How to Protect Potted Plants in Winter

Why is it that a plant that can survive the winter in your growing zone, oftentimes won’t overwinter well when planted in a container? Whether that be strawberries, asparagus, garlic, rosemary, or another something else, spring comes, and you find the outdoor potted plants have died.

Raised beds and in-ground gardens have a distinct advantage where containers tend to struggle. In this blog post, I’ll explain why it’s harder to overwinter perennials in containers and how to protect potted plants in winter.

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Dormant, Not Done

Just because your garden mainstays are going dormant for the winter, it doesn’t mean they’re done for! Give all your perennials the care they deserve, from the smallest perennial herbs to the tallest fruit trees, and they’ll reward you with another season of beautiful blooms and productive harvests.

Don’t start your winter hibernation just yet! Fall is a great time for preparing the garden for winter, learning how to amend soil around existing plants, fertilizing fruit trees and other perennials, and pruning blackberry and raspberry bushes. 

The Problem With Overwintering Container Plants

Insulation is everything when it comes to keeping your plants alive through winter! It makes all the difference between what survives in the ground, but not in a container garden.

The earth acts as a thermal mass source that radiates warmth up through the soil. Even when there is snow on the ground and the temperature drops below freezing, plants in the ground and in most raised beds are surrounded by lots of soil that acts as thermal mass to keep the temperatures warmer than containers that are exposed to air all around.

Containers allow the cold to penetrate the soil far more aggressively, freezing root fibers, and even killing the plant. The short and dirty of it is you have to find a way to keep container plants warmer longer, like the ground does. 

Best Types of Containers for Northern Gardens

Wood, some types of plastic, and fabric grow bags are generally the better choices for northern gardens over cinder blocks, unglazed terra cotta, and ceramic containers. The latter are porous, meaning they can absorb moisture, and in cold weather, that moisture can freeze and expand, which can lead to cracked containers.

One important note: just because a container can survive the winter, that doesn’t necessarily mean it can insulate and protect the plant roots inside it. Plants in smaller containers will be more affected by the cold, so make sure to protect smaller 3 or 5-gallon potted plants and Greenstalk planters from subzero temperatures!

How to Protect Potted Plants in Winter

Move Containers Into a Microclimate

Move containers into a warmer microclimate during cold weather. A microclimate is just a smaller area that has a different temperature or different growing conditions than the overall wider area. 

A microclimate might be right next to a brick wall, especially a south-facing one, that radiates heat throughout the night. Right next to that brick wall, there could be a little pocket of warmth with a temperature difference of 10° to 15°F. That warm pocket doesn’t extend out even 10 feet away, but you can tuck your containers right up next to it.

You could also have the reverse, if you place your containers on the north side of a building or something that blocks the sunlight. Those containers might get colder than if you had them out in the sun for part of the day.

Another microclimate I use is under the awning of our building. The sun comes up in the morning, warms the building up, and helps keep these plants 10° to 15° warmer, even though they are exposed.

An unheated garage or shed will keep the wind and cold air off, which can keep those plants 5° to 10° warmer, and an attached unheated garage will get some warmth transferred from the house, keeping the plants even warmer.

Using the same principle of microclimates, some people will recommend grouping all your potted plants together to help them survive the winter. This could be enough protection depending on the climate you live in. Placing pots together can shield them from the wind and slow heat loss somewhat, but this will not significantly insulate the pots if you have below-freezing temperatures for very long.

Watering Potted Plants in Winter

Here’s something that may surprise you. One of the top reasons that potted plants die throughout winter is that people don’t water them!

You see, plants that are in the ground or in raised beds are not just insulated against the cold, but they are also much more protected from drying out. And that’s not just because they get precipitation from the snowfall and rain.

Moisture from the surrounding soil and even from below will wick up and keep their roots moist, even during dry spells. The plants may be dormant, but they’re still alive. If you cover potted plants or take them indoors where they’re not exposed to any water at all, the soil will dry out, the roots will dehydrate, and the plants will die.

Their water consumption does go down in the winter, but they still need regular water to keep the roots hydrated. I like to water the perennial herbs I have in containers under the awning about once every 2 or 3 weeks.

Bring Plants Indoors

Sometimes, you just need to bring your potted plants indoors for the winter! Some people have indoor/outdoor cats; I have indoor/outdoor plants.

When I am considering when to bring plants indoors, I consider each type of potted plant on an individual basis. Cold-hardy perennials, like thyme, oregano, or strawberries will go dormant in the cold weather. I can leave them in pots outside all winter long if I make sure to follow steps 1 and 2 (keep them in a microclimate where the root fibers won't freeze and keep them adequately watered).

Other plants like my Dusty Miller, tropical plants, citrus trees, banana trees, or any other plant that doesn’t naturally live in a growing zone that gets cold winters, will just die if they are exposed to too cold temperatures. These plants needs to be taken inside and placed in a sunny window or under a grow light before their cold tolerance runs out.

Then, move them back outside when the springtime weather gets warm enough. I hope that this will help you keep your container plants alive for a second year, a third year, a fourth year, and beyond so they can be a huge asset to your garden!