How to overwinter annual geraniums

Geraniums (actually Pelargoniums, botanically) are more expensive than most annual plants we grow in our yards. So, when cooler weather reminds us that fall is here and winter not too far behind, it's natural not to want to lose them all to frost -- especially if we plan to buy and plant more next year.

Actually, it's pretty easy to overwinter annual, or zonal, geraniums and bring them back into shape for replanting next spring. And you have a choice of methods:

1. Grow them as houseplants. This works if you can provide the proper light and temperatures: You'll need a greenhouse, sunroom, or a south-facing window with a wide sill -- or can set up a spot in which to grow them under fluorescent lights. You don't have to use expensive grow lights; plain old fluorescents work just fine.

Ideal temperatures would be 65 to 70 degrees F. (18 to 21 C) during the day and about 55 F. (13 C) at night. Too-warm temps will cause leggy growth.

First, you have to pot up the geraniums you want to bring indoors. Fill as many six- to eight-inch plastic pots as you need with a moistened good-quality potting soil. (Don't try to get by with smaller pots; you won't be happy with the results.) Doug Green recommends that you cut the roots and top growth back first.

If your geraniums are growing in pots outdoors, make sure the soil is free of insects -- you don't want to bring any bugs indoors.

2. Take cuttings of your geraniums. This variation of No. 1 works well if you have the right light and temperatures for growing geraniums as indoor plants, but don't have enough room for six- to eight-inch pots -- or if you want to increase your stock of geraniums for next year.

Click here to read about how to make cuttings and here and here for video instructions, if you haven't done it before, or need some reminders. Make 4-inch cuttings and root them in sand, perlite, or vermiculite. Rig up a plastic bag to create a greenhouse effect, but make sure the bag doesn't touch the leaves. Once they're rooted (six to eight weeks), place on a sunny windowsill or outdoors.

Another alternative: About two months before your last frost date in spring, take cuttings from geranium plants that you brought indoors and treated as houseplants..

3. Let the geraniums go dormant and store in a cellar. This is the best method if you have lots of geraniums and nowhere to grow them as houseplants. (Unfortunately, heated basements, or basements that house a furnace, don't work.)

The typical way this is done is by removing all the soil from the roots and hanging the plants upside down from the rafters in a dark cellar. The temperature should be about 45 to 50 degrees F. (7 to 10 C). To avoid a mess on the floor, some gardeners hang the bare-root plants inside paper bags with air holes punched in them.

You can also let individual pots of geraniums go dormant in the cellar. Click here for directions.

I've always just left my bare-root plants hanging all winter and, typically, some of them make it through to spring and a few don't. The Iowa State University Extension Service suggests that you take them down a couple of times during the winter and soak the roots in water for a few hours. I can see how this would help, but can't imagine myself going to all that trouble.

A promising variation on dormant storage

But I've discovered another type of dormant storage that I can't wait to try. Garden Gate magazine has a series of four outstanding videos (click here to view them) on storing dormant geraniums in a box over winter, and how to "revive" them in the spring for new growth.

It's ideal for those who want to overwinter many geraniums in a relatively small space. And it doesn't require any special techniques. I'm pretty excited about trying this and would be interested to hear of other gardeners' experiences with it or other techniques for overwintering zonal geraniums.

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Judy Lowe is one of nine garden writers who blog regularly at Diggin' It. She's the author of 10 garden books. Her new book about herb theme gardens will be published in January 2011.