You can beat the frost and save your geraniums by taking them inside to overwinter, said an Extension horticulture educator.
“As soon as we get freezing temperatures, most unprotected annual geraniums will turn a mushy green and die,” said David Robson. “However, it’s possible to take those geraniums before they get nipped by a hard frost and overwinter them indoors.
“You can pot up the plants, take cuttings, or store the plants as bare root specimens. No matter what method you choose, understand that success isn’t guaranteed. The headaches and heartaches may not be worth it, especially since geraniums can easily be purchased every spring.”
It is important to make sure the plants you attempt to overwinter are vigorous, healthy, and insect and disease free.
“If you grew your geraniums in individual pots for the summer, you probably don’t have much to do except examine each plant carefully to make sure you’re not bringing insects indoors,” he said. “White flies, aphids, and mealy bugs can hide on a plant. Outdoors, they don’t cause many problems as there are predators to keep them in check. Indoors, though, those predators aren’t around.
“Make sure to check the soil to avoid bringing in other hitchhikers. Some gardeners will always re-pot the geranium in fresh houseplant soil. That might be a little unnecessary, though it practically guarantees no soil-born insects are brought indoors,” he said.
For plants in larger pots or in the ground, carefully dig the geranium and plant in a six or eight inch pot. Use potting soil instead of garden soil to avoid a soggy, heavy soil indoors. Prune back each plant by one-half.
“Geraniums need at least 10 to 12 hours of light indoors ideally,” he noted. “Place the plants in a bright south window or under fluorescent lights.
“Indoor temperatures are just as important. Geraniums, by nature, are more of an arid plant; they prefer warm—65 to 70 degrees F—day temperatures and cool night temperatures of 55 to 60 degrees F. Excessively warm temperatures may result in leggy plants.”
Using the cutting approach allows using smaller plants that take up less space, and have a better chance of acclimating to indoor light, temperature, and humidity levels.
“Take four to six inches of terminal growth and strip off the bottom two to three inches of leaves,” Robson said. “Dip each cutting in a rooting hormone. Stick the cuttings in sand, perlite or vermiculite up to the first set of leaves. Water thoroughly and place in a bright sunny window or under fluorescent lights. Cuttings should root in one or two months.
“When rooted, pot in a three or four inch pot and continue to grow until spring.”
The bare root approach is by far the easiest but also the least successful, Robson said. It involves digging the geraniums up, shaking most of the soil from the roots, and hanging upside down in a cool basement or dry crawl space where temperatures hover around 45 to 50 degrees F.
“Once a month, soak the roots for an hour or two in warm water,” he said. “Expect that leaves will probably turn brown, dry up and fall off. If all goes well, though, stems should remain green.
“In March, cut each plant back by half or to green, fleshy, solid stems. Pot each plant up and water thoroughly, placing the geraniums in a bright, sunny window. Plants should start budding out, sending out new shoots, and developing into attractive plants that can be set outside in May.”