Do perennial flowers reduce a gardener’s work? Perennials do eliminate the need to replant every year, but they’re not work-free, and that’s good. Gardening isn’t just about the end results, it’s also about enjoying the process, such as cleaning perennial flower beds in the fresh sunshine of a fine spring day.
How do we give perennial flowers their spring cleaning? The following are guidelines for readying perennials for the upcoming growing season.
The above-ground portions of perennial flowers, referred to as the “tops” in gardening language, are best left intact during winter, instead of clear-cutting in autumn. The muted brown shades and spent flowers in assorted shapes add winter interest to the snowy landscape. Perennials survive better with the tops left intact, as they catch insulating snow.
The hollow stems of perennial flowers provide an important winter nesting place for the small, native pollinating bees that are vital for ensuring crops of apples, cucumbers and other foods. Allowing perennial tops to remain during winter provides pollinator habitat.
Some perennials are better cut back in autumn, though, because the dead foliage turns to flattened mush by spring, including hosta, iris and daylilies, and clear-cutting in fall is easier — although spring is fine, if you didn’t get it done. Peonies and other disease-prone perennials are also better pruned back close to ground level in fall, with infected tops removed from the area.
When should we begin the spring cleanup of perennials? Beginning too early risks removing the stems of perennials in which pollinators are still nesting. If we wait too long, new growth begins at the perennial’s base, which makes removing old tops difficult without injuring the new growth.
There’s a good compromise. Entomologists suggest cutting back the perennial tops as you need before new growth appears at the base, but instead of disposing of the branches in which pollinators may be hibernating, stack the material in an out-of-the way spot at the rear of the flower bed, or next to the compost pile. By June the pollinators will have emerged, and the tops can be composted or disposed of.
A first step in spring cleanup is to pull away any protective winter mulch that we mounded over tender perennials like roses or chrysanthemums, or newly planted perennials to which we gave extra protection. As the weather moderates by early April, winter mulches should be pulled to the side, but kept handy in case a few days of frigid weather return.
Ornamental grasses should be among the first perennials we cut back to an inch above ground level, because new spring growth emerges early from the base. After cutting back, comb out old debris from the grass clump’s center with a rake or by hand.
As a rule, cut down most perennials to near ground level each spring. Some perennials become completely dead-brown down to the soil, so prune down leaving only an inch or so.
Other perennials have low-growing foliage that can remain semi-evergreen during winter, such as creeping phlox, bergenia, dianthus and heuchera coral bells. Remove any remnants of last year’s flowers and any dead-brown foliage. Growth that has remained green during winter can be trimmed back by half.
Some perennials grow back from woody stems, such as Russian sage and silver mound artemisia. Instead of cutting to ground level, allow 4 to 6 inches of the woody stalks to remain.
Many perennials can be divided or moved in spring. As a rule of thumb, perennials that bloom after July 1, such as tall garden phlox, hosta, chrysanthemum and monarda, can be divided or moved in spring. Perennials that bloom from early spring through late June are best delayed until late summer or fall for dividing or moving, including peonies, iris and bleeding heart. Daylilies and true lilies can be divided in spring or fall, although fall is preferred.
To move or divide perennials in spring, you can wait until a tiny amount of new growth appears, so you know you’re working with live material. Always keep roots moist and covered while digging and dividing. Roots exposed to air can quickly die.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org.