I planted lots of new perennials in my garden this summer and I'm wondering what I need to do to winterize it. Do I need to cut everything back? Should I cover any of the plants?
Herbaceous perennials provide an amazingly varied palette of flowers, foliage and plant forms. But these increasingly popular plants are also varied in their winter hardiness. Without a list of specific plants, I can give you only some basic suggestions for winterizing your perennial garden.
You should cut back any perennials that had showed signs of diseases on the flowers or the foliage. After you've cut them, be sure to remove all stems and leaves from the garden to reduce the chance of diseases overwintering. Don't throw diseased plants in the compost. Even actively managed home compost piles usually don't heat up enough to kill fungal spores and other pathogens.
Cutting back other healthy perennials is a matter of personal choice. Some gardeners like to clear out all stems and foliage in the fall because it saves them from doing it in the spring. Cutting back also can make it easier to apply winter mulch, where it's needed.
However, there are also some good reasons to leave at least some perennials standing over winter.
Perennials -- especially those that have interesting seed heads -- provide texture and visual interest in the winter garden. A dusting of fresh snow looks charming on the platterlike seed heads of tall sedums such as 'Autumn Joy' and the spiky brown centers of purple coneflower, among others. And many ornamental grasses, especially Miscanthus, show fluffy, long-lasting plumes through the winter months. As an added benefit, the seeds of a number of perennials provide food for birds, such as gold finches. Leaving the stems of perennials in the garden can also help with some natural insulation by catching and holding snow.
Some perennials are reliably winter hardy, even through winters without snow cover. Others are usually hardy, but can suffer when winter weather is especially cold or snowless. And then there are perennials that are marginally hardy in our region and will benefit from a generous winter mulch to get them through.
Snow is a terrific insulator. In many winters, natural snowfall provides as much protection as many perennials need. However, there are also plenty of winters when lack of snowfall leaves the soil open to deeper freezing and more potential for freeze/thaw cycles that can damage a perennial's root system.
Since we can't rely on snowfall, it's best to provide a layer of winter mulch to any perennials that were planted late in the season, have shallow root systems (such as coralbells or columbine) or are known to have limited hardiness (such as perennial hibiscus and some chrysanthemums).
Winter mulch should be applied after the ground starts to freeze, usually in mid-November in the Twin Cities. The purpose of the mulch is not to prevent the ground from freezing, but to moderate the temperature and reduce sudden freezing and thawing cycles. Dry straw is an excellent winter mulch, though it can be tedious to remove in the spring and can contain some weed seeds. Bags of dry leaves or pine needles also work well.
Nancy Rose is a horticulturist, writer and photographer. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.