Do I need to bring my potted plants inside during winter?

Small-space gardening has been a blessing during the pandemic. A few pots on apartment balconies, window boxes, decks, rooftops and stairwells brought beauty to our lives — and maybe even some delicious fruits, vegetables and fragrant herbs.

Earlier this year, the United Kingdom’s Chelsea Flower Show, one of the world’s most prestigious horticultural events, introduced two new categories intended to recognize the importance of small-space gardening. The results in the balcony and container gardens exhibits were predictably spectacular, but they begged a question: How do you protect plants in pots and boxes in locations exposed to the worst that winter may do?

Alexandra Noble, an award-winning garden designer based in London, created a balcony garden for this year’s show that combined small trees, ornamental grasses and shrubs. She recommends putting a layer of organic mulch on the soil in every pot and planter. Prune any dead or damaged branches, but leave seed heads just as they are.

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“I’m a big advocate of allowing structural seed heads of plants to remain over winter, providing forage for birds and other wildlife,” Noble said. “These seed heads also look beautiful when touched with frost on a winter morning.”

Many of those seed heads are sprouting from annuals, which only live one season, and perennials, which have the potential to thrive for several years. Gather some of those seeds, put them in a paper envelope, label each envelope, and store them in a cool, dry place, so you can replant the seeds in the winter.

Say goodbye to your tomatoes, peppers and other summer vegetables, however. “Unless you have an indoor greenhouse with ventilation, they won’t do well,” says Marta Caruso, container garden designer at the American Plant Landscape.

Some annuals, such as geraniums, have a fighting chance, along with perennials such as coleus, hibiscus and oleanders, and even those more accustomed to tropical climates.

How can you distinguish plants that can survive outdoors from those that must come in from the cold? Todd Rounsaville, a scientist with the National Arboretum, uses the Agriculture Department’s Plant Hardiness Zone Map when gardeners ask that question. Though the map’s data cannot predict the sudden temperature swings that can deliver balmy winter days followed by punishing frosts, “it’s a great place to start to get a sense of what plants are suitable for one’s region.”

Go to the website planthardiness.ars.usda.gov. Type in your Zip code, and the map will put you in a numbered, color-coded region indicating the average temperature of extremely cold days in a given year. Parts of D.C., for example, are in 7A, while others are in 7B; neighboring areas in Delaware, Maryland and Virginia vary from 7B to 6B.

“Winter survival can be more nuanced than the map alone,” Rounsaville says. “Microclimates,” or small areas within the larger zone, “can lead to localized sites that are slightly warmer or slightly cooler.”

If your plant came with a tag, the hardiness zone number might be there. You can also find the number online if you know the plant’s species, or you can use one of many free apps that will identify your plant from a cellphone photo.

You can also ask a horticultural extension specialist, according to Kirsten Ann Conrad, a master gardener and agriculture natural resources extension agent for Arlington County and the city of Alexandria. You’re in luck, Conrad says, if your plant is native to the general area where you’re living. “Bioregionally native plants require no special treatment to survive the winter.”

Others want a little attention. “For example, tea roses are cut back to short major stems and are sometimes then heavily mulched to protect the graft junction,” Conrad says.

But, as a general rule, a hardiness number of seven and lower (five is optimal) can stay outside, especially on balconies with southern exposure, where concrete will absorb heat from sunlight.

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Before your plants head into winter, examine their containers. If your pots are made of plastic, artificial fiber or wood “and they have nice-sized, unobstructed holes in the bottom so rainwater can drain out, you’re going to be okay when they get wet and freeze,” says Roger Davis, director of outdoor landscapes at Longwood Gardens in Kennett Square, Pa. “If you have ceramic or clay terra-cotta pots, glazed or not, and your drainage isn’t good, they’ll freeze and crack.”

Frozen water in pots can also damage the plants. “What you have to do is improve the drainage in all your containers,” Davis says. “Check to see if there is a hole at the bottom, and make sure that hole isn’t obstructed. Don’t put the pot in a saucer. Put chocks,” which are small pieces of wood that will elevate the bottom of the plant from the deck or concrete, “or some old pencils under the pot, so it won’t freeze to the surface.”

Then prune back to about a third of the plant’s height. Ornamental grasses can be cut even further, down almost to the level of the soil. If your balcony is on the breezy side, you can protect your plants by putting them behind tightly baled straw, or you can create a windbreak by securing burlap to the railing. Davis suggests insulating small pots and preventing them from being blown over by putting them into a larger pot and filling the gaps between with mulch.

Plants with a hardiness number of eight and higher will not fare well in a typical D.C. winter and should come in.

Before you bring a plant into your home, remove any weeds. Then check for insects, and cut away infested or diseased parts of the plant.

“Almost anything that’s coming in is going to need a haircut,” says Melissa Brizer, a greenhouse specialist at D.C.’s Dumbarton Oaks. Although she doesn’t prune citrus plants, she’ll trim others, because “it’s good for the plant, and, if you’re taking it into your home, you’re going to be looking at it for a while. If you don’t like that wonky branch, cut it off.”

Prune about one-third of the plant, then consider transplanting it to a slightly larger container with loose potting soil. Finish off the pot with pebbles, and water sparingly. “The humidity in a typical house is far less than what a plant is used to outside,” Brizer says. “So you may want to consider misting them every week or so, just to keep them happy.”

Jenn Pineau, a floral designer at Nature Composed in Middleburg, Va., advises against adding fertilizer. “With less sunlight and shorter days, we are called to rest in winter. Your houseplants are no different. Moving plants into your warm home and transplanting to more fertile soil will encourage an active growth phase,” Pineau says. “Without the necessary sunlight, this can result in leggy, soft stems that are attractive to pests. Try putting them in a space that’s protected from freezing temperatures, but not within the warmth of your home, with access to some sunlight. A garage with windows is a great spot.”

If your plant isn’t used to getting much rain or snow outside, water sparingly when it’s indoors. Pineau also recommends spending time with the plants you’ve left outside and, perhaps, planting bulbs, herbs and winter edibles. “We are accustomed to thinking the start of winter means the end of gardening, but that’s not the case.”

Bill Kent is a freelance writer living in Hardiness Zone 6B.

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