Putting the garden to bed: Cleanup guidelines are shifting

I used to laugh when people talked about "winter interest" in the garden. The only winter interest I wanted was a nice insulating blanket of snow for my perennials, some statuesque ornamental grasses waving in the frigid wind, and the knowledge that when spring came, I would not have to tramp around in the mud to cut down the soggy remains of last year's garden.

I learned this from my mother, who grew up in England and had definite ideas about how to clean up the garden for fall. Since she was the best gardener I've ever known, I've followed her example for most of my life.

But in recent years, my attitude has changed. The question about how to put the garden to bed, and whether to strip it of dead vegetation or leave plants standing until the snow melts, turns out to be far more complicated when you consider bigger questions like habitat for wildlife and beneficial insects.

Already I've seen goldfinches and warblers plucking at the dead blossoms of coneflowers and swinging from the stems of towering cup plants as they try to harvest seeds. While warblers fly south, birds like finches sometimes overwinter here — and benefit from having those seed-bearing plants through winter.

Birds aren't the only ones that will eat flower seed heads. I once looked out a window at the snowy garden on a below-zero day to see a vole scampering up the dead stem of a black-eyed Susan to chew on the seed head.

Some native bees overwinter and nest in the stems of dead perennials. With all the concern about pollinators, this is something gardeners who care about the environment should consider. Leaves and other plant litter in the garden can provide winter shelter for other beneficial insects as well.

Ornamental grasses, with stiff leaves and flowers that stand up well to snow, are a natural to leave standing because they look so good. Some ornamental grasses also suffer if they are cut down in fall, especially when plants are young. Wait until spring to cut them back. I never cut back woody plants like lavender or Russian sage, waiting until spring to see where the plants sprout, and then cutting stems back to that point. (Lavender is marginally hardy in the Twin Cities and benefits from mulching after the ground has frozen.) I leave chrysanthemums standing through winter, too. And the few evergreen plants we have in Minnesota, like bergenia, are best left alone until spring, when dead leaves can be pulled off.

The case for cleanup

So are there still arguments for cleaning up the garden? You bet there are. This year, there was an explosion of four-lined plant bugs in part of my perennial bed. The insects suck the chlorophyll from leaves, usually causing only cosmetic damage. But this year, their attacks on new globe thistle plants were so aggressive that the plants died.

One of the ways to fight four-lined plant bugs is to cut down the perennials they favor in the fall, removing all the leaves and stems that are host to their eggs. A bee balm near the dead globe thistle plants also was attacked, so this year I'll cut that back.

Another reason to clear plant material in the fall is to remove diseased foliage that could help disease spread next year. Plants like phlox that had powdery mildew this summer should be cut down to the ground, and any fallen leaves should be raked up and removed. The same is true with perennials like peonies that get leaf diseases.

There's no point in leaving dead annuals in the garden, so I always pull those. Cleanup is especially important in the vegetable garden, where diseases that plague tomatoes will linger in leaf litter or plants that are left standing. After pulling the dead plants, rake the bed to collect and remove as many leaves as possible.

So after we have a freeze this fall, I'll cut back my phlox, gooseneck loosestrife and other perennials that turn to mush over winter, as well as any plants that had disease or pest problems.

I plan to leave the ornamental grasses, black-eyed Susans, coneflower and other stiff-stemmed perennials with interesting or wildlife-friendly seed heads.

The last stage of putting the garden to bed is to mulch any tender perennials to protect them from freezing and thawing cycles that can result in heaving roots that kill the plant. Wait to cover those plants with leaves or straw until after the ground is frozen.

Mary Jane Smetanka is a Master Gardener and Minneapolis freelance writer.