth? And, if so, how far?