Fall clean-up: Clean out gutters after leaves fall so that you don’t end up with a lot of ice dams over winter. Rake up fallen leaves and compost them if there is minimal foliar disease. If you have had significant amounts of diseases on woody plants such as downy mildew, apple scab, rose blackspot, tar spot, powdery mildew, anthracnose, or buckeye/horsechestnut foliar diseases, take the leaves to the yard waste center where they can be hot-composted.
If you want to keep your diseased leaves on-site, spores will survive if you don’t hot-compost. Pile temperatures need to reach 135 degrees to kill most pathogens and weed seeds. Don’t compost leaves, twigs or fruit from black walnut or butternut trees. They contain juglone, which inhibits the growth of many plants and could affect plants growing wherever you deposit the compost. If you have finished compost in your compost pile, you can spread it and till it in to vegetable garden beds so they are all ready to go next spring, or do it in spring if you wish. Be sure to bring in any clay or ceramic pots that are still outside so they don’t crack as the temperatures continue to fall. Empty your rain barrels and unhook them from downspouts or cap them off so they don’t refill, freeze and crack during winter thaw events where there might be melt water running off the roof.
Perennials: Cut aster-family plants back to the ground after frost to remove any leaves with foliar diseases such as powdery mildew. This includes black-eyed Susans, Kalimeris, coneflowers, coreopsis, Shasta daisies, asters, Heleniums and perennial sunflowers, among others. The exception is garden chrysanthemums — those should be left at about 12 inches tall to provide enough material to protect the crowns overwinter. This helps substantially in reducing winter loss of mum plants.
Also cut down monarda (bee balm) and garden phlox, which are prone to powdery mildew, and herbaceous peonies which are susceptible to a variety of foliar diseases. If you have not cut back iris, balloon flowers, hostas and other perennials, now is the time to do that as well to avoid providing disease organisms and insect pests with overwintering shelters in plant debris.
If you didn’t already check iris rhizomes for borers, it is better to wait now since the rhizomes may not get fully re-rooted before the soil freezes. Ornamental grasses can either be cut back to about 4 inches in fall or left standing to cut back in early spring. Some species, such as Miscanthus (maiden grass) cultivars have seed heads that stay relatively attractive during winter unless there is a heavy snow. However, some cultivars tend to shatter, leaving pieces of stems and seed heads blowing all over the landscape. Most of our native grasses do not tend to remain attractive over winter, so prairie dropseed, switchgrass, big and little bluestem and Indiangrass can all be cut back. If you do save the cutting until spring, be sure to cut back the old stems before new ones start to grow. I can tell you from experience that it is a big pain to try and separate the new and old growth if you wait too long!
Die-back shrubs/perennials like Caryopteris (Blue false spirea), Perovskia (Russian sage) and butterfly bush should only be cut back to about 18 inches. The reason we cut back anything at all is to avoid branch breakage if there is wet, heavy snow. Since these plants do die back to an unpredictable degree, it is best to leave as much as possible, since any tissue that remains alive will produce buds and branches next spring. These plants bud out late in spring, around the middle to the end of May, so don’t give up on them too early.