If you live outside a tropical zone, there's a chance you can grow tropical plants in your garden. For the winter months, cold-hardy canna might survive in the ground. Transplant cold-hardy bulbs, rather than leave them in the ground, when the predicted temperatures over the winter months are much lower than usual, or when the ground in the area is wet during winter.
No Guarantees for Cold Hardiness
Canna bulbs are generally not cold hardy outside of tropical or warm winter areas. You can consider a canna bulb to be cold hardy if it can survive the average winter in your garden. Canna bulbs have a limited ability to enter a dormant state during periods of extended cold weather. If you can keep the bulbs relatively warm and dry, they may bloom in the summer.
Small differences in your property can affect how or if your plants will grow again in the spring. The bulbs need the soil to drain well to draw moisture away from them. The bulbs will be damaged if they rot while sitting in cold, wet soil. Transplant bulbs when you expect the conditions over the winter to be abnormally wet or abnormally cold in that spot.
Microclimates Make A Difference
If canna plants have done well over the winter in your garden in previous years, take note of any changes in the specific spot where you have planted the canna. For example, if there are any new structures or trees in the area, check to see if they block sunlight from reaching the ground. New gutter downspouts may drain into the area, creating more moisture than the ground can absorb.
Transplant the canna bulbs for the winter when you anticipate the new physical conditions will make it too cold or wet for the canna to survive. In this instance, you have created a microclimate, a small subsection that differs from the normal temperature highs and lows of the overall climate in your area.
Transplant After First Frost
The ideal time to transplant canna for the winter is immediately after the first frost of the season. Wait until the leaves of the plant turn brown. Dig up the bulbs before the ground freezes. Exercise caution when digging up the bulbs---because canna plants spread out during the growing season. In particular, if you have left the canna in the ground in previous years, you will have a lot more canna to dig up than you planted. Take care not to dig into the bulbs themselves. If possible, wash off the soil on the canna bulbs with a garden hose. Alternatively, let the canna dry in the sun and brush the remaining soil away. Avoid storing the bulbs while they are still covered in soil; the soil will cause the bulbs to rot. Store the bulbs in a cool, dark place. Cover them with peat moss or perlite.
Replant the canna bulbs in the spring after all danger of frost has passed in your area and when the nighttime temperatures average 50 degrees or more.