Gardening Basics

The Cold, Hard Facts on Protecting Potted Plants

Tips for helping your potted plants survive winter.

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Whether a containerized plant survives a cold winter depends on a number of factors in addition to temperature — among them, the size and type of pot, its microclimate and the type of plant. A given plant's roots are almost always less cold-hardy than its hardiness rating indicates. — photo courtesy of Denver Botanic Gardens

Winter container gardening is tough — you have to protect plants from wind, harsh sunlight and drying out. The biggest challenge, though, is guarding against root damage caused by rapidly fluctuating temperatures.

Even plants that are hardy to your zone can be hit hard when planted in a container in the winter. Although the top part of a plant has the ability to go dormant, the roots don't.

"Essentially any type of container exposes the roots to ambient temperatures," says Dr. Hannah Mathers, assistant professor in nursery and landscape extension at Ohio State University in Columbus. Mature roots can gradually get used to the cold, but young, immature roots can't. In containers, young roots grow on the outer part of the rootball. When exposed to the cold, young roots are unable to acclimate and die back.

And, young or old, the roots are usually not has hardy as the plant's top. American holly (Ilex opaca) is hardy to USDA Zone 5. The top part (stems and foliage) of the plant will survive to a temperature of about -20 F, but immature roots die at 23 degrees above zero, and mature roots at nine degrees. In the ground and insulated by the earth, that's usually no problem for the roots of hollies in Zone 5 where the average minimum temperature is -10 to -20 F. But in a container, root damage in American holly would begin to occur at 23 degrees if left unprotected — a drastic difference from -20 degrees.

When it comes to containers for the winter, the bigger the better. The surrounding soil helps provide insulation for plant roots. --photo courtesy of Denver Botanic Gardens

To protect delicate root systems, consider these tips from the experts:

  • Avoid exposing plants to the freeze/thaw cycle. Rapidly fluctuating temperatures (from cold to hot and back) can cause significant injury to roots and, as a result, may even heave the plant out of the pot. To avoid this, place pots on soil instead of pavement. For plants in the ground, the main source of heat for roots is the heat of the earth. For containers on pavement, the sun can warm the pavement considerably, elevating the temperature of the rootball. This is followed by a drop in temperatures at night. The fluctuation exposes containers on pavement to freezing and thawing.

  • When choosing a pot, bigger is better. "The larger the volume of the container, the better off the plant will be," Hannah says. The soil in a 15-gallon container will insulate the roots better than that in a one-gallon pot. The smaller container will freeze faster. A good rule of thumb: select a larger-sized pot that also has a thickness of one inch or more. Also, smaller pots dry out more quickly than large pots.

  • Plant the container as early as possible in order to allow plants to harden off. If plants are healthy and go into winter with mature roots, they have a much better chance to harden off and, as a result, will tolerate winter stress much better.

    One trick: Select container plants that are hardy to two zones, cooler than your hardiness zone.

  • Large-sized containers, like this whiskey barrel, can help to protect plant roots in cold temperatures. Make sure to select the right type of container suitable for overwintering container plants in your climate.

    Choosing the right pot

    Your climate determines which types of pots will do best in winter. Untreated porous containers, such as terra cotta and ceramic, tend to crack and break with freezing and thawing. In his USDA Zone 4 garden, Ray Mims, director of horticulture at the Denver Botanic Gardens, likes the look of his large terra-cotta pots and wants to use them all year long. So he treats the interior of his pots with pool paint. The coat of pool paint helps to prevent moisture from entering the porous surface and causing cracks from freezing and thawing.

    Wood containers can be very durable, depending on wood type and exposure to the elements. Non-porous containers, like concrete, plastic and metal containers, are great for their ability to withstand the elements. However, concrete and metal containers can be heavy and awkward to move once set in place. Plastic pots have the potential to crack over a period of time. Foam or resin pots come in a variety of looks and can be a lightweight alternative to concrete and metal. When using lightweight plastic, foam or resin pots, top-heavy plants can topple over when hit with strong wind, so be careful to avoid causing winter injury to plants.

    Be wary of using thin-walled pots and hanging baskets. The insulation in these containers may not be enough to protect the root zone. Plus, hanging baskets tend to dry out faster than their ground-level counterparts.

    If possible, avoid using saucers; collected water can freeze. Raise your pots on feet to provide good drainage.

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