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Fall garden tasks: Cutting back perennials

What am I supposed to do in my garden right now? Today’s topic: Cutting back herbaceous perennials (non-woody plants that return year after year)

So, I imagine you’re looking at your garden, noticing that everything is either turning brownish and crisp, slimy, or gorgeously fire-colored (in the case of some trees and shrubs). That means it’s a good time to begin fall cleanup! Now, I actually recommend you DO NOT get overly fastidious, which is good news to gardeners like myself. I’m tired after a long and fruitful growing season!

First things first, what do you leave? For the most part, you’ll want to leave any plants that have evergreen foliage–Bergenia cordifolia, Ajuga repens, Epimedium, Heuchera…the list goes on. Also, go ahead and leave those seedheads that will provide important food sources for birds in the coming months. Probably the best way to determine which seedheads should remain over winter, is simply to leave everything alone for this fall and winter, and take a moment each day to quietly observe your garden. To which plants do the birds seem to flock, and when? Over the last couple days, I’ve seen hummingbirds visit an old hanging basket to feed from the last fuschia blooms and try out the last blooms of Salvia x sylvestris ‘May Night,’ as well as a handful of black-eyed juncos perch on a low fence I’ve strung across the patio to eat the seeds from some Mexican feather grass (Nassella tenuissima). If you don’t lead the kind of life where you can spend hours staring out your window (or just a few minutes in the morning, then I’d advise you to go ahead and leave the seedheads of coneflowers (Echinacea and Rudbeckia), sunflowers, globe thistles (Echinops species), asters (Symphyotrichum species), goldenrod (Solidago species–S. canadensis has the added bonus of being a native plant), coreopsis, Sedum ‘Autumn Joy,’ and grasses. Not only do these plants provide food, they also provide shelter and water, when the morning dew collects in their nooks and crannies. Even some annuals will help feed the birds…zinnias especially, as well as marigolds and impatiens.

One note about leaving seedheads…you’ll probably want to cut back any plants that self-seed too readily in the garden, especially fennel, which is listed as a Class B noxious weed in Whatcom County.

So what should you cut back? As with determining evergreen foliage and beneficial seedheads, careful observation of your plants will provide clues as to what should stay and what should go. You can selectively cut back the parts of your perennials that show signs of disease, especially common in the PNW are fungal diseases like powdery mildew, leaf spot and rust. I’ve had to sacrifice the seedheads on an Sedum ‘Autumn Joy’ in the past, simply because it was too infested with disease. Make sure to dispose of diseased plant material in the trash, or in the city’s yard waste collection service, where it will be subjected to a hot compost process, to kill disease agents.

Any plant that produces new growth from underground, like peony and crocosmia, can be cut back to ground level. Plants that show new basal growth–aka new growth at the base of the plant, just above ground level–can be less severely cut back, with care to remove only the dead material, leaving any good-looking live growth. Hardy geraniums (Geranium sanguineum), bleeding heart (Dicentra spectabilis), and yarrow (Achillea millefolium) are good examples of plants that demonstrate new basal growth.

When you decide which plant material to remove, make sure you have a sharp pair of hand pruners. Wherever you decide to make the cut, make sure it’s clean and angled at about 45 degrees (to discourage water from pooling on the cut, and creating an environment that may foster disease growth). Also, tread lightly on your garden’s soil after all the rain we’ve been having. It’s easy to compact wet soil, which will destroy the tiny air pockets between soil grains. These miniscule spaces are crucial for air and water movement to plant roots. When I’ve had no choice but to work on wet soils, I’ve brought a thick piece of cardboard or a few short lengths of two-by-four, to create temporary walkways through the garden beds. They help to disperse my weight, and minimize soil compaction.

So what have I been up to in gardens around town? Lately, I’ve found myself cutting back a lot of slimy iris, daylily, hosta, and ligularia foliage. There’s been plenty of work pulling the fall flush of weeds, in preparation for mulching. And I’ve been getting a good deal of fall planting and transplanting accomplished. And don’t worry! I will discuss fall planting/transplanting and mulching in future posts. Because of course these projects are more complex than one might assume, and I have lots of opinions about them! But for now, have fun getting out into your garden and cutting back your herbaceous perennials with confidence and a certain je ne sais quoi! Your garden, and our local wildlife, thank you.