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Ask the Gardener: Fall is here. Do your perennials need a trim?

Ask the Expert Gardening
Adobe-Stock-Butterfly-Weed
You can scatter fresh seeds from self-seeders like this butterfly weed in your garden. Adobe Stock

What to do this week Remove fallen leaves from walkways, high-traffic areas, and lawns, where they can smother grass, but leave them in other areas of the yard to provide soil insulation, natural fertilizer, and homes for pollinators and other necessary insects that winter over in the leaf litter. Fertilize lawns with a light hand. Over-fertilized lawns are more prone to disease. Keep watering evergreens for two hours a week each to hydrate them for the winter. (Don’t bother watering trees that have dropped their leaves. They’ve gone dormant.) Bag invasives, weeds with seeds, and spent vegetables and annuals for your municipal compost. You can save seeds from old heirloom vegetable varieties, which breed true, but not from modern hybrid vegetables, which don’t. Notice which annuals (such as sweet alyssum) bloom late into the fall. Make a note in your garden diary to plant more of these next spring, along with a record of what you plant. Shop local nurseries for discounted hardy plants. Stick spring bulbs where holes open up from your selective cleanup and where you are planting new shrubs, trees, and perennials. You can also scatter fresh seeds from self-seeders in your garden such as perennial lobelia, foxglove, verbena bonariensis, butterfly weed, poppies, and nigella on open ground to sprout for a casual cottage garden effect next year.

Q. I’m wondering when I should cut back my perennials. Should I wait until the flowers and stems are dead or until spring to cut back for new growth? And how far from the ground should I cut?

R.T., Revere

A. “Around and around the house the leaves fall thick — but never fast, for they come circling down with a dead lightness that is somber and slow,’’ Charles Dickens wrote in “Bleak House.’’ Gardening in the backyard has been a source of health and sanity during quarantine, and I dread winter indoors. I am dragging out fall as long as possible. Instead of cutting and clearing the garden all at once, I am removing only diseased foliage, such as bearded iris, phlox, and peonies, to showcase the sparkling Korean mums and native asters still in bloom.

Don’t clean up your yard each fall as though you were picking up your parlor for guests. It’s enlightened to leave a little mess to support birds and other wildlife through the winter. Clean up your vegetable garden as the season winds down, but try to wait until spring to cut down your ornamental garden. When daytime highs stay mostly in the 50s in late March, cut your perennials back to a height of 3 inches. Also wait until spring to clean out birdhouses, which provide winter shelter for roosting birds. Don’t leave soil bare. Cover your vegetable plot with leaves or grass clippings.

Q. I made a stupid and very costly plant mistake! I purchased a cape plumbago, beautiful periwinkle flowers I thought were phlox. I later discovered it’s a tropical plant not suitable for coastal Massachusetts. Is there any way to keep it alive outside or winter it over?

J.G., Cohasset

A. It may survive if you cut it back and then cover the roots with about 5 inches of bark mulch. In late March, pull back the mulch to expose the soil underneath in two stages a week apart. Alternatively, you could dig up the entire plant and put it in a pot in a frost-free garage until May. Or you can try to grow it as an indoor houseplant in an east or west window with regular misting. But be aware that some plumbagos have toxic sap.

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