A triumph of the breeder's art, tuberous begonias at their biggest have flowers of salad-plate size in fanciful forms and bright colors, even with petal edges tipped in a contrasting color (picotee). These beautiful flowers that grow well in morning sun and light shade have been joined in recent years by new varieties with altogether more modest flowers but many more of them.
Description of tuberous begonia: The large-flowered tuberous begonias come with many flower types, both upright and pendulous, single or double-flowered, and with frilled or plain petals. Unlike their semperflorens cousins, tuberous begonias offer wide color choices: white, pink, rose, red, orange, and yellow. They grow upright with large, arrow-shaped leaves. Both the large- and small-flowered tuberous begonias alternately bear male (ravishingly beautiful) and female (single and smaller) flowers. The smaller-flowered tuberous begonias bear many flowers up to 3 inches in diameter.
Growing tuberous begonia: Tuberous begonias grow best in midday and afternoon shade; otherwise the foliage will scorch. They need rich, well-drained soil with high organic matter. Allow soil to dry between waterings. The large-flowered varieties easily become top-heavy and require judicious staking, while the smaller-flowered ones can usually support their own growth.
Powdery mildew is frequently a problem with tuberous begonias, especially if they are grown where the air around leaves and stems is stagnant. At the first signs of a white powder on leaves, spray with a fungicide.
Propagating tuberous begonia: Reproduce new plants from seed, tubers, or cuttings. Most of the big-flowered tuberous begonias are sold as named-variety tubers. When tiny, pink growth appears on the upper side (with a depression where last year's stem was attached), place the tuber with the hollow side up at soil level in a pot filled with packaged soil mix. Water well once to firm the tuber in the pot and provide a temperature of 65 degrees Fahrenheit.
As the top swells and grows, roots will be forming below the surface. Do not allow the soil to dry out, but avoid drenching until the leaves expand. Provide high light until time for planting outside (after all danger of frost has passed, the weather has settled, and the soil has warmed). Carefully plant at the same level as the begonia was growing in the pot.
Uses for tuberous begonia: Grow the large-flowered kinds as specimen plants in semi-shady locations. Pendulous varieties make good container plants. The new, small-flowered kinds (varieties include Memory, Non Stop, and Clips, all with separate colors) can be used for larger beds, in containers, and in hanging baskets. Watch container plantings to prevent drying out.
Tuberous begonia related species: Begonia boliviensis is a tuberous species with glowing orange flowers spurting narrow petals. Bonfire has arching stems and red-orange flowers. The iron cross begonia (Begonia masoniana), a widely grown indoor plant, makes a handsome foliage specimen for shade in summer planted directly into the ground or plunged in its own pot. The chartreuse leaves strongly marked with a chocolate-brown iron cross make a bold statement. Be sure to take this plant inside before cool weather starts because it is very frost-sensitive.
Begonia richmondensis exhibits a graceful, flowing habit, vigorous growth, handsome, glossy leaves, and copious flowers, making it a very popular hanging basket plant in many parts of the country. Flower buds are cherry-red opening to a bright pink. It is easily rooted from cuttings. Morning sun and afternoon shade are ideal. Rex begonias (Begonia rex) are foliage plants colored in every conceivable combination: steel-gray, red, pink, green, and with splashes of white. They do well outdoors in the summer in shady spots.
Elatior begonia (Begonia hiemalis) are hybrid begonias produced by crossing several species. One series is upright, good for planters, while the other has a flowing character and is ideal for hanging baskets and other containers to be viewed at eye level or above. Because much of the early development work was done by the Rieger firm in Germany, they are frequently known by this name. Flowers are 1 to 11/2 inches in diameter, single, semi-double, and double. Colors are red, orange, pink, and a luscious white that looks green when the light shines through it.
Tuberous begonia related varieties: Tubers of large-flowered varieties in separate colors and flower forms are usually available at garden centers and from specialists as named varieties. Smaller-flowered types are available as seed or started plants in garden centers. Cultivars come in a variety of colors and forms.
Scientific name of tuberous begonia: Begonia tuberhybrida