The most demanding part of tuberous begonia care is wintering the bulbs. Begonia bulbs require a dormancy period. That means they need to go to sleep at certain specific times. Here's how to put them to bed and how to wake them back up.
These tuberous Begonias have been used to form the top of this tree at Longwood Gardens. More of the same Begonias mulch the soil.
Tuberous Begonias growing in warm climates will go dormant in response to the changing day length in the fall.
Sometime around October or November the plants will turn yellow and drop their leaves. This is your queue to stop watering them. Once all the top growth falls away, it's time to dig the tubers up.
If they are in the ground, dig them up and store them dry over the winter. If they are in pots, leave the bulbs in the soil and just move the pots to a dry location.
Dig the bulbs up after the first frost/frosts has killed the top growth. Begonia bulbs buried in the ground can handle a light frost, but a hard freeze will kill them so be sure to get to this before real cold sets in.
Remove the soil from the tubers and lay them on newsprint inside your house or frost-free garage to let them dry thoroughly.
Check the bulbs for rotted spots before you store them.
If you see any rot, cut it out with a sharp knife. Dust the cut with a fungicide. If you don't have any, use ground cinnamon. It's a natural fungicide that is commonly used by orchid growers.
Store each bulb in a separate paper bag. Lunch bags will serve. Place the bags into a cardboard box for the winter. Store the box in a dark, dry, cool location. Storing them in this fashion creates airspace around each bulb and keeps any rot that might be in one from spreading to the others. Check the tubers occasionally to make sure they are dry.
Red Begonias bedded out with marigolds and Dusty Miller.
Another important aspect of tuberous Begonia care is reawakening them in the spring.
Timing is important here. You want to wait until the bulbs sprout before replanting them.
They should do this on their own after they've had enough sleep. If they don't--or if you get tired of waiting--move the box or pot to a warm room. The change in temperature should trigger sprouting within a couple of weeks.
One the bulbs sprout, plant them 5 inches apart in a shallow container filled with any free-draining potting mix. There should not be any fertilizer added to the soil.
Cover the bulbs completely with soil as they root from all sides. Water the flat and keep it at room temp. The sprouts need light in order to grow but they can't handle full sun.
Do not water the flat again until the soil begins to dry slightly.
When the plants are 5 inches tall, transplant them into their permanent locations.
By practicing the kind of tuberous Begonia care outlined here, you can enjoy the luscious blooms these bulbs produce for many years.