Bruce Bjerva, Star Tribune

Overwintering geraniums

  • Article by: Deb Brown
  • Contributing Writer
  • September 2, 2003 - 11:00 PM

There are many ways to overwinter geraniums

Some gardeners try to carry their geraniums from one summer to the next. They diligently dig the plants up just before frost, then store them in bags or boxes in the basement or hang them by their roots from the rafters. A cool, damp basement or root cellar is just the thing for holding geraniums in a state of suspended animation -- not quite dead, yet showing few signs of life -- until they mysteriously begin to send out new sprouts in late winter or early spring.

The problem is, most of our basements just aren't cool or damp enough anymore. And who among us is fortunate enough to have a functional root cellar?

Luckily, there are other ways to keep geraniums over the winter.

Plenty of sunshine

One method is to pot up the geraniums, carefully wash them to remove any insects clinging to the foliage, then bring them indoors several weeks before frost threatens, then set them in front of a bright, sunny window or patio door. This works well if you've got plenty of space and lots of sun all winter. Most of my sunny windows already have blooming plants in them, so there's not much room left for pots of geraniums.

Electric illumination

Another option would be to pot them up and set them on a table beneath a bank of fluorescent lights. A cool basement room -- or any spare room, for that matter -- would work just fine. Be sure to suspend the fluorescent light tubes no more than 12 inches above the foliage so that the geraniums receive enough light to stay healthy. It's a good idea to buy an inexpensive automatic timer to make sure the lights are on for at least 12 hours daily.

If the room is cool, the geraniums won't need to be watered very frequently. Wait to water until the soil feels dry about 1/2 inch below the surface, then water them thoroughly. Apply fertilizer only two or three times over the winter and mix the fertilizer at one-quarter strength. If you detect a noticeable increase in plant growth toward spring, you can increase the frequency of fertilizing and increase the dose to the label-recommended strength.

Cut a slip

If you don't have window space or room for lights, you can still overwinter geraniums by taking slips. Right now is the best time to take slips because the more actively the plants are growing and the more light is available, the more rapidly these cuttings will root. If you wait until temperatures hover near freezing, the geraniums will have become so hardened off that they'll root only with great reluctance, if at all.

Take stem cuttings roughly 3 to 5 to five inches long, with several leaves attached. Choose the tip portion of the healthiest-looking green stems. Remove any flower clusters or flower buds, which consume energy that could be used for making new roots. Then strip off the lowest leaf so you have about an inch of bare stem to stick into the rooting medium.

Though these slips might root in water, they'll develop a much sturdier root system if you root them in horticultural vermiculite or fresh potting soil. For added insurance, dip the cut end of each slip into rooting powder, then make a small hole in the vermiculite or potting soil to accommodate the slip, rather than pushing the stem in directly.

Keep the rooting medium constantly damp until roots form. In a few weeks, check for root development by tugging each cutting very gently. If you feel resistance, you know it's rooting. If not, wait a few more weeks and try again.

Once rooted, transfer the geraniums to small pots, then put them in a sunny window or under fluorescent lights, and treat them as you would larger plants. If you provide plenty of light, they should be just about ready to bloom by the time it's warm enough to put them outdoors next spring.

Deb Brown is a horticulturist with the University of Minnesota Extension Service Yard and Garden Line. For help with garden, plant and insect questions, call the Extension service at 612-624-4771.

© 2018 Star Tribune