How to Plant Tuberous Begonias

Start tuberous begonias indoors about eight weeks before your last frost, hollow side up and barely covered, then move them outdoors into shade after frost, 12–18 inches apart.

How to Plant Tuberous Begonias
When to plant
Start indoors about 8 weeks before your average last spring frost; move outdoors once all danger of frost has passed
Depth
Hollow (concave) side up, covered with ½–1 inch of soil
Spacing
12–18 inches apart in the ground
Sun
Partial shade; shaded for most of the day
Water
Keep evenly moist but never soggy; begonias are not drought tolerant. Water the soil, not the leaves
Soil
Rich in organic matter, moist and well-drained

Step by step

  1. About eight weeks before your last frost, find the top of each tuber. It's the side with the hollow or dent.
  2. Set the tubers hollow side up in a shallow tray of moist potting mix and cover them with ½–1 inch of mix.
  3. Keep them somewhere warm, about 70°F, and moist but not wet. Expect shoots in 5–6 weeks.
  4. Once shoots appear, move the tray into bright light. When stems are about 3 inches tall, feed with a balanced fertilizer at half strength, then keep feeding with a dilute balanced fertilizer through the season.
  5. Thin to the strongest shoots, two or three for a small tuber and 3–6 for a large one.
  6. Starting about two weeks before your last frost, harden the plants off by setting them outside in a sheltered, shady spot for a little longer each day.
  7. After frost, plant them into a spot that is shaded for most of the day, 12–18 inches apart, at the same depth they grew in the tray, and water the soil rather than the foliage to keep mildew away.

After bloom

At the end of August, water less and stop feeding. Dig the tubers when the foliage starts to yellow or after the first killing frost. Leave a few inches of stem on and dry them indoors, out of direct sun, for 1–2 weeks, until the stems come away easily. Brush off the soil, bed them in peat moss or vermiculite in a box, and store in a cool, dark place at 40–50°F where they can't freeze. Start them again next spring.
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