This article is appearing before most of us have had a killing frost, but take a moment to read this now so you’ll have an idea of what to do when the time comes. The processes described below are general and reliable instructions, but we’ve had luck simply wintering over dormant plants in the containers they were grown in, stored in cool, dark spaces which stay above 35F.
WAIT UNTIL AFTER there has been a killing frost, when the foliage has blackened and in some cases toppled over (you want to be sure active growth is halted). Then it is time to gently unearth the corms, tuberous roots and rhizomes of tender perennials such as Canna, Colocasia, Oxalis, Gladiolus, Dahlia, Salvia guaranitica, Mirabilis, Phygelius and others. Be careful, as cuts and breakage of fleshy tubers invite disease and decay. (If pieces of root become broken or cut, remove the affected divisions or treat with a garden fungicide to prevent fungal diseases from developing and spreading.) It is always a wise practice to sterilize shovels and forks after digging each clump, so as not to spread disease. Remember to write on a tag each variety of plant as they are dug, and attach. It is so easy to mix up root stocks, especially if you’re growing more than one selection of a certain plant.
The next thing you should do is clean and cure your roots. Wash off with a hose sprayer as much of the clinging soil as possible, which will rid you of possible pests and pathogens lingering in the soil. You may want to dip the clean roots in a fungicide solution at this point as a preventative action. (We may try the botanical all purpose Neem this year, which has fungicidal properties as well as insecticidal.) Lay the roots to cure or dry in a protected well ventilated area out of direct sunlight. This may take a few days or up to a week or two, depending on the plants, weather and temperature conditions.
In the meantime, locate some ventilated containers to store the roots, such as wooden crates or apple baskets. Plastic covered bins are easy to acquire, but be sure to puncture holes in the sides for ventilation. Be sure to include a label in each bin. First put down an insulating layer of clean wood shavings, almost dry peat moss, a well drained barky soil mix which is just a tad moist, or even sand, and lay down the roots, covering each layer with an inch of your medium, being careful to not to let the roots touch. Place the bins in a dark cool area that stays above freezing. A cool, slightly humid root cellar would be perfect, and if you live in an ancient house like we do, your cellar may be the perfect spot. Any basement, shed or garage that is dimly lit and cool will be fine, as long as it stays above freezing. You may want to check the bins occasionally to monitor if mold is beginning to develop. If you do catch fungal problems, immediately discard the affected roots and remove what is left that look healthy. Treat the healthy roots with a fungicide, (acts as a preventative) and place in a clean bin with fresh shavings, soil mix or sand.