Container gardening with ornamental shrubs and trees is a joy and not very difficult to do, provided you follow some basic guidelines. When growing plants in pots, planters and other types of containers that will be above ground, protection from wind, harsh sunlight and drying out are concerns. However, in this article we'll be focusing primarily on protecting plants growing in containers from freeze damage during a harsh winter.
To protect plants consider the following helpful tips. First, Choose The Right PlantsWhen selecting plants to grow outdoors in a pot, probably the first and foremost thing to consider is the cold hardiness of a specific plant you'd like to grow. Every type of plant has a low temperature threshold. Thankfully, we have the USDA Plant Hardiness Zones to help us understand which plants will most likely survive the average low winter temperatures at a specific location. Find Your USDA Zone >
Here's the catch. the USDA Zones are based on plants growing outdoors in the ground, not in containers. While the roots of plants growing in the ground are insulated by the surrounding soil, the roots of plants growing outdoors in above-ground containers are more exposed to the colder air temperatures, leaving them more susceptible to cold damage. Essentially, the roots of a plant aren't as cold hardy as the above-ground plant parts, which can often tolerate temperatures 20 degrees or more cooler than the roots.
For this reason, and based on our own experience and feedback from many customers over the years, we've come up with a way to determine whether or not an outdoor potted plant is likely to survive the winter in a specific location, without protection. It's not fail-safe, but when it comes to living things such as plants and Mother Nature, it's the best we can do and is almost as reliable as using the Zones for plants that will be growing in the ground. Simply put, when looking for plants to grow outdoors in containers that will be cold hardy in your area, select plants that are listed as cold hardy to at least one USDA Zones cooler than yours. Two zones cooler is even better.
Keep in mind that the higher the USDA Zone number, the warmer the climate. For example, if you're in USDA Zone 8 and would like to grow a Camellia plant in a container, you want to choose from Camellia varieties that are listed as cold hardy to USDA Zone 7, which has a colder climate than yours. Zone 6 would be even better. Doing so gives your plant a better chance of surviving the winter without freeze damage.
Even when we do everything we can to ensure protection from cold, sometimes Mother Nature will send us a super-harsh winter when temperatures drop below the threshold in your zone. In the event this happens, there are simple measures you can take to help prevent freeze damage.
Wrap The Pot. In the event unusually low temperatures are forecast, simply wrap the container in a heavy blanket, bubble wrap, or some other material that will provide insulation during the severe cold snap. These protective coverings will help to trap heat and keep it at the root zone. When temperatures rise to the safe level the blankets can be removed.
Wrap The Plant. The roots of plants growing in containers are not only more exposed to cold air temperatures, but warm temperatures as well. An unseasonably warm spell in winter can cause the flush of new, tender roots to develop much quicker than if they were growing in the ground. When this happens, sometimes we'll see leaf or flower buds begin to swell or even emerge on a plant. One way or another, when a warm spell during winter is forecast to be followed by a cold snap, make sure to protect not only the pot and the roots but also the top of the plant. Cover plants with plastic film, burlap, blankets or other cloth material at night. If you use plastic, to avoid scalding plants or forcing more bud growth, be sure to remove it during the day.
Inside Insulation. Before planting, you can line the interior walls of the pot with foam or foam peanuts. Since most of the roots of a plant growing in a container will be on the outside of the root ball and near the walls of the pot, the foam insulation will help to insulate the roots.
Pot In Pot. Insert your pot into a larger pot for added protection. This will work best if the larger pot has thick walls or added insulation. The space between the two pots can be filled with foam peanuts or some other type of insulation material.
Place Pots On Soil. The roots of plants in pots placed on the ground will have less exposure to the freeze/thaw cycle. 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.
Choose The Right Pot. When container gardening in cooler climates that experience freezing temperatures, it's a good idea to use non-porous pots, such as metal, plastic or synthetic, which absorb water. On the other hand, untreated porous containers, such as terra-cotta and ceramic, tend to crack and break with freezing and thawing. If you are in a climate where temperatures drop below freezing it's a good idea to paint the interior of porous 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. Even though they absorb water, wooden pots are usually fine because they won't crack. Just make sure to choose cedar, redwood or other woods known to be more resistant to rot and therefore longer lasting.
The Bigger The Pot The Better. The extra soil in a larger container, such as a 15 gallon pot, will insulate the roots better than those in a 1 gallon pot. Simply put, the roots in the smaller pot will freeze faster. Also try to select a thicker container. A pot side walls of 1-inch thickness or more will help to insulate roots.
Plant Early. Plant earlier in the growing season to allow plants to harden off before winter arrives. Plants that go into winter dormancy with more mature roots are much less likely to suffer cold damage in winter. As mentioned earlier, select container plants that are hardy to at least one zone cooler than your hardiness zone. Two zones cooler is even better!
Elevate To Alleviate. Potted ornamental plants usually go dormant during winter so won't drink as much water. Therefore, there won't be the need for saucers under the bottom of the pot. Instead, set your pots on "pot feet' or bricks to elevate them above the ground or surface. Saucers hold water that will freeze when temperatures drop low and the ice will impede the flow of water through the drainage holes. The result is a constantly soggy soil, which often leads to the development of root rot, a disease that is often fatal to plants. No matter what time of year, keep in mind that you want to keep the flow of water going to alleviate stress on your potted plants.
Location, Location. If a plant growing in a container is known to be marginally cold hardy in your area it's best to place it in an area that will provide protection from cold winter winds. Containers placed on the south or east side of a home or other structure will decrease the plants exposure to west and north winds.
Mulch Is Good. Adding a layer of straw, wood mulch or rotted leaves to the soil surface in containers will help to provide extra insulation from cold.
Water Is Important. Though dormant ornamental plants don't drink much water, you want to keep an occasional eye on soil moisture, especially when there's been drought. Check soil moisture at least once a week during dry periods while you container plants are in winter dormancy. When the soil is dry and freezing temperatures occur the roots of plants are much more susceptible to freeze-drying. So, if freezing temperatures are forecast, check all your container plants to make sure the soil is moist. Moisture in soil will form an ice sheath around roots that actually serves as insulation, insulating the roots to 32 degrees. Snow does the same thing. After a snow we often see temperatures drop much lower that prior to or during the snow. Instead of removing snow from container plants and the soil surface leave it there for insulation!
Overwintering Indoors. If severe, much colder than normal temperatures are forecast, and you're afraid that your beloved and prized container plant may be damaged by freeze, there's the option of moving it to a warmer or less-cold interior space, then moving it back outdoors when temperatures rise above the danger zone. An unheated space such as a garage or an insulated shed is perfect.
Group Therapy. If you have many potted plants outdoors, group them together. Like humans and animals, plants can help to keep each other warmer when grouped close together. The most cold hardy plants can be placed around the outside of the grouping to help protect the less cold hardy plants from the cold and harsh winds that cause the freezing.
Bury The Pot. Dig a hole in the ground and place the potted plant in the hole. This will help to keep the roots of the plant at the warmer ground temperature.
Avoid Late Pruning. Cease pruning plants 60 days prior to the average first-frost date in your area. Late pruning can promote new growth that can be damaged by a frost.