Tuberous begonias can thrive for most of the year in United States Department of Agriculture plant hardiness zones 3 to 10. Because tuberous begonias have tender tubers, though, they cannot survive the winters outdoors in most climates. This is why they usually need winter storage if you want to save them from season to season. Otherwise, you’ll have to grow them as annuals.
Zones 3 to 8
In the late summer and early fall, slow down on irrigation and stop fertilizing them in the summer. After a frost or two, your tuberous begonias will turn yellow and die back to the ground. This is the time to get them out of the soil before a hard frost kills them. Use a garden fork and lift the tubers out of the soil. They are planted just below the surface of the soil so you won’t have to dig far. Examine the tubers and throw out any that look rotten or diseased.
Cut the foliage down to about 5 inches and let the tubers cure inside, out of any direct sunlight. You’ll know they are dry when you can pull the foliage from the tubers. Also, pull off the fibrous roots and brush off any soil. Don’t wash the tubers. Place the tubers along with some peat moss or vermiculite in a plastic bag with holes, similar to ones that potatoes come in from the store. Store in a cool, dark place in your home, attic or garage, ideally around 40 to 50 degrees Fahrenheit. You can store them in the refrigerator, if desired. Check periodically and throw out tubers that are shriveling, rotting, withering or softening.
Replant tubers in the spring after the threat of frost is past. A partially shady location is ideal since tuberous begonias can burn under the hot sun. In warmer climates, they should be well-shaded in the afternoon, but exposed to the sun in the morning. Soil should be well-draining. Mixing in several inches of compost or another organic matter into the soil bed will create an environment suitable for tuberous begonias. Plant tubers just below the surface of the soil, about 6 inches apart. Water them in with a 1/2 inch of water after planting.
Zones 9 and 10
In zones 9 and 10, you do not have to dig up tuberous begonias. Rather, slow down watering in the fall and don’t fertilize. In zone 9, cover your tuberous begonia bed with about 3 to 4 inches of mulch, such as leaves or straw. This will protect them if you have a colder than normal winter. In zone 10, you can leave them be for the winter.