by Ken Lain, the mountain gardener
This week’s column is an extensive resource of perennial flowers that should be cut back now. Local gardeners have given each a commonly used name rather than the scientific Latin name, so this list is alphabetized accordingly. I’ve included resources that explain garden zones on each flower mentioned. Where possible, online shopping opportunities are linked directly to the grower. To reduce shipping costs of larger specimens, some can be ordered, delivered, and picked up directly from the garden center, still, the references are linked to each.
While it is tempting to cut back the whole flower garden in the fall, it can be nice to leave some perennials standing throughout the winter months. The seeds of echinacea and rudbeckia attract and feed the birds, while sedums hold onto snow like frosting on the gardens. Some perennial plants like the protection their foliage provides for their crowns, like asclepias (butterfly weed), chrysanthemums, and heuchera (coral bells); they all fare best if cleaned up in February and March. The perennial plants mentioned below need to be pruned sooner rather than later.
A garden friend just shot a great Youtube video on how and when to prune back perennial flowers. Viewing it may be easier than reading this column!
The perennial flowers on this list are not attractive after a killing freeze. Also, recurrent problems with pests and diseases which overwinter in the dead foliage and reemerge in spring can kill blossoms. They are best cut down in the fall. If diseased, throw away the leaves. Do NOT use them for compost.
How to Prune Mountain Perennial Plants
Bachelor’s Button (Centaurea Montana)
Tend to become black and unsightly with the first frost and should be cut back in Autumn. If sheared in late summer when their bloom is finished, the basal growth should remain until spring. Centaurea grows best in USDA zones 3 through 8.
The tall foliage of bearded Iris becomes rangy early in the season. By autumn, it becomes a refuge for iris borers, fungal diseases, and aphids. Cut them back after a killing frost and dispose of the trimmed foliage; do not compost it. Irises grow best in USDA zones 3 through 10.
Even the most resistant varieties of monarda can succumb to mildew. When that happens, it’s essential to cut them back early, so the winter garden soil is exposed to mildew-killing cold. If your Beebalm had issues this season, leave the seed heads for the birds but prune back after the New Year. Beebalm grows best in USDA zones 4 through 9.
Prune to avoid borers and to keep the foliage from collapsing and causing the crown to rot. Lily grows best in USDA zones 5 through 10.
Gaillardia is one robust plant, but cutting back the spent stems seems to improve its hardiness even more and enhance its vigor. Gaillardia grows best in USDA zones 3 through 10.
Bronze fennel has increased in popularity and can be found accenting many local gardens. The foliage provides food for swallowtail caterpillars, whose voracity can leave the stems completely stripped by autumn. If that is the case, it is no longer providing any useful service and can be cut back to the ground. Fennel grows best in USDA zones 5 through 9.
Catmint responds well to severe pruning throughout the season. Cut back spent flowers after each bloom cycle to force repeat blooming. The foliage will be damaged by winter cold and needs to be cut back in early winter. Catmint grows best in USDA zones 3 through 8.
Remove foliage showing mildew and leaf miner damage, and remove any debris around the base of the plants. Columbine emerges early in spring gardens, so cut them back by the end of the year. Columbine grows best in USDA zones 3 through 9.
It is hard to kill corydalis. Deadhead often, and control its spreading habit by cutting this blooming perennial after the first killing frost of the season. Corydalis grows best in USDA zones 5 through 7.
The flowers of crocosmia fall off naturally once blooming has finished, but the seed heads offer interest. The foliage eventually goes down in the garden because of winter snows, and that’s the best time to cut it back. Crocosmia grows best in USDA zones 6 through 10.
Daylilies respond well to shearing. Cut them back as soon as killing frost turns their foliage brown. This will avoid a messy cleanup in spring. Daylily grows best in USDA zones 3 through 9.
False Indigo (Baptisia Australis)
Named 2010 perennial of the year, false indigo often grows so tall it separates in the middle and needs staking. For the sake of aesthetics,cut it back after a hard freeze takes it back to ground level. False indigo grows best in USDA zones 3 through 9.
Golden Marguerite (Anthemis Tinctoria)
By late summer, golden marguerite daisies have finished blooming and are fading. Pruning to the crown will encourage new basal growth that helps protect and sustain them through the winter. Marguerite grows best in USDA zones 3 through 7.
Goldenstar (Chrysogonum Virginianum)
Makes a good underplanting for shrubs and trees. It often has problems with powdery mildew, and if so, remove and destroy foliage in the fall. Goldenstar grows best in USDA zones 5 through 8.
Ground Clematis (Clematis Recta)
This is a clump-forming clematis that blooms late summer into fall. It produces attractive seed heads, but when hit by a frost, it shrivels down to the ground. It flowers on new growth, so do not be afraid to clean it up in the fall. Clematis grows best in USDA zones 3 through 7.
