Tuberous begonias are valued for their spectacular, showy blooms, providing continuous color in shaded areas throughout the summer. Since these tropical plants cannot tolerate frost, they are commonly started indoors several weeks before the outdoor growing season. Starting them early ensures a head start and earlier flowering. Successfully growing these magnificent flowers begins with the proper handling and planting of the dormant, fleshy tuber. This guide provides a step-by-step approach to preparing, planting, and caring for begonia tubers.
Timing and Necessary Supplies
Waking up dormant begonia tubers begins indoors, typically during late winter or early spring. To ensure the plants are mature enough to bloom early, start the tubers approximately eight to twelve weeks before the average last expected frost date in your region. This head start allows the tuber to break dormancy and develop a robust root system before being exposed to outdoor conditions.
When acquiring tubers, inspect them carefully to select only those that are firm and appear healthy, avoiding any that feel soft or show signs of mold. The necessary planting medium should be a light, well-drained potting mix, as heavy garden soil retains too much moisture and can lead to rot. Shallow trays or small pots, ideally three to four inches in diameter, should be prepared, making sure all containers have adequate drainage holes.
The Planting Process
Before planting the tubers, the potting mix should be moistened slightly to ensure it is damp but not soaking wet. Overly wet conditions are a primary cause of tuber rot before the plant develops roots and shoots. Fill the chosen container with the prepared potting mix, leaving approximately an inch of space below the rim.
The most important step in the planting process is correctly identifying the top and bottom of the tuber. A begonia tuber is not perfectly round; it is bowl-shaped, with one side being rounded and convex, and the other side being slightly concave or hollowed out. The concave, indented side is the top, as this is where the growth points, often called “eyes,” will emerge. The rounded side is the bottom, where the roots will form.
Place the tuber directly onto the surface of the moistened soil with the concave side facing upward. Gently press the tuber into the soil so that it is secure. Plant the tuber shallowly, either barely covering it with a thin layer of soil or leaving the top rim visible. Covering the tuber deeply obstructs oxygen flow and increases the risk of rot.
Initial Care and Transplanting Outdoors
Initial Indoor Care
After planting, place the containers in a warm location that receives bright, indirect light, avoiding direct sun which can scorch developing shoots. A consistent temperature of around 70°F stimulates the tuber into active growth. Watering must be done with extreme caution during this initial stage, as the dormant tuber is highly susceptible to fungal diseases and decay.
Water the soil very sparingly until the first signs of growth appear, which may take several weeks. Keep the potting mix just barely moist, not wet, to encourage root development without causing rot. Once shoots and leaves appear, move the plants to a location with more light, such as a bright windowsill or under a grow light. Watering can then be increased slightly, maintaining consistently damp but not soggy soil.
Transplanting Outdoors
Young begonia plants must be gradually acclimated to outdoor conditions before being permanently moved outside. This process, known as hardening off, involves taking the plants outdoors for increasingly longer periods over one to two weeks, initially placing them in a sheltered, shaded spot. Final transplanting should not occur until all danger of frost has passed and nighttime temperatures are reliably above 50°F to 60°F. Once transplanted, the final location should offer filtered or bright, indirect light, as direct afternoon sun can burn the foliage and damage the blooms.