More and more gardeners are growing plants in containers these days: pots, flower boxes, smart pots, etc. It’s very handy, as it lets you grow plants in spots where there is no possibility of digging a planting hole, such as on a deck, balcony or rooftop. Of course, most of the plants grown in containers are either annuals or plants treated as annuals—tomatoes, begonias, petunias, sweet peppers, etc.—and no one expects them to survive the winter at any rate. Either you can just let them freeze and replace them the following year, or you can bring those that are tender perennials indoors (or bring in cuttings) indoors and grow them as houseplants over the winter.
But what about hardy plants grown in pots? Shrubs, perennials, conifers, fruit trees, etc. Most of these are hardy enough to survive local winter conditions… that is, if they’re grown in the ground. But in a pot? That changes everything! You can’t bring them indoors, as hardy plants need a cold winter in order to thrive, yet it can be so cold outdoors in a pot during the winter that even hardy plants die.
Mostly a Problem in Colder Climates
In hardiness zone 8 and above, or any area where winter cold comes in short bursts quickly followed by above-freezing temperatures, there’s no great difference between temperatures in pots and in the ground and most plants that will survive the winter in the ground will do about equally as well in pots. It’s where winter cold is deeper and longer lasting that plants in containers have a harder time surviving and some sort of special winter care becomes necessary.
Winter Is Tough on Potted Plants
Sadly, growing in a pot is more stressful to plants than growing in the ground, and that’s especially true in winter. Here’s why:
First, the soil in pots freezes more deeply than soil in the ground nearby. It generally drops to the same temperature as the surrounding air, while in-ground plants profit from “bottom heat,” heat given off by the soil below (the soil is an excellent source of geothermal heat). This can make a huge difference! The soil temperature in the ground can be more than 20 °F (10 °C) warmer just a few inches deep compared to the air on a very cold day.
Also, plant roots are usually much less hardy than plant parts that are exposed to the air. Since roots are typically protected from the worst of cold weather simply by being underground (remember ground heat), they’ve never had to adapt to extreme cold. For example, above-ground parts of black spruce (Picea mariana) can tolerate -58 °F (-50 °C) in midwinter, yet their roots will die if exposed to -5˚ F (-20 °C) temperatures. In other words, the aerial part is hardy to zone 1 whereas the roots are only hardy to zone 6! That’s a huge difference!
That was, of course, an extreme case, but nevertheless if you garden in containers, you have to assume the roots of a hardy plant will probably be one or two zones less hardy than its leaves and stems.
Added to that is the fact that the roots of a potted plant have to put up with some pretty wild temperature swings while in-ground roots usually undergo only very slow and very moderate changes. There are several reasons for this, not the least of which is ground heat, already mentioned. Also, though, it takes longer to heat or cool a large mass of soil (in the ground) than a small one (in a pot), plus soil is quite naturally a good insulator. As a result, soil temperatures in the root zone of plants growing in the ground tend to drop slowly in the fall, remain cold but very stable in winter, then to warm up, very gradually, in the spring. Compare that to potted plants, with their minimal soil mass and little to no bottom heat. They undergo regular and rapid temperature changes, usually including repeated cycles of freezing and thawing, and those are never good for plants.
There are nevertheless several methods you can use to successfully overwinter hardy container plants. Here are a few:
- Choose a large pot. The larger the pot, the greater soil mass and the less the plant will undergo temperature swings.
- Insulate the pot. At planting time, probably the previous spring, plan for the coming winter and line the inside of the pot with moisture-resistant insulation, such as waterproof foam panels. Indeed, some pots have built-in insulation. This insulation works not so much to keep the soil in the pot warmer (if the air is cold and remains so for long periods, soil temperatures will still drop) as to prevent those nasty temperature swings.
- Choose Extra Hardy Plants. Prefer as container plants ones that are at least one and preferably two zones hardier than the basic hardiness zone of your region. In other words, if you live in zone 5, for example, only grow plants adapted to zones 1, 2 and 3 in pots, or, at the limit, zone 4. If you apply that simple rule, you eliminate most of the problems of overwintering plants in pots. Notably, now that cold is no longer a concern, you can leave pots out in the open and use them as decorative elements for the winter months. Alpine plants (naturally acclimatized to deep freezing in their original environment) and conifers are particularly resistant to winter cold when grown in a pot … when you choose ones at least one zone hardier than your local climate.
- Place plants in a garage, shed or some other shelter for the winter. If the shelter is somewhat heated, all the better, but even when it isn’t, temperature swings will be considerably reduced compared to what pots experience in the open air. Do remember that such plants will likely need watering occasionally, even in winter.
