Tuberous begonias are highly valued for their vibrant, continuous color displays throughout the summer, making them a popular choice for brightening shady areas. These flowering plants originate from tubers, which are modified underground storage stems, rather than true bulbs. Proper preparation and planting technique are necessary to ensure a robust growing season and abundant blooms. Understanding the precise timing and depth for planting these tubers is the foundation for successful cultivation.
When and How to Start Begonia Tubers Indoors
Starting begonia tubers indoors gives them a necessary head start, resulting in earlier and more substantial summer flowering, especially in regions with shorter growing seasons. Begin this process eight to ten weeks before the average last expected spring frost date. This ensures the plants are well-developed and ready for outdoor conditions once the weather is reliably warm.
Select shallow trays or pots with adequate drainage holes. The growing medium must be light and well-draining to prevent rot. Use a mixture containing peat moss, perlite, or a high-quality potting mix to maintain moisture without becoming waterlogged.
Place the trays in a warm environment where temperatures remain consistently between 60°F and 70°F. This warmth coaxes the dormant tuber into active growth. Once sprouts appear, position them in bright, indirect light to encourage sturdy stem development.
Water lightly around the edges of the growing medium to keep it barely moist. Avoid letting water pool in the center of the tuber, as overwatering causes rot before roots establish. Sprouts should become visible within two to six weeks.
Correct Planting Depth and Placement
The orientation and depth of begonia tubers directly impacts root and shoot development. Tubers are not perfectly round; they feature a slightly concave (indented) side and a rounded (convex) side. The growing points, or eyes, from which the stems emerge, are located on the concave side, which must face upward when planted.
The rounded, convex side of the tuber is where the roots develop, so this side must be placed down into the growing medium. Plant the tuber just below the soil surface, covering it with no more than half an inch of soil. Some cultivators recommend leaving the top slightly exposed until the first sprouts are visible, then adding a thin layer of soil.
If starting in pots, select a container size appropriate for the tuber, with a minimum six-inch pot recommended for a single plant. Excellent drainage is a requirement for begonia health. When transplanting sprouted tubers, be gentle with the newly formed, brittle stems to prevent breakage.
Transitioning Plants Outdoors and Ongoing Care
Before moving young begonia plants outdoors, they must undergo “hardening off” to acclimate them to harsher conditions like wind and fluctuating temperatures. Begin this process approximately two weeks before the last expected frost date. Initially, move the pots outdoors to a sheltered, shaded location for only a few hours each day, bringing them back inside at night. Over the two-week period, incrementally increase the duration they spend outdoors and gradually expose them to brighter light.
The final outdoor planting should only occur after all danger of frost has passed and nighttime temperatures are consistently above 50°F. Tuberous begonias thrive in partial shade, preferring bright, dappled light or morning sun. They must be protected from the intense heat of the afternoon, as full sun will burn the leaves.
Ongoing care centers on careful watering and feeding. The soil must be kept consistently moist but never soggy, as waterlogged conditions quickly lead to tuber rot. Allow the top inch or two of soil to dry slightly between waterings. Once established, apply a balanced, water-soluble fertilizer every few weeks throughout the growing season to support continuous flowering.