Most gardeners that I talk with share the belief that cleaning up the garden as much as possible in the fall is necessary, not just for aesthetic value, but for control of insects and diseases, as well. Others simply like to get the clean-up work out of the way now, instead of waiting until spring. Make hay while the sun shines!
For generations, the standard practice has been to tidy up in the fall by cutting everything in the garden back to the within 3 inches of the ground. A new way of thinking now prevails. Yes, we do benefit by cutting plants like hosta back before they turn to moosh. And we absolutely must remove any vegetation that may harbor disease or provide cover for undesirable insect pests. But a number of perennial stems, seedheads, ornamental grasses and, in some cases, foliage can be aesthetically pleasing when left standing in winter.
There are several reasons for cutting back herbaceous perennials (those with top growth that dies back to the ground each year and that regrow their stems and foliage the following season) and discarding or composting that plant material in the fall. Dead foliage and plant debris can be a sanctuary for overwintering insects and diseases that will emerge at the same time that new plant growth begins in the spring. And cutting back now will greatly reduce the possibility of damaging emerging plants or parts of plants with rakes or other gardening tools in the spring. What's more, should winter temperatures fluctuate wildly, as they sometimes do, causing the ground to heave as it thaws and freezes, the additional weight of wet, dead foliage will increase the possibility of those plants crowns and roots being lifted out of the ground.
What you leave standing in your garden is up to you. Some gardeners enjoy seeing standing perennials in the winter. Others don't. Anything that you think may add winter interest can remain. Some sedum varieties, for example, are quite cold tolerant and remain extremely attractive right into winter. Although they are not very colorful, faded astilbe blooms remain downy and quite pretty, especially when seen reaching up through snow cover on a bright, sunny morning.
Ornamental grasses can be engaging, too. As can plants with brightly colored bark or stems. Plants with eye-catching seedheads have appeal as well, not just because they stand out in the stark winter landscape, but because they are a source of winter food for songbirds.
Evergreen perennials, especially those with brightly colored berries, add dimension as the snows come and go in late fall. If any evergreen foliage appears brown or scruffy in the spring, you can trim it up then.
Whether or not perennials should be heavily mulched for the winter is often a matter of personal preference, too. Most gardeners that I speak with grow only perennials cold hardy enough to overwinter without additional mulch. Attempting to overwinter plants that are only marginally hardy for this area, or those that are newly planted or recently divided, may call for an additional two to four inches of mulch. That mulch, however, should be removed gradually in the spring. Overwintering delicate perennials will require even more mulch.
Using as light a mulch as possible is recommended. Shredded leaves or straw are quite good, as are pine, spruce, fir or hemlock boughs. Avoid whole leaves. They will mat down and may promote rotting of otherwise healthy plants. And hay can be full of weed seeds. Keep in mind too, that thick carpets of mulch make comfortable winter homes for rodents, some of which may find your perennials quite tasty and extremely nutritious.
Tidying up the garden on a crisp autumn day with winter beauty and wildlife in mind can be extremely rewarding. Fall is also the time to ready any new beds you may wish to plant in the spring. Then you can relax by a wood fire, enjoying your winter landscape while thumbing through the newest seed and plant catalogs and thinking about spring.