The seasonal shift from summer to winter requires perennial maintenance. Deciding which plants to prune and which to leave standing in the fall is necessary for a tidy landscape and healthy growth next spring. Fall pruning, when done correctly, prepares the perennial’s crown for dormancy, ensuring its long-term vigor.
Determining Factors for Fall Pruning
The decision to cut back a perennial in autumn rests on three main criteria: disease and pest control, the nature of the foliage, and the plant’s need for winter protection. Perennials that showed signs of disease, such as powdery mildew or rust, should be a priority for removal to prevent pathogens from overwintering and re-infecting new growth in spring.
Sanitation is a primary driver for fall cleanup, especially for plants known to harbor pests or diseases within their dying leaves. Beyond disease, the structural integrity of the plant matters; herbaceous perennials that become mushy and collapse after a hard frost can funnel water and ice directly into the crown, which may lead to rot. Conversely, plants with semi-woody stems or a strong, upright habit often benefit from being left intact to protect the crown from harsh winter conditions.
Specific Perennials to Cut Back Before Winter
Perennials that benefit most from a fall cutback are those prone to fungal issues or those that host overwintering pests. Peonies (Paeonia), for example, are susceptible to botrytis blight; removing all foliage down to the ground in the fall reduces the spread of this fungus. Debris from diseased plants should be discarded, not composted, as typical compost heat may not kill all pathogens.
Hostas should be cut back after a hard frost blackens their leaves, as the dead foliage provides shelter for slug eggs. Removing the leaves disrupts the slug life cycle, reducing the population that emerges in spring. Similarly, daylilies (Hemerocallis) are often cut back to remove the foliage that collapses into a wet mass, which can harbor thrips and other pests near the crown.
Other plants benefit from fall pruning due to disease susceptibility. Bee Balm (Monarda) and Garden Phlox (Phlox paniculata) are highly susceptible to powdery mildew; cutting them back removes the mildew-covered material, ensuring a clean start the following season. Bearded Irises (Iris germanica) are also important, as the iris borer overwinters as eggs on old foliage. Cutting the fan-shaped leaves back to four to six inches and disposing of the debris can eliminate this pest.
Perennials That Must Remain Standing Until Spring
A large number of healthy perennials should be left standing through the winter to provide insulation, food, and shelter for wildlife. The dried stems and leaves of these plants act as a natural mulch, trapping snow and insulating the plant’s crown from damaging freeze-thaw cycles. This is particularly important for marginally hardy plants, where the extra layer of protection can ensure winter survival.
Many common garden favorites are best left untouched until late winter or early spring. Ornamental grasses, such as Miscanthus and Calamagrostis, have stiff, upright structures that catch snow, which is one of the best natural insulators for the root system. Cutting them down in the fall can create hollow stems that funnel water into the crown, potentially leading to fatal crown rot.
Seed Heads and Wildlife
Leaving seed heads intact provides food for birds during the winter months. Coneflowers (Echinacea), Black-Eyed Susans (Rudbeckia), and Sedum (Stonecrop) are excellent examples, as their seed heads are favorites of goldfinches and other small birds.
Semi-Woody Protection
Some perennials, like Lavender and Russian Sage (Perovskia), are considered semi-woody. Their old stems protect the new growth and the crown. Cutting them back hard in the fall can expose them to winter desiccation and damage.
Proper Timing and Cutting Technique
The timing for fall perennial cutback should occur only after the plant has entered dormancy. This is generally after the first hard frosts have killed back the foliage, causing it to turn brown or black. Waiting until this point ensures that the plant has fully withdrawn its energy from the leaves and stems back into the roots and crown for storage.
When cutting, use sharp, sterilized bypass pruners, which make a clean, non-crushing cut, to prevent disease transmission. For most herbaceous perennials, cut the stems back to about two to three inches above the soil line. Leaving a short stub helps mark the plant’s location, preventing accidental damage when working in the garden before new growth appears.