More than 1,300 species of begonias (Begonia spp.) exist, all native to subtropical or tropical regions. The begonias most gardeners grow are perennials, plants that live for three or more years. Less commonly grown begonias are annuals---plants that complete their life cycles, from seed to reproduction, in one year.
Begonias grown as houseplants are considered by horticulturists as "tender perennials," meaning they persist but are susceptible or tender to temperatures that reach or drop below 00 degrees Celsius. Outdoors, wax/fibrous begonias are short-lived perennials and tuberous begonias are perennial but go dormant in winter and re-sprout in spring.
Most begonias are perennials, but a few, such as Begonia hirtella, are annuals and must be propagated annually from seed, according to Mark Tebbitt, the author of "Begonias."
Frost causes the fleshy, succulent leaves and stems of begonias to rupture and blacken. Depending on severity and duration of cold, a begonia will regrow from unharmed buds on lower stems or from the roots. Root tubers or rhizomes survive slightly lower temperatures because soil insulates them.
Although many begonias are perennial, because they are used as seasonal bedding plants in summertime they are sometimes called annuals by gardeners. Fall frosts kill the plants. Without frost, these "annuals" would reveal their perennial nature.