Are Geraniums Cold Hardy?

On the one hand, true -- or hardy -- geraniums (Geranium spp.), commonly known as cranesbills, tolerate cold in U.S. Department of Agriculture plant hardiness zones 3 through 8, depending on variety. On the other hand, plants commonly known as geraniums, (Pelargonium spp.) survive winter outdoors only in USDA zones 9 through 10, depending on variety. Elsewhere, they grow as annuals unless taken indoors for the winter.

Generally, people and Pel geraniums like the same range of temperatures: 70 to 80 degrees Fahrenheit during the day and no lower than 50 degrees at night.

When the temperature drops below 40 degrees F, their growth slows considerably. Pel geraniums survive at 32 degrees, but more than a few hours at 27 F is a death sentence for most of them.

Cranesbills hardy to USDA zones 3 and 4 withstand cold down to minus 40 and minus 30 degrees F, respectively.

There's no need to lift Pel geraniums for the winter if a surprise early frost is in the forecast. Use simple, temporary measures to get them through the first cold snap, and they'll reward you with several more weeks of garden color:

Move potted Pel geraniums indoors.

Water in-ground Pels well before the temperature starts dropping. Extra moisture insulates their cells.

Protect the plants with overturned flower pots or cardboard boxes.

Cover large plantings with burlap,  bedsheets or other lightweight fabric. 

If possible, spread the fabric over a supporting frame so it doesn't damage the geraniums' blooms or leaves.

Use fabric only if it will remain dry during the night. Otherwise, moisture evaporating from the cloth lowers the temperature around the plants even more.

If nothing else is available, pile newspaper around the plants. Whatever protection you use, remove it in the morning so the sun can warm the soil.

Pel geraniums overwintering indoors need thorough preparation. While they're still in the ground, cut them back by one-third to one-half and examine them for bugs or disease. Discard the infested or sick plants and dig up the healthy ones.

Plant them in pots of sterilized commercial potting mix at the same depth they were in the soil. Use pots with drainage holes. Water well after planting to settle the potting mix.

Keep the overwintering geraniums in a cool room -- about 60 degrees F -- with plenty of direct sun.

Water whenever the surface of the potting medium becomes dry to the touch. When new growth appears, pinch each shoot back to a leaf node one or two times. It encourages bushy plants.

Fertilize once in mid-spring. Use a solution of 1 teaspoon of 20-20-20 fertilizer dissolved in 1 gallon of water. When the chances of a spring frost have ended, move the plants back outside.

Most cranesbills sail through winter without any protection. Two notable, widely grown exceptions are "Johnson's Blue" (Geranium ibericum "Johnson's Blue") and "Brookside" (Geranium "Brookside"). In USDA zones 3 and 4, both benefit from a 3- to 4-inch layer of organic mulch. Good choices are shredded leaves, pine bark or wood chips.