How to prepare your container garden for winter

When it comes to tending to contained vegetation, everything has to be reconsidered, and ensuring potted plants survive the transitions between seasons can be particularly challenging.Illustration by JOANNA Turner

As housing options and sizes have shifted in Canada over the last decade, earning your green thumb has increasingly come to mean cultivating outdoor container crops. Amber Stafford, the founder and owner of Toronto-based landscaping company Garden Party, has seen a shift in the type of support clients need. “We regularly work for people in condos with balconies or larger terraces,” Stafford says. “A lot of them have very small lots that are mostly hardscape.”

When it comes to tending to contained vegetation, everything – from watering to what has the best chance of thriving – has to be reconsidered, and ensuring potted plants survive the transitions between seasons can be particularly challenging. Given the diversity of Canada’s climate and how weather patterns are shifting, container gardening needs to be more adaptive than ever before. The swell of interest in developing outdoor spaces focused on native species and pollinator plants requires planter gardeners to pay even closer attention to achieving a delicate balance.

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Part of the beauty (and the burden) of agrarianism is feeling your way through all these unknown and shifting variables. Several books provide a roadmap to help get a planter garden going, including Isabelle Palmer’s Modern Container Gardening: How to Create a Stylish Small-Space Garden Anywhere and Kevin Espiritu’s Field Guide to Urban Gardening: How to Grow Plants, No Matter Where You Live. Advice for the end of the growing season, however, can be scarce. As thoughts turn to the fate of the bounty of blooms and veggies that fill our terracotta pots and raised beds now, we’ve compiled advice on how to prepare your planters for winter so they come back stronger than ever next spring.

What plants can I bring inside, and how do I do that safely?

If you’re gazing at your hard-fought growth and wincing at the thought of getting rid of it all as temperatures cool, consider this: some plants can come inside for temperate TLC.

Plants that aren’t winter-hardy, such as tender perennials, can be moved inside for the winter, says Rebecca Last-Guenette, coordinator at Master Gardeners of Ottawa-Carleton. She names garden classics like pelargoniums, aka geraniums; begonias and coleus; tender herbs such as rosemary and lemon grass; tropical accent plants such as hibiscus, oleanders, palms, pothos, crotons, bromeliads, cordylines, caladiums, and alocasias as candidates for cold-weather co-habitation.

If you’re going to attempt this, proceed with a plan. “Always practice good sanitation when bringing plants in from a summer outside,” Last-Guenette says. “Begin by inspecting them carefully for insects. Wipe down leaves to remove dust, debris and insect eggs, including spider egg cases. Be sure to check the undersides as well as the tops of leaves. If possible, spray the whole plant with a strong jet of water.” This can involve putting plants in the shower for a good cleaning, Stafford says.

If you have the room indoors to keep plants separated during this transition time, Last-Guenette notes that a brief quarantine can also help you to avoid introducing pests to your environment. “If you don’t have the space,” she says, you can “enclose the incoming plant in a large plastic bag and watch for signs of any bug infestations.”

For those with other types of garden goodies, Last-Guenette says that tender bulbs such as dahlias and calla lillies “can be cut back after the first frost, then lifted from their container to be stored as tubers in a cool, dark spot for the winter.”

How do I give a plant left outside the best chance of surviving?

“You don’t,” Last-Guenette says. “In most parts of Canada, plants simply will not survive outside in containers unless the containers are huge or insulated. Even then, there’s a good chance that cold will infiltrate through the walls of the pot or container, killing the roots and, therefore, the whole plant.”

When it comes to containers, Stafford says that materials like terracotta can potentially crack and break over the winter, so if you’re planning on keeping planters outside, look for ones made from wood, metal or fiberglass and that have drainage holes. “If there’s water sitting and freezing in the pot, that can cause it to expand and crack,” she says.

If I decide to bring some plants inside, how do I keep them comfortable?

“As we move past the fall equinox on Sept. 22, days become shorter and the light levels are weaker,” Last-Guenette says. “Most plants will naturally begin to slow their growth and some may become a bit leggy with longer stems and sparser foliage.”

You can cut back this leggy foliage to compensate. “I don’t save the whole plant,” she says. “I take cuttings from plants like coleus, rosemary and pelargoniums and root them in water so they don’t take up as much room.”

Supplemental lighting, Last-Guenette adds, can be offered to tropical plants, “but many houseplants need this dormant period to remain healthy. [And] you can reduce watering as well since a plant’s growth naturally slows with lower light levels.”

“Another thing to consider is humidity,” Stafford says. “When we have the heat on inside, obviously the air is really dry. You want to think about reproducing the tropical climate that a plant comes from.” Simply having a small spritz bottle on hand can make a difference.

Once I’ve picked the last of my vegetables, is there anything I need to do to prepare the soil in my raised beds for winter and growing season next year?

After you’ve enjoyed the final crop of tomatoes, peppers, cucumber and other earthly delights, Last-Guenette says that you can add a “top dressing to your veggie boxes with compost”, or plant a winter cover crop such as arugula, peas , radish or mustard greens. “It is a great way to prepare the soil for next year and maybe even get a bit of extra productivity out of those boxes,” she says.

There are even some additions you can make which will improve your garden over time. “Peas and other legumes will help by adding nitrogen to the soil,” Last-Guenette says. “Brassicas such as the mustard greens and arugula help to deter soil-dwelling pests like herbaceous nematodes.”

But if you opt for a winter cover crop, she says, “select one that is winter-killed, or not hardy, so you don’t have to weed it out next year.”

How should I prepare for reviving my garden outside next spring?

It may feel like a far way off, but a new growing season will be upon us before we know it. To get ahead of the game, Last-Guenette says that you can “start some annuals indoors well before the last frost; this will give them a head start on the season.” Plus, you can start tender bulbs under lights as early as mid-February.

A few important tips Last-Guenette offers are to harden off (or gradually expose) plants to outdoor conditions as they switch back from inside to the outdoors. And don’t start fertilizing your plants until after the spring equinox on March 21. “I find that slow-release fertilizer is a boon for any container plants,” she adds.

And finally, “as much as possible, keep plants in the same container,” she urges. “Transplant shock will set many plants back and combined with the transition from indoors to outdoors, this can make them very unhappy. If you must transplant, try to do it mid-winter, or better yet, in the middle of the growing season when the plant is already accustomed to its environment.”

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