The onset of fall and the approach of winter doesn't have to mean the end of all the beautiful annual plants you've come to love in your outdoor garden. With a bit of effort, you can enjoy some of these plants on your windowsills all winter long—or at least keep them alive to bring back outdoors in spring. There are three methods of overwintering your favorite annual plants, and the method you choose depends on just what kind of "annual" species you are dealing with—a true annual or tender perennial that is being grown as an annual.
What Is an Annual?
The term "annual" technically refers to a plant that completes its lifecycle, then dies in a single growing season. Some true annual flower species include pansy, larkspur, most types of snapdragon, carnation (pinks), and zinnia. However, in common practice, the term "annual" is also used to refer to many "tender" perennials that are grown as annuals in cooler climates. In warmer climates, these tender perennials will continue to grow year-round, often achieving a size that rivals that of a woody shrub. In colder climates, though, they will die off, roots and all, when freezing temps arrive—hence, they are often considered to be annuals and are sold under that label.
When to Overwinter Annuals
Generally speaking, the annual plants you want to protect or propagate should be brought indoors as the growing season begins to wane and before cold weather threatens to kill them. The timing can vary a little depending on the species, as some plants will continue to thrive even up to the first light frosts, while others need to be brought inside as soon as nighttime temperatures begin to dip below 50 degrees Fahrenheit. A little research will tell you the lowest temperatures your plants can tolerate.
Taking stem clippings to root is usually most successful if you do it while the plants are still actively growing, but not in their peak flowering periods.
Before Getting Started
The plants that adapt best to moving indoors as intact specimens for continued growth are the tender perennials that are grown as annuals. These include popular garden plants such as geraniums, coleus, wax begonias, heliotrope, and impatiens. When grown as container plants, the entire pot can often be brought indoors to continue actively growing through the winter. It may also be possible to dig up and transplant these species into pots to move indoors, though this is not always successful. If you move a growing plant indoors, you will get to enjoy the plant's beauty throughout the entire winter.
Another way to overwinter tender perennials is by letting them go partially dormant and placing them in a sheltered location until the weather warms again in spring. Sometimes the plants are left in pots to overwinter this way in a dormant condition, but more often the roots are dug up and stored until spring. Geraniums (Pelargonium), dahlia, and tuberous begonia are tender perennials that are often overwintered as dormant roots.
But the species that are "true annuals" can't be overwintered using either of these methods, as they are genetically programmed to flower and die in a single growing season. If you try to bring such a potted plant indoors, it will simply turn brown and die as it completes its lifecycle. Fortunately, there is another method of overwintering a true annual (which also works for some tender perennials): taking stem clippings and rooting them to create new plants. This is essentially a method of cloned propagation that creates a brand-new plant that is genetically identical to the parent plant.
Failure Is Common
Overwintering annual plants can be a tricky proposition, as you may well find that some plants just aren't friendly to any of the methods. And there is the issue of providing the right growing conditions. Many plants require a lot of direct sunlight, and this can be difficult or impossible to provide during the short days of winter. Supplementing with artificial lighting is sometimes an option, but some plants don't really like grow lights and will overwinter only if you can find a spot where they get direct sunlight for virtually the entire shortened day.
Further, indoor winter conditions are often quite dry, and plants that prefer high humidity may not overwinter indoors unless you can find a way to keep humidity levels high. At the same time, you need to protect them from fungal diseases and pests—some of which are more problematic when growing indoors. Spider mites, for example, aren't a serious problem for outdoor impatiens but can be a plague for the same plants once they are moved indoors.
Bottom line: Be willing to experiment with different annual plants and different methods, and don't be too surprised if some of your experiments don't succeed.
The materials you need to overwinter annual plants will vary depending on the method you choose, but you may need some or all of the following:
Equipment / Tools
- Grow lights (as needed)
- Plant pots
- General-purpose potting mix
- Plant fertilizer (as needed)
- Rooting hormone (optional)
- Plastic bags
- Paper bags
How to Overwinter Annuals in Pots
Annual plants that are being grown in containers can often be moved indoors as they are. It's also possible to transplant garden specimens into pots for moving indoors, using this method:
Cut Back the Plant
Whether they are already in pots or are being transplanted from the garden into containers, it's best to cut back the plant's foliage by about one-third before moving it. This will reduce the plant's energy consumption and reduce the stress it suffers when it moves indoors.
Transplant into a Pot
For garden plants, carefully dig up the plant's root ball and move it to its own pot filled with commercial potting mix. It's best not to transfer too much ordinary garden soil, as it may contain soil pathogens and doesn't drain as efficiently as a commercial potting mix.
Acclimate the Plant
It can be a fairly stressful event when an outdoor annual is moved indoors, so help the plant acclimate by giving it increasingly long visits to the indoor environment over a period of a week or two. One way to do this is by moving it indoors at night, then back outdoors during the daylight hours until the plant has grown accustomed to the indoor temperature and air quality.
Find a Suitable Indoor Location
Each plant species has its own ideal requirements for indoor growing, but an outdoor specimen moved indoors often struggles in the lower light conditions. Thus, it's usually best to give it the brightest location you can find. Windows or patio doors that enjoy direct sunlight are good locations for many plants, provided they don't experience frigid drafts.
Where you can't provide enough natural light, supplementing with artificial grow lights is an option. Remember that most plants will require longer exposure if they are getting most of their light from artificial sources. A plant that needs six hours of sunlight, for example, may require as much as 12 to 14 hours if receiving artificial light only.
