AMES, Iowa – Many perennials make it through winter just fine with no special attention. But in Iowa gardens, some perennials require a little attention just before the ground freezes. Horticulturists with Iowa State University Extension and Outreach tell how to prepare and provide perennials with winter protection. To have additional yard and garden questions answered, contact the ISU Hortline at 515-294-3108 or email firstname.lastname@example.org.
Chrysanthemums are shallow-rooted plants. Repeated freezing and thawing of the soil during the winter months can heave plants out of the ground and cause severe damage or even death. Gardeners can increase the odds of their mums surviving the winter by applying a mulch in fall. Mulching helps eliminate the alternate freezing-thawing cycles that can heave plants out of the soil. Apply the mulch in late fall, typically late November in central Iowa. Do not cut back the plants prior to mulching. Simply cover the plants with several inches of mulch. Suitable mulching materials include clean (weed-free) straw, pine needles and evergreen branches. Leaves are not a good mulch as they tend to mat down and don’t provide adequate protection. The mulch should remain in place until early to mid-April.
Perennial plants can be cut back in fall after the foliage has been destroyed by a hard freeze. Compost the plant debris. Most established perennials (those in the ground one or more years) do not require winter protection. However, a few perennials, such as garden mums, can be damaged in winter and should be mulched in fall to prevent injury. Perennials planted in late summer/early fall should also be mulched. Good mulching materials are clean, weed-free straw and pine needles. A 6- to 8-inch-layer of mulch should be adequate for most perennials. Mid- to late November is usually a good time to mulch perennial beds in Iowa.
The best way to overwinter perennials in pots is to dig holes in the ground in a sheltered location. The depth of the holes should be slightly less than the height of the containers. Set the pots in the ground and then place soil around the pots. For additional protection, place several inches of straw or pine needles over the potted perennials. In spring, the perennials can be dug up and planted in their permanent location.
Many ornamental grasses provide color, sound and movement to the winter landscape. Because of these winter features, cut back ornamental grasses in April in Iowa. Cut back the grasses to within 2 to 4 inches of the ground with hand shears, lopping shears or hedge trimmers.