8 Rules for Overwintering Plants in a Garage or Cellar

5 November 2020, written by Barbara Pleasant

Every year at this time, I am surprised by the motley group of containers that has accumulated in my cellar for safekeeping through winter. This year there are the amaryllis bulbs I’ve been growing for ten years, the shamrock (oxalis) that summered outdoors, and sheared-back pots of chives, rosemary, and sage. Even though the space is chilly and dark, they will be safe and happy through the winter there, with minimal care.

I’ve been enjoying overwintering plants in garages and other cold spaces for many years, and while I’ve had many successes, there have been failures, too. Here are a few tips for overwintering plants in a garage or cellar I learned through trial and error.

1. Find a Good Overwintering Place

To be sure plants know it is winter, you need a dry space where temperatures stay above 45°F (7°C) but below 60°F (15°C). I once had a house with an attached unheated garage that worked well. In another house I used a minimally heated outbuilding, and now I have a cellar. Light is not a factor for dormant plants, though weak winter light won’t hurt them.

2. Clean Up Containers Before Bringing Them Inside

With plants that have grown in pots outdoors all summer, I assume the top half inch of soil has gone salty, so I scrape it off and scatter it in the yard. Then I snip off dead plant parts, give the pots a quick wipe with a damp cloth, and move them into cold storage. After a few days, I check for signs of unwanted insects, earthworms or earwigs.

“Geraniums” — After a month of cool rest, many geraniums will resume growth and bloom all winter

3. Don’t Over-water Overwintering Bulbs

Many of the best plants for overwintering in a garage or cellar have bulbous roots designed to store nutrients and moisture through a dormant period. Amaryllis, oxalis, cannas, dahlias, and tuberous begonias hold moisture in their fleshy bulbs, tubers and corms, so very little supplemental water is needed. I like to let the pots dry out thoroughly in early winter, and then dribble a little water into the pots every week or so in January and February.

4. Let Overwintering Plants Rest

Chives and geraniums benefit from a short dormancy period, so I trim back the plants and let them rest for a month or so before bringing them back to life in a warm, sunny window. Many other perennials can do with a longer rest period, and you can control when the plants start growing again by keeping them cool and dark. I let my amaryllis and oxalis rest through the holidays, and restart them in January for late winter fireworks. Container-grown asters and chrysanthemums purchased for autumn display can be overwintered in a garage or cellar, then transplanted outdoors when the weather warms in spring.

5. Avoid Temperature Fluctuations

Reduce the guesswork about watering by covering your whole collection of pots with an old blanket or flannel sheet. Also cover the plants when you temporarily heat the space for other uses. A cloth cover will reduce temperature changes and block light that might confuse the resting plants.

“Pot-grown — Pot-grown herbs and flowers like rosemary, sage and chrysanthemum are easy to overwinter in a garage or cellar

6. Provide Good Ventilation for Happier Herbs

Some windows leak cold air, which compromises energy efficiency but feels pleasant and natural to rosemary, sage, and other half-hardy herbs. Exposure to chilly drafts seems to prevent rosemary powdery mildew, while meeting the plants’ needs for limited winter chilling.

7. Tackle Overwintering Pests Promptly

A few pests can persist in overwintered plants, so it’s important to trim off dead foliage before putting plants into storage. Be watchful for the webs made by spider mites, which can make a mess of resting strawberries that are holding only a few leaves. When you notice spider mites or any other pest, immediately move the plant away from the others to stop the pest’s spread.

8. Force Some Freesias

When I have lost overwintering plants, it has usually been because I forgot about them too long and let them dry to toast. This never happens in years when I pot up a few freesias and let them chill in the cellar for a month, because the awakening corms demand to be checked upon once a week. Adherence to duty brings beautiful, fragrant blossoms when the plants are moved upstairs and allowed to grow in a sunny winter windowsill. My other overwintering plants get better care by being friends with freesias.