How to Protect Tender Bulbs in Winter
Winter temperatures kill tender bulbs such as cannas, gladiolas, and dahlias. To protect tender bulbs and save them for next year's garden, you need to lift them from the ground and store indoors. Tender bulbs can survive light frosts that don’t last for weeks on end. Even so, the first time the temperature dips much below 30°F (–1°C), the leaves will turn to black mush.
(In comparison, hardy bulbs can withstand winter cold and freeze. Winter-hardy bulbs include tulips, daffodils, and crocuses.)
When the leaves on tender bulbs turn to mush, that’s your signal to take the following steps:
Carefully dig the bulbs out of the garden.
Gently shake off loose dirt and clip back the mushy leaves.
For the next few days, leave the roots out in a dry area.
Shake off any additional crumbly dry dirt that comes off easily.
Pack the roots in a cardboard box lined with a perforated plastic bag and fill with dry peat moss, wood shavings, buckwheat hulls, vermiculite, or any other similar material.
Make sure that individual root clumps don’t touch — keep them separated with packing material. That way, if one clump does start to rot, the rot won’t spread to all the stored plants.
Punch a few holes in the plastic so excess moisture can escape, but not so many holes that the stored roots will dry up.
Keep the box (or boxes) in a cool, dry, place. For example, use the back wall (the one adjoining the house) of an attached, unheated garage.
Check every couple of months to see that the material isn’t wet and that the roots aren’t shriveling from dryness. Along about early to mid-March or April, depending on your location, you can unpack the tubers and rhizomes to pot them up. (Sometimes they will have already begun to grow, sort of insisting that you pot them or lose them.) Otherwise, wait until two weeks before your frost-free date to plant dormant cannas and dahlias in the garden.