Begonia (Tuberous Begonias)

Pronunciation: beg-own'ee-uh

Tuberous Begonias are frost-tender plants thriving in bright light with little or no direct sun. Given an early start, they put on a glorious display all summer. We grow them in pots and hanging baskets, but they can be planted in the ground after frost danger passes. Unless in a frost-free climate, lift and store tuberous roots in fall to carry through winter.

Starting Tubers in Flats or Pots

To jump-start a short season, plant tubers on arrival in flats (shallow containers with drainage) or 4–5″ pots. Use light, well-drained potting mix (soilless mix 3:1 with builder's sand). Moisten mix, fill container to 1½″ from rim. Place tubers hollow side up on mix, space 2–4″ apart in flats (one per 4–5″ pot), cover with ½″ mix. Water sparingly, place in bright indirect light. Keep mix moist not soggy. To hasten growth, set on heating mat or radiator (with magazines to prevent overheating). Unsprouted tubers show growth in 2–6 weeks.

Transplanting

After first 2 leaves emerge, transplant flat-started tubers into 4–5″ pots or, if frost danger passed, into lightly shaded outdoor bed amended with organic matter. Lift carefully, set tuber top 1–2″ below surface. In ground, position so leaf points aim at viewer (blooms face same direction). Move 4–5″ pots to 7–10″ pots when roots fill; optionally to 12″ pots for spectacular plants. Set outdoors in bright, not sunny, location after frost threat. Will not grow well in deep shade.

Staking

When upright varieties reach 4–6″, push heavy 18–20″ bamboo stake (or cushioned steel Begonia stakes) into mix on side opposite leaf points. Place stake a few inches from main stem to avoid tuber injury. Fasten with garden twine or soft plastic tape in figure-8. Add ties as plant grows.

Summer Care

Thrives in evenly moist but well-drained soil. Soggy soil causes stems to snap at base. Fertilize monthly with balanced (20-20-20) water-soluble fertilizer. Remove spent flowers by cutting close to stem with sharp knife. Powdery mildew (white powder on leaves) is the main disease. Prevention: free air circulation. If prone, spray homemade fungicide: 1 tbsp baking soda + 2–3 drops insecticidal soap per gallon tepid water every 10 days in hot, humid weather. Once mildew appears, spray commercial fungicide promptly.

Overwintering

Allow growth through November (or until frost) to store energy. Force container plants into dormancy by gradually withholding water. Dig ground plants with soil ball, dry in shed/garage. When stems break free, shake off soil, cure tubers in sun ~4 days. Store in dry peat moss or sand in open flats in cool (45–50°F), dry place. Replant in late winter as above.

Hanging Basket Begonias

Trailing varieties need similar care but look best in shallow suspended containers. One tuber in a 12″ container makes a spectacular display. Note: If stems grow upright and refuse to trail, plants need more light.