Growing Green: Some perennials make the cut
Most of the perennials in the garden are finished blooming and it’s time to throw in the towel for the growing season.
Some perennials, however, can be left standing and this begs the question, “to cut or not to cut?”
It’s easy to make a decision with annuals. After the first frost when they are blackened and looking ugly, pull them out and throw them in the compost bin.
Likewise, clean up plant debris from the vegetable garden.
When asked what to do with perennials, as with many gardening questions, the answer is: “It depends.”
In this installment of the “Growing Green” column, we’ll provide tips on when to cut and when to wait.
Perennials that add interest to the winter landscape can be left standing. During a snowy winter, some plants provide stunning interest in the form of height and structure.
Not only evergreens and the skeletons of shrubs, but also grasses and standing perennial seedpods display beautiful winter artistry.
Ornamental grasses are most dramatic in the winter landscape with their tall plumes. Switchgrass, zebra grass and feather reed grass can be left standing until spring, then cut them back before the new shoots appear.
Siberian iris and blue false indigo have interesting elongated black seedpods that stand out against the snow. Allow the seeds of sedum ‘Autumn Joy’ and Joe Pye weed to display their large, round lacy globes all winter long.
Many birds rely on the seedheads of dried perennials for food. It is common to see goldfinches in a stand of purple coneflower enjoying the seed treats. The seeds of black-eyed Susan and oxeye sunflower are other favorites. In addition, overwintering birds find protection in plant stubs and groundcovers.
Beneficial insects may hide in or near native plants for the winter either as pupae, caterpillars or eggs. The plants provide shelter from their predators such as birds or spiders.
The Lehigh Valley region’s native butterfly, the viceroy, rolls itself in a leaf and drops to the ground where it stays until spring, providing a good reason not to rake plant debris from around your perennials.
Don’t cut back marginally-hardy perennials like garden mums, anise hyssop, red-hot poker and Montauk daisy. They are more likely to survive the cold of winter if you leave their tops to collect leaves and snow for insulation and moisture.
There is no need to cut back hardy geraniums, heucheras, hellebores and dianthus. Tidy them up in the spring as needed.
You should cut back plants with disease or insect pest problems to reduce the chance of infection the following season. Bee balm and phlox with powdery mildew are examples. Even resistant varieties of bee balm and phlox can become infected in bad weather so cut them all back.
Remember to destroy, not compost, diseased stems and leaves.
It is important to cut back hostas and remove their leaves from the ground as soon as the frost takes them. Dead hosta leaves harbor slug eggs that will hatch and ruin next year’s greenery.
Cut back plants with browning or blackened foliage and bare stalks that don’t add anything visually to the winter garden: peonies, daylilies and speedwell for example.
Late in the season, some plants, including yarrow, Shasta daisy and globe thistle grow new basal leaves. Cut off the stalks without disturbing this new growth.
When cutting down a plant, leave about two inches above the soil to mark its location. This is especially important for plants that emerge late such as butterfly weed, rose mallow and balloon flower. You will be less likely to dig into them accidentally before they appear in spring if you can see a portion of their stalks.
Don’t be in a hurry to rush outside and cut plants back. Unless the plant is diseased or infected, wait until several hard frosts have killed back the tops.
In the spring, the plant sends up energy from its roots to produce beautiful foliage and blooms. Allow the roots time to reclaim that energy from the dying plant, keeping it strong for re-emergence in the spring.
For many perennials, leaving plant tops overwinter is fine and may be preferable.
Spotted Lanternfly update: Be on the lookout for spotted lanternfly egg masses. For information on what to look for and how to destroy the eggs go to:
“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners.