Begonias (Begonia spp.) are generally tender plants that do not survive the winter in areas where temperatures fall below freezing. Wax begonias (Begonia semperflorens) are hardy only in U.S. Department of Agriculture plant hardiness zones 10 to 11 and are usually grown as annuals. Among the other types of begonias commonly grown in gardens, some may be encouraged to come back year after year.
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Tuberous begonias such as hardy begonia (Begonia grandis) require a dormant period, even in USDA plant hardiness zones 6 to 10, where the species is winter hardy. In cold climates, the plant's dormancy is triggered by the fall's first light frost, which will cause the foliage to die back. In frost-free climates, the plant will likely go dormant on its own in October or November, or you can force the plant to go dormant by withholding water beginning in the fall. In frost-free climates where tuberous begonias are grown in the ground, the biggest problem during winter is keeping the begonia tubers dry. Therefore, digging the tubers as you would in northern climates assures the tubers won't rot.
When the plant's foliage has yellowed, dig up the tubers and gently clean the dirt from them. Allow the tubers to dry, then place each one in a paper bag. (Storing them individually helps prevent possible spread of disease or pests.) Store the tubers in a dark, cool place indoors through the winter and replant in the spring after frost danger has passed. If tuberous begonias are grown in containers in frost-free locations, gardeners can stop watering during the winter months and store in a location where the plant won't receive water, or turn the container on its side.
Some species of rhizomatus begonias, including Mount Emei begonia (Begonia emeiensis) and Taiwan begonia (Begonia chitoensis) are winter hardy in USDA zones 7 to 10. Those species that can't tolerate freezing temperatures may be moved indoors for the winter if they're grown in pots.
Some rhizomatus species enter a period of dormancy, characterized by little or no growth, during the winter. Those that do not go dormant should be lightly watered so the soil never dries out completely. Fertilize lightly through the winter; for example, dissolve a 1/4 teaspoon of 24-8-6 houseplant fertilizer in a gallon of water and water the plant with the solution every two weeks. Those species that go dormant should be infrequently watered but not fertilized.
In zones where they're winter hardy, rhizomatus begonias will die back to the ground after the first frost in the fall. The root systems of the plants are more likely to survive the winter where temperatures fall below freezing if they're protected by a layer of mulch or compost, especially when their rhizomes are close to the surface of the soil.
If you're moving begonias indoors for the winter, check the plants for the presence of insect pests before you bring them inside. Mix 2 1/2 to 5 tablespoons of insecticidal soap in a gallon of water in a hand sprayer. Spray the entire plant, making sure to soak all leaf surfaces, both the tops and the undersides.