Tuberous begonias (Begonia x tuberhybrida) are attractive flowering plants for partially shaded, protected sites in the home landscape. They are suitable for pots, window boxes, hanging baskets, and beds or borders. Tuberous begonias are available in a variety of flower colors, forms, and growth habits. Blossoms may be single or double, plain or ruffled, and are available in white, pink, red, orange, yellow, and bicolors. Plants may be upright or trailing.
While tuberous begonias can be grown from seeds, most home gardeners select tubers or plants. When purchasing tubers, select those that are large and firm. Small tubers produce smaller plants with fewer flowers. Dry, shriveled tubers are unlikely to grow.
Tubers can be started indoors or planted directly outdoors in May. Since flowering typically begins 12 to 14 weeks after the tubers are planted, most home gardeners start them indoors to maximize their bloom period.
Plant tubers indoors about 8 weeks before the average last spring frost in your area. Start tuberous begonias in pots or other suitable containers. All containers should have drainage holes in the bottom. Use a well-drained potting mix. When planting the tubers, place the concave or indented side upward. The rounded side is the bottom. Cover the tubers with 1/2 to 1 inch of potting soil. Water well. Then place the containers in a warm, 70 F location. Since the tubers are susceptible to rotting, keep the potting soil moist, but not wet. Once the tubers sprout, move the plants to a sunny window or place under fluorescent lights. Plants that don't get adequate light will be tall and spindly. Fertilize the plants with a diluted liquid fertilizer solution once every 2 weeks. To acclimate the plants to outdoor conditions, harden the plants outdoors 7 to 10 days before planting. Initially, place the plants in a shady, protected location. The tuberous begonia foliage will burn if immediately placed in direct sun. Strong winds can easily tear the foliage or break the brittle stems. During the hardening procedure, gradually expose the plants to longer periods of sun and light winds.
When purchasing plants, select stocky plants with green, healthy foliage. As with tuberous begonias started indoors, harden the plants outdoors for a few days before planting.
Plant tuberous begonias outdoors after the danger of frost is past. In central Iowa, mid-May is an appropriate planting date. Tuberous begonias perform best in moist, well-drained soils in partial shade. Choose a site sheltered from strong winds. In the home landscape, sites that receive morning sun and afternoon shade are often excellent planting sites. Use a well-drained potting mix when planting tuberous begonias in window boxes, hanging baskets, and other containers. When planting tuberous begonias, place the plants at the same depth they grew in the container. Planting them deeper than previously grown may encourage the buried portion of the stem to rot.
Once planted, watering, fertilizing, and deadheading are the primary maintenance practices. Tuberous begonias should be watered weekly in beds and borders in dry weather. Plants in containers will have to be watered more often. Check container-grown plants regularly (daily or every other day) and water as needed. Fertilize tuberous begonias every 2 weeks with a diluted fertilizer solution. Remove spent flowers to improve their appearance and encourage additional blooms.
In Iowa, tuberous begonias will not survive the winter outdoors. These tender perennials must be dug up in the fall and stored indoors over winter. Carefully dig up the tuberous begonias within a few days of a killing frost. Leave a small amount of soil around each tuber. Cut off the stems about 1 inch above the tubers. Place the tubers in a cool, dry area to cure for 2 to 3 weeks. After curing, shake off the remaining soil, then bury the tubers in dry peat moss, vermiculite, or sawdust. Store the tubers in an area with a temperature of 40 to 50 F. Do not allow the tubers to freeze.
As mentioned earlier, tuberous begonias can be grown from seeds. However, it can be rather challenging. Tuberous begonia seeds are tiny. There are approximately one million seeds per ounce. To make the seeds easier to handle and sow, most seed companies coat the seeds with clay or a similar material. Seeds should be started in January as flowering typically occurs 18 to 20 weeks after sowing.
Whether purchased as seeds, tubers, or plants, tuberous begonias are beautiful additions to partially shaded sites in the home landscape.
This article originally appeared in the 2/27/2004 issue.