Among the many species of begonia (Begonia spp.), some are very tender and will not survive freezing temperatures even with protection, but some have robust underground root systems that allow them to make it through the winter in climates where temperatures fall well below freezing.
Wax begonia (Begonia semperflorens) tolerates heat and drought, and it performs well through the summer in hot climates. It does not, however, tolerate cold, and it is winter hardy only in U.S. Department of Agriculture plant hardiness zones 10 to 11. Freezing temperatures will kill the plant, and in areas where frost is possible, it can be grown outdoors only between the last frost in the spring and the first frost in the fall. In cool climates, container-grown wax begonias may be brought indoors to overwinter.
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Most tuberous begonia species are hardy only in USDA zones 9 to 11 and will die during freezes. The species commonly called hardy begonia (Begonia grandis), however, is able to survive winters in USDA zones 6 to 10. Fall frosts and freezes will cause the above-ground portion of the plant to die back, but the plant will regrow in the spring from underground tubers that go dormant through the winter. The plant will also produce new tubers in its leaf axils during the growing season; these young tubers drop to the ground in the fall and will sprout in the spring.
In the colder parts of its range, hardy begonia may not be reliably winter hardy, and it will benefit from a thick layer of mulch over its tubers through the winter.
Rhizomatous begonias grow and spread via horizontal stems called rhizomes, and some rhizomatous species are nearly as cold tolerant as hardy begonia. The Mount Emei begonia (Begonia emeiensis) and Taiwan begonia (Begonia chitoensis), for example, are winter hardy in USDA zones 7 to 10.
Like hardy begonia, these species drop their leaves when exposed to frost, but their rhizomes can survive when temperatures fall below freezing. Because the rhizomes are often very close to the surface of the soil, though, they are more likely to survive if they're covered with a layer of mulch in the winter.