From the folks at Botanical Interests: (a few comments by me in italics)
"You may have nestled your dormant vegetable beds down for a good winter's sleep with a nice blanket of mulch. But, don't forget your perennial flowers and herbs!
If you live in an area with hard frost, you should wait to mulch your perennials until after the ground freezes (typically after several hard frosts and when the soil is impenetrable by a shovel). To get them through the winter, the goal is to mulch them to retain some moisture around the roots and prevent upheaval from the soil from fluctuating freeze/thaw cycles. A few inches of shredded leaves or bark, weed-free grass clippings, straw, or evergreen boughs will tuck them in nicely until spring. The exact best time to do this mulching can really vary in Skillin's Country. I would say we are looking at a couple of more weeks before the ground "binds up" enough to be worthy of mulching.
Some gardeners like to tidy their garden up in fall, cutting back all dead or declining perennial foliage. There may be an aesthetic to that, but consider the benefits of leaving at least some of the stalks and seed heads intact. If you live in an area that gets snow, the stalks can help trap snow at the crown, insulating it and providing moisture as it melts. Also, many flower seed heads provide food for birds through the winter. I know, I know many of you want to trim those gardens all the way back. But Botanical Interests really gives some good reasons to wait on cutting back some of your material.
If have really dry winters with infrequent rain or snow, you can increase the chances for your perennials to survive if you give them a little water once or twice a month during warmer days. (Watercress is one perennial variety that definitely needs supplemental water to survive.)
Do you have perennials in containers? Cluster them in a sheltered area and add a little mulch on top. If the plants are hardy in your zone, the most common thing that damages them during the winter isn't extreme cold—it's lack of moisture. So, be sure to give them a little drink on warm winter days. Good advice but in Skillin's Country I almost always recommend planting your perennials in the ground for the winter.
As your caring for your perennial flowers, remember the perennial herbs too. If you have catnip, chives, garlic chives, feverfew, lavender, lemon balm, lovage, marjoram, mint, mitsuba, oregano, sage, sorrel, thyme, or watercress, they will also benefit from some winter protection. "
November 15, 2010