Solutions for Wintering Container Plants

Filed Under: Gardening
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With Winter here and more cold weather on its way, its time to visit your garden and make sure your ready! There are also lots of annuals that look great and will last well past the first frost. Some will even look fabulous and sculptural with a coating of snow. Try combining different heights and forms, or go simple and pot a single dramatic plant in a beautiful container.

Capital Garden Downing Street PlanterTry some of these cold-loving annuals:

  • Flowering cabbages
  • Flowering kales
  • Annual grasses
  • Sages
  • Flaxes
  • Pansies
  • Creeping wirevine
  • New Zealand Flax

In general, plants can survive in containers if they are rated as 2 USDA Hardiness zones lower than the zone you are gardening in. So if you are in Zone 5, you’d be safe with plants labeled hardy to Zone 3, but how many of them are there? However gardeners in Zones 7 and 8 can get by with minimal loss on chilly nights if they stick to the much wider variety of plants considered Zone 5 and above.

Another thing to consider is that once the ground freezes under the container, water cannot escape the bottom of the pot. The container will thaw before the ground does and if you get a few rainy days, the water will stand in the pot, either rotting the roots or turning into an ice cube when the weather chills again. Avoid this by tilting the pots slightly.

Some tips to give your container plants a bit of an advantage in making it through the winter include:
  • The more soil in the pot, the better insulated the roots will be. If possible, you could simply slip the existing container into a larger container and fill the side with soil or mulch.
  • Cluster several containers together and move them to a sheltered spot, such as near the house or a south facing wall.
  • Encircle the containers with chicken wire and fill with leaves or mulch. Once the ground freezes, add mulch to the tops of the plants.
  • Consider using a cold frame or create a make shift cold frame by surrounding the containers with pales of hay and covering them with an old window or glass door or a sheet of plexiglass. Keep an eye on your plants if the weather warms. It can heat up quickly under glass. Lift the cover if temperatures are going above about 40 degrees F. and remember to close it at night.
  • For marginally hardy small trees and shrubs, you can protect them from frigid winds by driving 3-4 stakes around their perimeter, about 8-12 inches from the branches, and then wrapping them with burlap. Don’t let the burlap touch the leaves or needles or they could suffer more frost damage than if left unprotected. You could use a cage of chicken wire instead of the stakes.

Even with all your best efforts, you may lose a plant or two to circumstances beyond your control. But you’d be surprised how many will make it. 

Planters to store in an indoor sheltered place for the winter months:

  • Terra cotta: there is a 100% guarantee that moisture from any source will increase the likelihood that terra cotta will crack. If not, my experience has proven that even though a pot may make it through the winter, the terra cotta will start to flake, especially around the rim. Ultimately, the pots do crack.
  • Glass: if you have container gardening pots made from glass, bring them inside, too. Though glass will not break from water penetrating the container itself, it will most likely break if it contains very soggy soil. The soil will expand as it freezes solid. The same is true of even the best ceramic containers, but it moves toward “less likely” as the degree of vitrification increases.
  • Glazed pots (exterior only glazed): same as terra cotta.
  • Glazed pots (interior and exterior glazed): these are not as likely to break in the winter. But, you’re gambling if you’ve got them planted and left outside.

And finally, make sure your container is strong enough to make it through winter. The more porous your container is, the more likely it will crack during winter. Planters that are great year round include styrofoam, plastic, polyurethane, fiberglass, wood, hypertufa and concrete.