Because tender perennials can't survive harsh Canadian winters unassisted, many people treat them like annuals, buying new plants year after year. But with a little care and the right conditions, you can overwinter tender perennials for years, saving money and earning a sense of horticultural pride.
Just what are tender perennials? Some are showy flowers with fleshy underground rhizomes, while others are familiar garden herbs suited to a warm, dry Mediterranean climate. Overwintering these somewhat delicate plants isn't difficult, but the technique varies with the root system.
Bulbs and tubers and corms, oh my!
Cannas, dahlias, gladiolas, and tuberous begonias are the most common flowering tender perennials. Whether they grow from a bulb, corm or tuber, all these flowers require the same basic care to overwinter.
The secret is timing: Frost kills active growth, so leave the plants in the ground until after the first light frost. But don't wait too long: A hard frost can kill some tender perennials, and once the plant goes dormant, rot-causing microbes can quickly attack. After the first frost, trim the foliage to a few inches from the ground. Carefully dig up the root bulb and inspect it for signs of insects or fungus. Only overwinter healthy plants.
Allow the bulb, corm or tuber to dry inside where it's warm for a couple of weeks. Ensure the drying area has good air circulation. If the plant is a corm (like gladiolus), remove the dried foliage and cut off the old corms from the bottom. Once dried, bury in either dry sand, sphagnum peat moss or vermiculite.
The right temperature—between 7°C and 10°C—is key, since normal room temperatures are too hot. To provide the critical rest the plants need, store them in an unheated spare room or basement area. A root cellar is ideal.