Outdoor container gardens usually are planted with annual plant species that are simply discarded in the late fall as frost approaches, then replanted anew each spring with fresh plants. But many perennial plants also can be grown in containers on decks and patios. Roses, hibiscus, and many other perennial plants are expensive enough that it makes sense to keep them alive through the winter, if possible, rather than discarding them. But over-wintering perennial plants growing in containers is not easy. Even species that are technically cold-hardy in your growing zone experience very harsh conditions when they sit outdoors in exposed pots over the winter. Many species that over-winter just fine when planted in the ground will die if they are planted in pots—unless, that is, you use some special techniques.
The Problem With Over-Wintering Perennials in Pots
Because container-grown plants are exposed to the air on all sides, winter temperatures affect them more radically than they do for plants that are growing in garden soil. Containers do not enjoy the insulating properties of the earth, so they become cold quicker than the ground does, and in winter they may become considerably colder than the earth. This means that plants growing in containers need to be exceptionally hardy in order to survive intact through the winter outdoors. If you live in a zone 5 garden, for example, a plant listed for zone 5 hardiness will very likely perish if over-wintered in a pot, even though similar specimens planted in the garden do just fine.
The accepted rule of thumb is that a perennial plant should be rated two climate zones colder than your climate in order to be dependably hardy in a container planting. A zone 5 gardener, for example, should fill his or her containers with perennials rated for zone 3 or colder in order to over-winter those plants outdoors without special measures. With special measures, though, you may find it possible to successfully winter some perennials that match your climate's hardiness zone. Methods to protect borderline perennials growing in pots include:
- Digging and storing the tubers or bulbs until the following spring
- Burying the entire pot in the ground for the winter
- Sheltering the plant in place outdoors
- Moving the entire pot indoors or to another sheltered location
When to Over-Winter Plants in Containers
Whatever special measures you take to over-winter perennials growing in pots should be put into action a week or so before the expected first frost of the late fall or winter. While some plants will survive light frosts, others (especially those suited to warm climates) will die for good as soon as their cells freeze.
That's not to say that a perennial that freezes will necessarily die down to the roots. Depending on their level of hardiness, some potted plants will respond to the first frost by going into their normal dormancy, just as do garden plants. But while they may survive nicely down to perhaps 15 degrees Fahrenheit, when winter temperatures drop below this, their roots will then die completely. So the best time to prepare for winter is before the deep freeze begins.
What You'll Need
The materials and tools required for over-wintering potted perennials vary depending on the methods you use, but you will need one or more of the following:
- Shovel or trowel
- Insulating materials (such as straw or leaves)
- Chicken wire (where necessary)
- Stakes (where necessary)
Burying Potted Plants
If your perennials are hardy in your climate zone, you can buy the entire container in the ground to keep them over the winter. They will experience the same protection from the earth as any other perennial plant planted in garden soil. This is perhaps the best of all solutions, provided you have the garden space and the energy to dig down all your potted plants. But remember that this method works only for perennials that are hardy in your climate zone. You can't expect a tender tropical plant rated for zone 9 to survive when buried for the winter in a zone 4 garden.
- Choose a relatively sheltered location, then dig a hole in the earth that is slightly deeper than the container.
- Spread a layer of gravel in the bottom of the hole. This will facilitate drainage in the spring as the soil in the pot thaws. The loose soil of potted plants typically thaws a little faster than surrounding garden soil, which means that drainage can be an issue.
- Set the pot into the hole, then backfill around the pot. Ideally, the rim of the pot should be slightly lower than the surrounding ground. Spread a thin layer of garden soil over the top of the pot.
- Winterize the buried plant in the same fashion as you would for any similar plant growing in the ground. For example, if the standard recommendation is to mulch over a particular garden perennial for winter, also do this with the buried potted plant. This can involve spreading leaves, straw, or compost over the entire garden, including the spot where you've buried the pot.
- When spring comes, your potted plant should be lifted from the ground as soon as the ground thaws and new growth is beginning to appear. You don't want a buried plant to soak in water from spring rains any longer than is necessary. Move the plant to its location on the deck or patio.
Digging and Storing Bulbs, Corms, or Tubers
If your container gardens contain true bulbs, such as tulips or daffodils, corms such as gladiolas, or tubers such as dahlias, one strategy you can use is to dig up the root structures and store them for the winter in a cool, dry location, then replant them back into outdoor containers the following spring. This is not always successful, as some stored bulbs can develop rot or become desiccated over the winter months. Many gardeners count themselves as lucky if 80 percent of their bulbs survive for replanting.
