Round Robin: Temperature Control

ATLANTA, Georgia–Last year, I was due at a photography session on December 11. As the photo stylist, I needed a good supply of mature “summer leaves” of Skeleton Rose geranium for the shoot. My plants had grown large and full in intensively planted raised beds near the house, keeping company with many other varieties of scented geraniums, including Snowflake, Rober’s Lemon Rose, Coconut, Lime, Lemon Crispum, Velvet Peppermint, Ginger, and Old-Fashioned Rose. But the first frost was threatening nightly, even though the days hovered around a balmy 66°F. To prevent any disasters those last few nights before the shoot, I ran an extension cord out the back window and spread an electric blanket right over the tops of the scented geraniums. I set the thermostat on medium and hoped for the best. It worked fine! And sure enough, the morning before the photo shoot, frost hit. Basil in another part of the garden turned black, geranium branches peeking outside the edges of the blanket turned brown, but beneath the fuzzy blue blanket, the leaves were fresh and green, fragrant and velvety. And the pictures were perfect.

Each fall, I clean out the cold frame and test my soil-heating mat–a length of plastic webbing with a low-voltage heating cable snaking back and forth in the mesh. Plugged into an electrical power source, the mat keeps the temperature inside the frame above freezing, turning the cold frame into a hotbed.

I set flats of cuttings on the heating mat; the bottom heat is a perfect way to root almost any cutting. I have found that taking cuttings is the easiest and most trouble-free way to ensure that my tender, half-hardy, and borderline herbs and shrubs overwinter. Instead of digging up and potting overgrown, mature plants in cold weather, I simply take 6-inch-long cuttings of new growth, strip off the bottom leaves, and insert the cuttings halfway into a flat filled with a mixture of vermiculite, perlite, and milled sphagnum moss. My flats are 5-inch-deep Styrofoam box, used to ship grapes to grocery stores; they provide a deep root run and enable me to propagate almost anything. I don’t bother with hormone rooting powders, but I do take care to water the cuttings thoroughly and to keep them moist throughout the winter. The cold frame is fitted with an automatic ventilating system which lifts the lid of the frame when the heat buildup inside reaches a critical point and allows fresh, cool air inside so that a more even temperature is maintained. Plant disease is seldom a problem. December is chilly, and January is cold. Because the flats hold a good volume of rooting mix, watering only every couple of weeks is usually sufficient. First, the cuttings callus over. Then, small roots emerge from the callus and from the nodes where the leaves were stripped off. By late winter, when it’s time to pot up the cuttings in preparation for spring, the roots have grown long and tangled. I get the pots and potting soil ready first, then lift the cuttings from the flats and try to gently tease the roots apart. The pots of rooted cuttings go back into the cold frame until the day in March when I pull the plug and the hotbed reverts to a cold frame.

I care for all my scented geraniums this way. During some mild winters, old plants left in the ground survive and start up again from the roots in April, but if they don’t prove hardy, the cuttings cover my losses. Rosemary and prostrate rosemary also respond beautifully to this treatment; lemon verbena, the tender lavenders, heliotrope, Mexican mint marigold, creeping gardenia, and jasmine all root well.

A few plants are too tender for this technique; the Madagascar jasmine (Stephanotis floribunda), which doesn’t like temperatures in the forties, spends the winter in the north-facing bay window in the house. Other plants don’t work because of their size. The lemon tree and the lemon eucalyptus are too big to fit in the cold frame. The bay trees, grown large in big terra-cotta pots over the years, need to be dragged into the garage if the weather forecasters predict lows of near 5°F, but this happened only one night last year and only for a week the year before that.

This is not my first cold frame/ hotbed. In graduate school at Rutgers, I had one that derived its source of heat from fresh pig manure! I’m just as glad to use electricity today.