Tips for Growing Tuberous Begonias

For bold color in shady gardens, it’s hard to beat tuberous begonias. These lush plants have attractive foliage and big, rose-like flowers that bloom continuously from midsummer to frost. Planted in window boxes, hanging baskets, decorative urns or garden beds, they make it easy to dress up any outdoor living space.

When to Start the Tubers

Tuberous begonias are cold-sensitive plants that need an early start to reach their full flowering potential. For this reason, the tubers are usually started indoors in late winter. With a two-month head start, your begonias will be ready to begin flowering almost as soon as you put them outdoors. Depending on where you live, that means 5 or 6 months of carefree summer color.

Plan to grow the tubers indoors for about 8 weeks. The chart on this page will tell you approximately when to plant the tubers. It’s based on growing zones, so if you don’t know your growing zone, click here.

What the Tubers Look Like

Begonia tubers are firm, brown and slightly hairy with an irregular shape. If you look closely, you will see that one side is domed and the other is slightly concave. Most of the tuber’s eyes (which will sprout into stems and leaves) are clustered inside the concave area. When you plant the tubers, they should rest on the domed side, with the eyes facing up. For photos, see here: Tuberous Begonias: Which End is Up?

For best results, encourage the tubers to sprout before planting. Simply set them on a tray in a warm place with indirect light. When you see sprouts, they are ready to plant.

How to Plant Tuberous Begonias

To save space indoors, you can get the tubers started in small pots (peat pots work well) or a shallow tray. Put 2-3” of moistened growing mix into the container and set the tubers on top (sprouts up) about 3” apart. Cover with another 1" of moist growing mix. Water well to settle the tubers.

Indoor Growing Tips

Begonias love warmth and humidity. Grow the tubers in a warm (70 to 75°F) place and cover the pots loosely with clear plastic to help retain moisture. Moist air and damp soil will encourage quick sprouting and rapid growth. Though the soil should be moist, keep the tubers themselves relatively dry so they don’t rot.

When sprouts are about an inch long, move the pots to a warm, sunny windowsill or put them under grow lights. Water as needed, keeping the soil barely moist, not wet.

Outdoor Growing Tips

When your begonias have 3 or more leaves, you can transplant them into larger containers. Be very gentle when handling them as it’s easy to break off a stem.

Plant one tuber per 6” pot or 2-3 tubers for a 12” pot. If you are planting the tubers into a window box or garden bed, plant them approximately 8” apart. Using slightly bigger pots that hold more soil will make it easier to keep up with summertime watering.

Don’t be too eager to move your begonias outdoors. Wait until the nights are relatively warm (above 50°F) and there’s no threat of frost.

Tuberous begonias are sensitive to the mid-day sun, which can burn the leaves and flowers. Give them partial shade (a half day of the morning or late afternoon sun is perfect) or grow them in filtered light beneath a deciduous shade tree.

Water your begonias when the soil is dry to the touch. Check often during dry, hot weather. Removing spent blooms and wilted leaves will help to keep your begonias healthy and encourage more flowers. For maximum flower production, apply a liquid fertilizer every 2 to 3 weeks.

At the end of the growing season, as days get shorter and overnight temperatures get cooler, your begonias will flower less and the foliage may begin to yellow. If you want to keep the tubers and replant them next year, move the plants to a protected location and let them gradually die back. Dig the tubers and let them air dry for a week. Then store them in peat moss, in a cool, dark, frost-free place.

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