We get a lot of calls from clients in the autumn, asking when/whether they should cut back their perennial plants. While there is no clear-cut answer to that question the following guidelines may be of assistance:
Plants that offer winter interest should be left standing. Ornamental grasses are the most obvious members of this group, but there is also a subtle beauty in the standing seed-heads of such plants as Sedum, Coneflowers, Allium, and Siberian Iris.
Most perennials will need to be cut down to ground level sometime before the new growth emerges in spring.
If there is any evidence of insect or disease damage on the plant then I advocate cutting it down in the fall and disposing of the foliage, to minimize the risk of overwintering disease organisms or undesirable insects. As a rule, there are a few perennials whose foliage I always remove in fall. These include Hosta (their soggy foliage makes an excellent hiding place for overwintering slugs), Paeonies (prone to a variety of foliage diseases), and Daylilies. If you wait until after the first killing frost you should be able to simply pull the spent foliage off both Hosta and Daylilies, sparing you the joint punishing tedium of cutting the plants down with hand pruners.
If you have any concerns about the winter hardiness of a perennial plant, I recommend leaving it standing through the winter months. The remaining top growth does offer some winter protection to the crown.
Many perennials will self-sow if seed heads are left standing. This may be a plus or a minus, depending on your gardening style, so time your cleanup accordingly. Some short-lived perennials will continue in your garden through their seedling and often the gardener does not even realize that their original plant has disappeared, but its place has been taken by its offspring.
Leaving perennials standing over the winter also benefits wildlife in several ways. Seed-heads of many species provide food for native birds. The standing hollow stems provide shelter for overwintering desirable insects, which are also a valuable food source for birds.
It is often simply the gardener’s schedule that determines whether fall or spring cleanup is best. While we believe in leaving foliage standing to benefit beneficial insects and native birds, we do cut down our perennial beds here at Heyden’s Gardens in fall or winter. We are just too busy with a myriad of other preparatory tasks to have time to complete the task in spring.
There is yet another option, and that is to forgo seasonal cleanup altogether. Certainly, that is how our native plants grow in their natural environment. This is an environmentally sound alternative and could be a viable option for a naturalized area. It is strictly a matter of personal taste, but in my experience, most gardeners prefer a slightly more manicured look in their garden beds.