Should perennials be cut back in fall? It depends.
FARGO — What's the first thought that comes to mind if someone mentions cutting back? Fewer trips through the buffet line? Less snacking between meals?
Gardening has a dialect all its own with words like pinching, deadheading, slip, crown and cutting back. So, when a questioner asked about cutting back, I knew they weren't reducing their caffeine consumption.
The email photo showed a mailbox on a wooden post surrounded by a nice planting of daylilies with the question, "Do you recommend cutting these back this fall?" I couldn't resist replying that daylilies can be cut back, but the mailbox is best left standing.
Longtime gardeners sometimes forget that new gardeners might not know the nuances of some phrases. In gardening dialect, "cutting back" means reducing the size of a plant. We can cut back a leggy geranium to promote branching. An overgrown potentilla can be cut back in early spring to rejuvenate.
When we hear discussions of cutting back in fall, it's usually about whether the tops of perennial plants should be pruned off. Another gardening term, "tops" aren't just the tallest point, as in the top of a building, rather "cutting back the tops" of perennials means cutting all above-ground material to 1 or 2 inches above ground level.
Some perennial tops are best cut back in fall, while other types winter best with their tops left intact.
Cut these back in fall
• Peonies are susceptible to foliage diseases and blossom blights. After several frosts, prune tops back to near soil level and dispose in garbage. Good sanitation helps lessen peony diseases by removing leaves and stems where disease organisms survive winter. Other diseased perennials are likewise best pruned back.
• Hosta foliage becomes mushy and more difficult to remove if left over winter. About the time of the first frost, cut hosta foliage down to an inch or two above soil level.
• Iris foliage likewise becomes limp and difficult to handle by next spring, so it's best removed around frost time. Iris can be divided August through September, and foliage is cut back at that time to a fan shape 2 to 3 inches high.
• Daylily leaves collapse during winter, so pruning them off after a few light fall frosts is less messy than waiting until spring. Old daylily foliage should be cut down every year or the accumulation of old leaves can choke out new growth.
Leave most intact
The tops of most perennials, with the exception of those listed, are best left intact during winter instead of cutting back. The stems and dried flowers add interest to the winter landscape and provide habitat for birds.
The most important reason for leaving perennial tops in place is the added winter protection they provide. The stems catch and hold snow, which is a good insulator, especially important during cold winters with little snow and in windswept areas. Tops reduce alternating freezing and thawing of soil, which injures perennials.
For types that are borderline in hardiness, intact tops hold protective mulch applied in late fall. Roses are generally best pruned back in spring. Fall pruning can be done if winter protection requires shortening stems.
If perennial tops are removed in fall, will they die? No. Although it's recommended to leave them in place until spring, perennials will usually survive if cut back. But we occasionally have a "test" winter, and intact tops can mean the difference between survival and failure on some types. Some perennials, like mums, always winter best with tops left in place.
When leaving perennial tops intact during winter, cut them back in spring before new growth emerges from ground level.