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How to save your plants for next season

There’s nothing tricky about growing plants in containers as long as you offer them good soil, food and water. But because a plant growing in the ground is more protected from severe cold (and alternate freezing and thawing) than one in a container, the real challenge is overwintering those potted specimens so they can adorn your landscape next spring.

As with any game plan, you need to know your players. Some plants are easier to overwinter than others, while some are more trouble than they’re worth. How cold your winters are and to what lengths you’re willing to go to protect your containers are also factors. Here are methods I recommend for saving your plants for next season.

Hardy and borderline-hardy herbaceous perennials

These plants usually die back and are dormant in winter, while their roots sleep until it’s time for new growth next spring. Examples include hostas, shasta daisies, heuchera, astilbe, lady’s mantle and daylilies. The goal is to maintain dormancy and provide a winter environment that’s within their hardiness zone.

After a couple of light frosts, water plants well and choose one of the following storage options:

Leave as is. If the pot is large and the plant is at least one hardiness zone below your area (i.e., if you live in Zone 5, herbaceous perennials in containers need to be hardy to Zone 4 or lower), the likelihood of successfully overwintering the pot outdoors is high. A large container holds more soil, which helps insulate roots and keeps soil temperatures consistent. However, when sun hits the sides of a container, especially a dark-coloured one, alternate freezing and thawing may trick the plant into thinking it’s spring and trigger early growth, when it’s merely a warm day in February.

Store borderline-hardy plants or those in small containers in an unheated garage or shed. Because the plants are dormant, light isn’t required, but check periodically to make sure the soil isn’t bone-dry. When growth resumes in late winter/early spring, reintroduce plants to normal growing conditions outdoors by gradually exposing containers to the elements for increasing periods of time.

Find an area, such as the vegetable garden, where you can sink the pot in the ground up to its rim so roots will be better insulated. Cover the entire plant with about five centimeters of winter mulch, such as shredded bark or leaves. In spring, remove mulch and lift out your container.

Hardy deciduous shrubs, small trees and vines

The stems, branches and trunks of these plants persist over winter, but the leaves drop off in the fall. Examples include miniature roses, rose standards, Japanese maples, clematis and dwarf lilacs. The goal is to keep plants dormant and within the range of winter temperatures they would tolerate if grown in the garden.

Use large containers and plants that are at least two zones hardier than recommended for your area. These woody plants have above-ground branches that hold next year’s flower and leaf buds, making them more vulnerable to winter winds than herbaceous perennials.

For the best protection, store them in an unheated garage, against the warmest wall. If practical, place them in a garbage bag loosely filled with dry leaves for even more insulation (leave the top open for air circulation). Keep in a dark part of the garage; light may trigger growth too early. Once a month, check soil to make sure it’s not bone-dry, but do not overwater, as this could cause plants to rot or break dormancy.

Hardy broadleafed and needle evergreens

These types of plants transpire (lose water) during winter, and when temperatures remain below freezing for long periods, root balls freeze solid and water is unavailable to the plant, causing leaf damage or possibly death. Examples include English holly, boxwoods, English ivy, cedars, junipers and yews. The goal is to keep them within their hardiness zone and prevent desiccation from winter winds.

Move pots to an area where they will be protected from strong winter sun and winds, and erect a burlap screen around them (don’t allow burlap to rest on foliage). Keep plants well watered until freeze-up and check frequently throughout the winter to make sure soil is moist. Thorough watering prior to freezing temperatures and again in March and April, when the root balls are most prone to thawing—and drying out—is crucial.

Tip

Don’t forget to assess the winter hardiness of your pots. Soil-filled containers exposed to long-term freezing may expand and crack. Those made of earthenware, ceramic and terracotta (unless designated as frost-resistant) are especially vulnerable; concrete, wood, plastic and metal are more durable, but the last two materials offer little insulation for plant roots.