And you also need to know your species, which will require a bit of research. Some bulbs require a winter chilling period to "reset" themselves. This type of bulb (tulips, for example), will require that you move them to a chilled location at some point during the winter if you want them to rebloom the next spring. Tulip bulbs dug up and stored at room temperature usually do not rebloom successfully, but if you move them from indoors to a cold garage for a few weeks in late winter, they perform just fine. Refrigerating these bulbs at the right time can also work.
Sheltering Plants in Place
What really kills plants in winter is not so much the low temperatures, but rather the rapid cycle from high temperatures to very low temperatures. If you can provide shelter that prevents these rapid temperature swings, your potted perennial plants stand a much better chance of survival. There are several ways to do this:
- Cluster your pots together in a sheltered corner of your yard, such as against a house or garage wall. These pots will be sheltered from winds that can rapidly drop temperatures, and will also enjoy some amount of heat radiation from the house.
- You may want to provide additional shelter for a group of clustered pots in the form of a tarp or by heaping straw or another insulating material over the group. This, too, will moderate the temperature swings and increase the likelihood of your perennials surviving the winter. This may not be the most attractive look in your landscape, but it's a good solution where keeping your plants alive is important. You may be able to find an out-of-the-way location where a group of pots draped with a tarp is relatively unobtrusive.
- Many gardeners in cold climates find it possible to over-winter potted roses and other flowering shrubs by building insulating silos around the pots with chicken wire and stakes, then filling these silos with loose leaves or straw.
Many people imagine that sensitive plants are best placed on the south side of a home where they will get a lot of winter sunshine, but this is exactly the wrong strategy. What actually kills most plants in winter is the rapid swings from thaw to deep frost and back to thaw. When they placed in a sunny location, borderline perennials often die because of this thaw-frost-thaw-frost cycle. For this reason, borderline plants are much better positioned in a shady, sheltered location for the winter, where the temperature swings will be slow and less dramatic. A borderline perennial is much better placed in a position where it will remain moderately frozen all winter until spring offers a slow thaw, rather than in a sunny place where the plant thaws slightly each day only to plummet back into the deep freeze at night.
Moving Potted Plants Indoors
A technique that is sometimes successful is to move potted perennials indoors for the winter. In fact, some plants grown as annuals in northern climates are actually perennials in warmer zones, and some of these can often simply be moved indoors for the winter. Coleus, geraniums (pelargoniums), and begonias are some examples of tropical perennials that can usually be moved indoors, pot and all, for winter. Small potted fruit trees that spend the summer on a deck or patio can also be moved indoors for the winter.
The trick to success here is to make sure you have an indoor spot with plenty of light. This can be hard to come by in the wintertime when the sun is low in the sky and the days are short. Further complicating the issue is the fact that indoor conditions are often very dry in the wintertime, and many of these plants are tropicals that need humid conditions in order to thrive. But if you can find the right indoor location, or are able to artificially provide the necessary light and humidity, moving potted perennial indoors is a good strategy.
Remember, though, that this technique works best with tropical perennials that will continue growing through the winter. Perennials that require winter dieback and dormancy period shouldn't be brought indoors for the winter. For those, providing outdoor shelter is the best strategy.
Tips for Over-Wintering Perennials in Pots
- When the ground freezes under a container, water will not be able to escape through the bottom of the pot. In spring, 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 to ice when the weather chills again. Avoid this by tilting the pots slightly.
- 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. Materials such as untreated terra cotta can absorb water, which will expand when frozen, cracking the pot. Plastic containers are usually resilient enough to tolerate the expansion and contraction.
- The more soil in the pot, the better insulated the roots will be. If possible, you can slip the existing pot into a larger container and fill the sides with soil or mulch.
- Consider using a cold frame or create a makeshift cold frame by surrounding the containers with bales 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 outdoor temperatures are rising above about 40 degrees Fahrenheit. and remember to close the cold frame at night.
- For marginally hardy small trees and shrubs, you can protect them from the drying effect of frigid winds by driving 3 or 4 stakes around their perimeter, about 8 to 12 inches from the branches, and then wrap 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 also use a cage of chicken wire instead of the stakes.
- Another way to preserve plants over the winter is to take stem cuttings and root the cuttings indoors. After a few weeks (or sometimes a couple of months) the rooted cuttings can be transferred to small pots filled with potting soil. By spring, these will be ready to transfer back to their larger outdoor pots. These "over-wintered" plants may not be the same specimens, but they will be visually identical.