Overwintering annuals is not part of a typical garden cycle. Usually, you plant them in spring, nurture them through summer, then rip them up and compost (or dispose) them in fall. This is the circle of life for most plants grown as annuals. But…if you’re willing to set aside some indoor real estate in a sunny area, you may be able to give some of those plants a second chance at outdoor life come spring.
You Say Annual, I say Perennial…it all depends
What you can overwinter are actually ‘tender perennials’ rather than true annuals.
A short botanical vocabulary lesson:
Annual – a plant with its entire life cycle of seed to flower, back to seed, in one growing season. They burst into bloom for an extended period, giving their all, then dying away once they’ve produced their seeds for future generations. Sometime before winter, the entire plant (roots, stems, and leaves) die completely. True annuals are one-season wonders in any climate zone and aren’t candidates for overwintering. Some popular examples of annuals are pansies, zinnias, cosmos, and African or French marigolds.
Perennial – a plant that lives for at least two years, often dying back to the ground in winter before coming back from its roots the following spring. Perennials may bloom for a limited time each year, but have the advantage of not needing to be replanted each spring. Some examples of perennials are peonies, clematis, coneflowers, and butterfly bushes.
Tender Perennial – a plant that lives as a perennial in warmer zones, but is treated as an annual in colder zones. These are the plants that can sometimes be overwintered indoors in colder zones, then transplanted back outside in spring. Some examples are begonias, hibiscus, geraniums, and coleus, among others.
Overwintering Annuals: Bringing the Outdoors, In
Just as you bring your outdoor tender perennials inside, so must you bring outdoor light and moisture indoors for successful overwintering. This means placing your potted plants in a bright room with south or southwestern facing windows. If that’s not possible, or you live in an area of limited winter sunshine, grow-lights can provide needed artificial sunlight.
Heated indoor winter air is often much too dry for plants to thrive. So keep them away from heat vents, and mist the leaves several times a week. A more expensive, but very effective, solution is a room-size humidifier inside a closed plant room. And if you have enough space, a small interior greenhouse is ideal.
Another method of providing moisture is to set your potted plants on a layer of clean gravel inside a shallow container. Water placed in the gravel evaporates upward into the plants while the gravel keeps the roots from being constantly wet. We don’t like wet feet and neither do our plants!
Prepare Your Pots
Your winter guests must have clean pots. If you purchase new plastic ones, give them a quick rinse before planting. If you opt for new terra cotta pots, soak them in a container of water for about 10 minutes first. This keeps the clay pots from absorbing the moisture meant for the transplants.
Re-using old pots is fine, but some added preparation will ensure they provide your plants a good home. Empty out any leftover soil and use a stiff brush to remove any salt deposits. Rinse out the pots, then submerge them in a container of 9 parts water to 1 part unscented household bleach. After soaking for 10 minutes, remove, rinse well with clear water and air dry. Terra cotta pots should be re-submerged in clear, clean water for another 10 minutes to remove bleach from the pores of the clay, and then air dried.
Dig or Cut?
You have your choice of digging plants out of the garden when you’re overwintering annuals indoors, or taking cuttings of existing plants. Of course cuttings take more planning and time-sensitive preparation.
Take your cuttings from healthy and vigorously growing plants in midsummer, but avoid taking cuttings that are actively blooming. If you must, pinch the blossom or bud as you take the cutting. Using a clean, sharp blade, take cuttings of three to five inches and remove any leaves from the lower half, inserting the bottom third of the stem into a pot of fresh, very moist potting soil. Some gardeners treat them with root hormone to stimulate growth. (If taking more than one cutting, clean blade with soapy water or rubbing alcohol between cuttings.)
Make a mini-greenhouse by placing a plastic bag over each pot, supporting it with stakes, twigs or skewers to keep the plastic from coming in contact with the plant. Place the pots in a bright place, but not in direct sunlight, as this can cook your cuttings. Some gardeners use rooting hormone when taking cuttings, but it is not a necessity. In three to four weeks the cuttings will have rooted; you can remove the plastic bag and move your pots to a sunny window.
If transplanting whole plants, this must be done before frost has damaged any of the foliage. Choose only the healthiest and inspect them for signs of mold, mildew, virus or damage from pests. Use a sharp trowel or shovel to dig around and under the roots, loosening the soil and removing a good portion of the roots along with the soil. Gently shake off most of the excess soil and place in a pot with fresh potting soil, watering them well. A small amount of water should come out of the drain hole, after which the drain tray should be emptied.
Since you will be moving your plants into a lower light environment, it is best to acclimate them first. Move the newly potted plants to a shadier outdoor spot for a couple weeks and watch for signs of stress. If frost is expected, cover the plants, or move them into the garage or other protected area overnight. Once acclimated, it is best to prune your plants before bringing them indoors. Most plants can be cut back by half without threatening the health of the plant. Inspect them one last time for debris and dead or dying foliage, removing these before bringing them inside. If bugs are visible, remove them, and soak the plant with a mixture of one tablespoon of all-natural soap to one quart of water.
Food and Water Needs
The growth of most over-wintering plants will slow down, and not require regular feeding. If your plants are vigorously growing, however, they may be fed lightly once a month or so. As long as the air around them is kept as moist as possible, your plants will not need as much water as they did outdoors. Only water when the top inch of the soil is dry and then water until a small amount exits the drainage hole. Allow the top of the soil to dry again before watering. Never let your pots sit in excess water. In fact, indoor plants are killed from overwatering more than any other type of mistake.
Here Comes the Sun
As spring draws near and the days grow longer again, your overwintered plants may start putting out fresh growth. This is the time to feed lightly with a water-soluble plant food and prune back any long and leggy stems. Monitor water requirements more carefully now as new growth means thirstier plants. Lightly pinching the first signs of new growth will encourage more branching and a more beautiful plant.
Once the last expected frost has passed, you need to reacquaint your winter guests with the great outdoors. You need to re-acclimate your plants over about a two-week period.
Put them out during the day, gradually moving them from a shady spot to where they will reside for the season. Bring them in at night in the beginning, slowly leaving them out for longer periods after sunset, until they are acclimated to nighttime temperatures.
Once the soil temperature reaches 50°F and the nighttime temps are regularly 50 or above, it should be safe to put them back in the ground. Simply loosen the soil and transplant, soil and all, into a hole that is about twice the size of the pot and at the same depth. Backfill the planting hole with garden soil, water well, and mulch to discourage weed growth and to retain moisture.
It’s a Journey
Not every attempt at overwintering tender perennials will be successful but, if nothing else, it keeps your gardener-brain and heart engaged during the long, dark days of winter. Give it a try!