- Take the plant out of its container and plant it in the ground for winter. Add a good layer of mulch (always useful to protect freshly disturbed roots from temperature swings) and your plant will be much better able to tolerate cold temperatures. In the spring, you just have to repot it.
- Plant the plant in the soil, pot and all. The result will be the same as point 5 and you won’t have to repot, although you’ll likely have to give the pot a thorough cleaning come spring.
- Remove the saucer. If you normally place your pots on a saucer during the summer, remove it for the winter, otherwise the roots are likely to soak in cold water throughout the winter or, in the coldest climates, the water in the saucer will freeze solid for the winter, preventing any drainage. And roots that soak in water all winter are likely to rot.
- Raise the container off hard surfaces. Sometimes a thin layer of ice forms under the pots placed directly on a hard surface such as wood or concrete, preventing any drainage. The result is that the plant dies, not from the cold, but from rot because the soil remains constantly soggy. Just raising the pot only ¼ inch (6 mm) above the surface is enough. You can use pot feet, sold for that purpose, or raise the pot on slats of wood or a few flat stones.
- Set the pot on the north or east side of the house. This may seem counterintuitive, as these exposures are colder than south or west exposures, but winter sun causes more damage to container plants than does persistent cold, as it often leads to the soil thawing during the day, then freezing again at night. By sheltering the plants from the winter sun, the soil temperature will remain more stable. Also, snow tends to build up more and last longer in north or east exposures … and snow is an excellent insulator!
- Place the pot against a wall that radiates some heat, such as the wall of the house. This will be even more effective when combined with point 11.
- Insulate the pot against the elements to protect the soil inside against temperature swings and allow it to retain more heat. Several methods are possible. You can, for example, form a wire mesh “cage” and fill it with fall leaves or straw or surround the pot with bales of straw or plastic bags filled with fall leaves. You can also wrap the container in burlap, geotextile designed for winter protection, bubble wrap, an old blanket, etc. and thus gain some protection. Allow snow to build up around the pot or add more when it’s available for even better insulation.
- Insulate the foliage using one of the methods outlined in point 11. This will reduce damage to aerial parts due to temperature swings, but … remember that the plant still needs to breathe. Avoid covering the plant itself in airproof wrapping, like bubble wrap or plastic sheeting. Leaves, straw and snow are the best insulators, as they allow air circulation while keeping cold winds out.
- Group your pots together. This reduces access to cold wind and facilitates insulation (points 11 and 12) because you can cover the pots collectively. Place the hardiest plants on the outside so they surround the least cold-tolerant ones for even better protection.
The Right Pot
If you want to keep perennials or shrubs in pots all year, you’ll need a container that can withstand freezing and in particular repeated freezing and thawing. Ceramic and terracotta pots, for example, tend to crack quickly if left outdoors all winter, especially with a mass of soil inside that expands as it freezes, but containers made of wood, concrete, metal, fiberglass, resin or plastic are generally more weather resistant.
One suggestion: use valuable pots outdoors only as a cache-pots and bring them back indoors over the winter, growing your plants in simple “grow pots” of little value. Then you place the grow pots inside the cache-pots for the summer and on the ground for the winter. If a grow pot cracks in the winter, it won’t cost much to replace!
While plants may be dormant in the winter, they aren’t dead. You’ll need to keep watering your container plants during the winter if Mother Nature doesn’t do so. Their watering needs will be greatly reduced due to their dormancy and reduced evaporation due to the cold, but still they mustn’t dry out completely. Conifers and broadleaf evergreens, in particular, continue lose quite a bit of water to evapotranspiration during the winter and this lost water must be replaced. Water on days when the temperature is above freezing, preferably in the morning so that water can penetrate before freezing temperatures return at night.
Note that moist soil insulates roots better than does dry soil, because when the soil is damp, its air spaces fill with water and water is a much better insulator than air.
What I Do
Above I’ve listed a whole panoply of suggestions on how to protect your container plants from the cold, but personally, there are only two that I apply to outdoor containers in the very cold area where I live (zone 4). During my apartment years, when my only garden space was a balcony, I learned two things that I’ve applied ever since: to stick to extra-hardy plants (point 3) and to raise the pots above the balcony floor (point 8).
As for watering, that turns out not to be a problem in my area, since everything is covered with snow throughout the winter and I can therefore count on melting snow to take care of watering needs.
Thank you Mother Nature!