There are many reasons to want to overwinter annuals indoors. You may have grown some new plants that you want to preserve for next season, or you may want the challenge and the sense of pride that comes with being able to say, I did it! Some gardeners simply want to save money, while many more just can’t stand to be without plants for the entire fall and winter, therefore moving as much of the garden indoors as possible.
The plants that adapt best to indoor container life are tender perennials, which are the perennial plants that won’t survive the winter in your climate Zone; therefore you typically grow them as an annual. These plants include, but are not limited to: Coleus, Cuphea, Delosperma, French Lavender, Gaura, Geraniums, Geum, Hardy Hibiscus, Heliotrope, Herbs, Impatiens, Monarda, Salvia, Sedum, Sempervivum, Swedish Ivy, True Geranium and Verbena. The fun is in the trying, so even if you’re unsure, you have nothing to lose by experimenting.
What will your outdoor plants require to survive inside?
They’ll need sunlight and moisture, just like they do outside. The problem most gardeners face is not enough sunny windows. Southern facing windows are ideal, though many gardeners rely upon grow lights to provide the necessary hours of artificial sunlight. Moisture is also critical. Winter air is very dry to begin with and heated winter air is the worst. Most plants will not tolerate such low moisture levels, even if you have a whole-house humidifier, though a room-sized humidifier and a regularly closed door may do the trick. Most experts will recommend using clean gravel with water in shallow containers under the plants. The gravel allows the plant to sit above the water, keeping the roots from being constantly too wet, while its evaporation provides the necessary humidity. Some gardeners simply mist the leaves a couple of times a week, while others house their overwintering plants in small, interior greenhouses.
The first step:
Start with clean pots. Take inventory of how many plants and the pot sizes you may need. If you are purchasing new pots, plastic ones will need no more than a quick rinse. If using terra cotta pots, soaking them in a container of water for about 10 minutes before planting will keep the pot from wicking the moisture immediately from transplants, ensuring the soil does not get overly dry during that first critical day.
Used pots, whether you’re using your own, ones you’ve purchased at yard sales or pots you’ve been given, will usually have salt deposits and old, possibly contaminated soil. Use a stiff brush, old toothbrush or butter knife to remove as much as you can and then soak the pots in a large container filled with 1 part unscented household bleach to 9 parts water. Be sure the pots are entirely submerged and let soak for 10 minutes. This will kill any diseases that may be lurking. Plastic pots can then be rinsed with clear water and air dried. Terra cotta pots should be re-submerged in a container of clear, clean water for another 10 minutes to remove the bleach from the pores of the clay, and then air dried. Performing this task each spring as you move the plants back outside will prevent cross contamination and make the fall transplant easier.
Plants or cuttings?
You have your choice of digging plants out of the garden to overwinter indoors, or taking cuttings of existing plants. Rooting the cuttings from late-season or frost-damaged plants is most likely an exercise in futility. Planning ahead, taking your cuttings from healthy and vigorously growing plants in midsummer, will provide the best opportunity for success. Avoid attempting to transplant or root cuttings from any plant that has symptoms of disease or pests. If you just have to have that plant for next season, quarantine it well away from any others until you are sure the problem is resolved.
Also, avoid taking cuttings that are actively blooming. If you must, pinch the blossom or bud as you take the cutting. Use a clean, sharp pair of scissors or knife, cleaning it between each cutting with soapy water or rubbing alcohol. This prevents spreading any possible disease from one plant to another. Take cuttings of three to five inches and remove any leaves from the lower half of the cutting, inserting the bottom third of the stem into a pot of fresh, very moist potting soil. 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, you must accomplish this before frost has damaged any of the foliage. Choose only the healthiest and inspect them carefully for signs of mold, mildew, a 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 them to a shadier outdoor spot for a couple of weeks and watch for signs of stress. If frost is expected, cover the plants to protect them, or move them into the garage or another protected area overnight during this time of acclimation. 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. Again, you will want to use clean shears or a sharp knife, cleaning the utensils after cutting each plant. Inspect them one last time for debris and dead or dying foliage, removing these before bringing them inside. Visible bugs should be removed and the plant thoroughly soaked with a mixture of one tablespoon of all-natural soap to one quart of water.
Food and water
Most plants being overwintered will not require regular feeding. If so inclined, feed them right after you’ve acclimated them and before bringing them in. Vigorously growing plants can be fed lightly once a month or so, if necessary. Your plants also may 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, letting the top dry again before watering. Never let your pots sit in excess water; most plants do not thrive well with wet feet. In fact, indoor plants are killed from overwatering more than any other type of neglect or tender loving care.
As spring arrives
As the days grow longer again, your overwintered annuals will most likely start putting out fresh growth. This is the time to feed lightly with a water-soluble plant food and to prune back any long and leggy stems that have been reaching for the sun. 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.
You will want to re-acclimate your plants over the next couple of weeks, after the last frost date for the season. Put them out during the day, gradually moving them from a shady spot to where they will reside for the season over a period of two to three weeks. Bring them in at night in the beginning, slowly leaving them out for longer periods as the sun goes down until they are acclimated to the 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.
Above all else, enjoy! Every plant may not survive, but the fun is in the trying. Learn from both your failures and your successes, and share what you learn with us, on our Facebook page, or with your family and friends. Gardening and plant care are an enjoyable way to connect with people from all walks of life.