Some plants may also struggle in the normally dry conditions of indoor winter environments. Where necessary, provide added humidity through the use of a humidifier or other method.
Care for the Plant
Like other houseplants, potted outdoor annuals moved indoors are often susceptible to different pests and diseases. They need to be handled as you would any other houseplant. Most annuals grown indoors over winter will be in a semi-dormant stage that calls for reduced watering and little to no feeding through the winter. Inspect the plant regularly for common indoor pests, such as spider mites and scale insects. And pick off yellow or dried leaves as they appear.
How to Overwinter Annuals by Taking Cuttings
Another way to overwinter annuals—and the only viable method for true annuals—is to take cuttings from your existing plants. This can also be the best method for tender perennials that don't adapt well to transplanting, such as coleus and begonia.
Take a Cutting
Take 3- to 5-inch cuttings from vigorously growing plants in mid- to late summer. (some plants can be snipped as late as mid-fall). If possible, take the cuttings from non-flowering shoots. If you do have to take them from flowering shoots, cut or pinch off any blooms or buds from each cutting. This will allow the cutting to direct its energy into producing roots.
Root the Cutting
Remove any leaves from the lower half of each cutting, and insert the bottom third of the stem into a container of moist potting soil. You can dip the cut end in rooting hormone if you like, but most cuttings will root without it.
Cover the Cutting
Place a plastic bag over the pot, supporting it with skewers, twigs, or stakes to keep the plastic up off of the plant. Set the pot in location with plenty of bright indirect light, but preferably not in direct sunlight.
Wait for Roots
Monitor the covered pot. Keep the potting soil barely damp but not saturated. If condensation appears on the plastic, open it temporarily to air out excess moisture. In about three to four weeks, the cuttings should be rooted. If you tug on the cutting and find that it holds firm, the root system is well developed. Then, you can remove the plastic bag and place the pot by a sunny window or under grow lights. You can also transplant the rooted cutting into a larger pot to continue growing it.
Care for the Plant
Treat your overwintering plants as you would any houseplant. Make sure they have plenty of light, water them according to their species requirements, and keep an eye on them for any pest or disease problems. They typically won't need feeding during the winter. But if you like, you can start feeding them in the late winter or early spring with a liquid feed, such as vermicompost tea.
Move the Plant Back Outdoors
When it's time to move your newly rooted plants back outside, give them a chance to acclimate to outdoor conditions before moving them there permanently. After the danger of frost has passed, move the pot outdoors each day for a gradually increasing number of hours. Do this for at least a week. Then, you can either keep your plants in their containers outside or plant them in your garden.
How Overwinter Dormant Bare-Root Plants
Some tender perennials are often grown as annuals in regions with cold winters, but they can also be overwintered using a special technique: The tough fibrous or tuberous roots can be dug up and stored as bare roots, then resurrected in the spring. Geraniums (Pelargonium), dahlias, and tuberous begonia are three such plants often handled this way.
Dig Up the Roots
Just before, or immediately after, the first frost of fall/winter, dig up the plant and shake off the soil from the roots.
Geraniums are best dug up before the first frost; dahlias roots often overwinter best if they are left in the ground for a week or two after frost kills the foliage. Tuberous begonias should be allowed to die back completely, then the root structure can be dug up for storage.
Store the Roots
Most plants handled this way store best in a dark, dry location where temperatures remain at 45 to 50 degrees Fahrenheit, but this can vary depending on the species.
- With geraniums, the entire plant, including the stem and root ball, can be stored hanging from a rafter, or can be wrapped in newspaper and stored on a shelf.
- With dahlias, separate the tuberous roots from the stems and store the roots in a plain paper bag filled with slightly damp (not sopping) peat moss, saw dust, or vermiculite. Dahlia roots can become desiccated and non-viable if they are allowed to dry out completely.
- With tuberous begonias, simply store the roots in a paper bag stored in a dry location. Tuberous begonia roots like a location that's a bit warmer (no less than 50 degrees Fahrenheit), and many gardeners like to sprinkle some fungicide powder in the bag before storing them. Begonia roots do not like moisture at all, so make sure they stay dry.
Monitor the Dormant Roots
Check the dormant roots monthly and tend them in whatever way is appropriate for the species.
- With dahlia roots, which are susceptible to desiccation, make sure to slightly moisten the storage media (peat moss or vermiculite) if it becomes bone dry.
- For geranium roots, soak the plants in water for an hour or so once a month, then redry them and hang them back up to store. Make sure to snip off any foliage or root sections that are clearly diseased or rotting.
- Tuberous begonias should be inspected periodically; discard any tubers that are becoming excessively soft or mushy. Don't allow begonia roots to be stored moist, as they are very susceptible to rot.
Resurrect the Roots
About six weeks before the last frost date, plant the roots in a container filled with potting mix. Geraniums should be planted so the first two nodes of the stem are buried; these will develop into new roots. Dahlia roots and tuberous begonia roots should be planted at the normal depth.
Place the newly planted specimens in a sunny window and give them plenty of water as they sprout new growth. Feeding can begin as soon as there is a good amount of new green growth.
When spring conditions have reached optimal levels for the plant species, your overwintered plants can be transplanted into the outdoor garden or into outdoor containers. Before moving them outdoors permanently, It's a good idea to "harden off" these tender perennials by giving them increasingly long outdoor visits over a period of a week or two, until they are fully acclimated to the new environment.
Optimal climate conditions vary for different species, but in general, most plants are safe to plant outdoors when nighttime temperatures remain reliably above 50 degrees Fahrenheit.