Begonia (Begonia spp.) is an enormous plant family. The term does not describe a species of plant but a genus that includes more than 1,500 different varieties, each with its own bloom period.
The most widely grown begonias in this country are wax begonias (Begonia x semperflorens-cultorum), plants often grown as annuals, and tuberous begonias (Begonia x tuberhybrida), which grow from bulbs. Both flower all summer long. But many lesser-known begonia species also decorate a garden.
Wax begonias are the bright blooming bedding plants one most often sees in garden beds and flower boxes. They get their common name from their shiny, waxy leaves. The plants can be grown as perennials in U.S. Department of Agriculture plant hardiness zones 8 through 10 and as colorful summer annuals in cooler zones. They bloom all summer long, from July through the first frosts.
Tuberous begonias (Begonia tuberhybrida) offer big blooms in tropical colors that grow well in hard-to-plant areas of garden shade. These begonias are hardy in USDA zones 9 through 11, offering white, yellow, orange, rose, red and pink flowers from mid-July through frosts. For even longer blooms, try the patented Nonstop Mix tuberous begonia (Begonia x tuberhybrida Nonstop Mix), hardy in USDA zones 9 through 11. The flowers are full doubles -- up to 4 1/2 inches wide -- in a mix of bright colors that bloom without stopping for four solid months from July through October.
Don't expect blossoms on rex begonias (Begonia rex-coltorum) because these are plants grown not for their flowers, which are insignificant, but for their fantastic foliage that mixes shapes, textures and colors such as greens, reds, pinks, silvers, grays, lavenders and a wind-dark maroon. Rex begonias thrive outside only in USDA zones 10 and 11.
If you are looking for a begonia that blooms all year long and you live in a warm region, try the shrub begonia commonly called sweet begonia (Begonia odorata ‘Alba’). This succulent begonia is a perennial in USDA zones 10 through 11. It grows to 3 feet high and 4 feet wide and, in warm climates, is covered with fragrant white flower clusters 12 months out of 12. Expect more blossoms in the cooler months of the year, from fall to late spring, because the plant produces fewer flowers in hot, humid summers.
When they call it hardy begonia, they aren't kidding. Hardy begonia (Begonia evansiana syn Begonia grandis) is an herbaceous perennial whose tuberous roots can live out winters in USDA zones 6 and 7. The foliage forms a loose mound about 2 feet in each direction. Pink pendant flower clusters cover the plant from July until October.
Snip off faded flower heads of hardy begonias to extend the bloom period. Sterilize the pruners first by soaking in a mixture of 1 part denatured alcohol to 1 part water.