Hardy Begonia (Begonia Grandis)
Frost blackens and collapses the foliage of begonias. The unkempt foliage brings on crown rot that will cause damage to the plant. The foliage should be removed before compacting snow can cause even more damage. Begonia grows best in USDA zones 6 through 9.
Perennial Sunflower (Helianthus)
These perennial members of the sunflower family usually finish blooming toward the end of summer and then begin to fade. Deadheading does not improve their appearance, and the tall stems are guaranteed to break and flop. Cut back to the ground for aesthetics alone. Helianthus grows best in USDA zones 5 through 9.
Hollyhock Mallow (Malva Alcea)
Hollyhock is very survivable. Cut back these flowers or they will repopulate everywhere. Or, help them grow in other parts of the garden by spreading the seedheads. Hollyhock grows best in USDA zones 4 through 8.
Favored by certain beetles and often defoliated by the end of autumn. The foliage turns black and unattractive after a frost so cut back at the end of autumn. Anemone grows best in USDA zones 4 through 8.
Predominantly grown for its foliage and tall yellow flowers. Frost turns this perennial into a heap of black mush. Feel free to cut it back. Ligularia grows best in USDA zones 4 through 8.
Lilyleaf Ladybell (Adenophora Lilifolia)
An elegant plant making spires of white-eyed blue flowers in early and mid-summer. Cut off the flower heads when blooming is complete. Foliage often remains fresh until spring. Ladybell grows best in USDA zones 4 through 8.
Masterwort (Astrantia Major)
Pinch spent flowers off this perennial bloomer to keep it blooming. If hot weather causes the foliage to yellow, don’t be afraid to cut this plant back to the crown. Allow new growth to remain through winter. If there is no yellowing, leave the plant for spring cleaning. Astrantia grows best in USDA zones 4 through 7.
Meadow Rue (Thalictrum Aquilegiifolium)
Performance-wise, it does not really matter when you cut back meadow rue. But once it is finished flowering for the season, pruning in the fall is one less thing to do in the spring. However, some varieties will self-seed. If that is desirable, let it go until spring. Rue grows best in USDA zones 5 through 8.
Painted Daisy (Tanacetum Coccineum)
Painted daisies quickly rot in wet soil. Prune in the fall to prevent the foliage from flopping over onto itself and acting as a damp mulch. Daisy grows best in USDA zones 3 through 7.
Penstemon does not like soggy soil and should be planted a little higher in the ground than most plants. The foliage usually declines toward the end of summer and can be trimmed back, inducing new basal growth that is sufficient to mulch the plants through winter. Penstemon grows best in USDA zones 5 through 9.
It is vital to cut back peony foliage in fall because it is so prone to mildew. Infected foliage can be removed and disposed of in late summer. Healthy foliage turns a beautiful golden in autumn and can be removed once frost burns the plant back to the ground. Peony grows best in USDA zones 3 through 8.
Phlox is prone to powdery mildew when grown in shady gardens; even resistant varieties can become infected during a wet monsoon season. Prune and destroy all foliage and stems when frost finally burns the foliage back to the garden soil. Phlox grows best in USDA zones 4 through 8.
Plume Poppy (Macleaya Cordata)
Plume poppies are nearly impossible to kill. Cut these back before they go to seed, or there will be plume poppies throughout the landscape! Macleaya grows best in USDA zones 3 through 8.
Salvia is a natural in mountain landscapes and benefits from cutting back spent flowers for repeat blooming. When this perennial stops flowering in autumn, cut the whole plant back to the new basal growth. Salvia grows best in USDA zones 3 through 8.
Siberian Bugloss (Brunnera Macrophylla)
Its foliage turns black and unattractive with the first frost. Fall cleanup is preferable if only to keep the gardens looking clean and ready for spring growth. Brunnera grows best in USDA zones 3 through 8.
Sneezeweed (Helenium Autumnale)
Vibrant flowers on upright stems wave enthusiastically above a compact mound of green foliage. Mid-fall is when this perennial bloomer finishes flowering. To prevent mildew issues, cut back and remove foliage at Winter’s start. Helenium grows best in USDA zones 3 through 8.
Solomon’s Seal (Polygonatum Odoratum)
Solomon’s seal disappears on its own after the first hard freeze. If the stems remain, the plant can be pruned back to the ground. Polygonatum grows best in USDA zones 3 through 9.
Speedwell (Veronica Spicata)
As blooms fade in fall, this perennial can be cut back to the ground. If left until spring, the black foliage will uglify your gardens. Speedwell grows best in USDA zones 3 through 8.
Yarrow does not like to sit in cold, wet soil. By fall most of its blooms are spent and the foliage is flopping over and often is diseased. Prune back in early autumn, and new basal growth will fill in before frost. Yarrow grows best in USDA zones 4 through 8.
Until next week, I’ll be here at Watters Garden Center, helping gardeners with Christmas trees and pruning projects.