School has started, the first football game is history, can autumn be far behind? Many gardeners are redirecting their efforts from flower production to ensuring the survival of their tender perennials. Tender perennials are not winter hardy in our part of the country, but with a little help from us, they can survive for many years. Tender perennials are those plants that need to be dug from the soil in the fall and wintered over in a frost-free location. Luckily we don't have to keep the entire plant, only the bulb, corm, or tuber are stored. New growth occurs from these structures after replanting next spring.
Popular tender perennials considered tender in Iowa include gladioli, cannas, dahlias, tuberous begonias and caladiums. Other lesser known tender perennials include the Peruvian lily also known as alstroemeria, agapanthus, freesia, and calla lilies. Survival of tender perennials requires more attention than simply digging the particular storage organ and putting it in a box in the basement. Specific storage conditions must be met to successfully store the plants through the winter.
Dig tender perennials just before or soon after a killing frost. If left until after a frost, the foliage will be killed and the storage organ will need to be dug within a few days to prevent rot-causing organisms from entering through the damaged stem.
Corm producing plants such as gladioli, corn lily, and bugle lily can be stored in a similar fashion. Before digging, trim the foliage to within a few inches of the corm. Dig carefully to prevent damaging the corm. Brush off remaining soil. Allow the corms to cure for several weeks in a dry location with good air circulation. After drying, cut or twist off the foliage and discard the shriveled remains of last year's corm. A new corm is produced each year on top of the old one. Store the corms in old nylon stockings or onion bags at temperatures between 35 and 45 degrees Fahrenheit. The bulbs of Siberian lily and Mexican shellflower can be treated the same way.
Dahlia, alstroemeria, and gloriosa lily produce tuberous roots. To store these plants over the winter, trim back the foliage of the plant to within a few inches after the first light frost. Dig carefully to avoid injury. With some soil attached, pack the roots between 2 and 3 inch layers of vermiculite, peat moss, sawdust, or wood shavings. Store at 35 to 45 degrees. Check frequently to remove those that shrivel or rot.
The tubers of tuberous begonias and elephant's ear (Colocasia esculenta) and the rhizomes of calla lilies should be dug before a hard frost. Cut the tops back and allow 2 inches of stem to remain. Dry for 2 to 3 weeks in a frost-free location, shake off the soil and remove the dried stem. Pack in peat moss, vermiculite, sawdust, or wood shavings and store at 45 to 55 degrees. Caladium (Caladium xhortulanum) tubers should be cured for a week in a warm location and stored in packing material at a temperature around 60 degrees.
Canna and butterfly lily rhizomes should be dug after the foliage has been killed by a frost. Cut the stems back to about 4 inches above the soil. Dig the rhizomes and dry in a frost-free location for about two weeks. Place the roots in shallow boxes; they do not require covering. Store at 45 to 50 degrees.
The storage organs of most tender perennials multiply quite quickly in the garden. It is important to leave them intact until spring. Any injury incurred prior to storage will increase the chances for rot to occur. In the spring cut the rhizomes and tubers apart making sure at least one or two dormant buds are present on each section. Share the extras with relatives, neighbors, and friends.
The major problem homeowners have in storing these tender perennials is finding a location with the correct temperature. Many of us no longer have an unheated basement or extra bedroom in which to store the tender perennials adequately. Normal interior temperatures are too warm. Most garages, even though attached, will be too cold for survival. If this is your situation, it would be best to grow these plants as annuals instead of perennials.
This article originally appeared in the September 13, 1996 issue, p. 157.