Prepare Your Tuberous Begonias for Frost: Five Essential Steps to Save Them

In spring garden centers stock a tempting variety of brightly colored tuberous begonias that are hard to resist. Gardeners have a tremendous choice of flower color in shades of orange, pink, yellow, white and bi-colored.

But these lush plants don’t grow over winter and must be protected. The tubers can be saved and planted again the next spring for another year of showy color. The tuberous begonias should not be confused with the low-growing wax begonias often called annual bedding begonias that don’t respond to winter protection.

“There are so many choices for tuberous begonias,” said Nicole Sanchez, Oregon State University Extension Service horticulturist. “They come in so many colors and there are a lot of new salmon/orange varieties on the market now. In my experience, the yellow ones are always a little less vigorous than other colors.”

Tuberous begonias should be grown in bright shade (too much sun will burn them and too little will make them leggy) and watered only when the soil dries out completely.

“We can easily love them to death,” Sanchez said. “The biggest problem with tuberous begonias is operator error in the form of overwatering. Lots of water is already stored in the tubers and stems so be careful to check moisture in the soil at least an inch in, not just on the top. But there’s a fine line between letting them dry out well and oops! it dried out too much. This is where I most often fail with begonias.”

Tuberous begonias don’t like potting soils that contain peat or sphagnum moss that hold lots of water, she added. All of that being said, if they are outside in hanging baskets, they will still need to be watered every day during hot weather.

Begonias are heavy feeders and will bloom larger and longer if they get regular fertilizer, Sanchez said. She recommends slow-release fertilizers for begonias, which are easier to apply and make it harder to over fertilize. Over-fed plants often get leggy and have weak stems.

Tuberous begonias contain both male and female flowers. The male flowers are the showier blooms. Sanchez recommends removing female flowers so all the plant’s energy goes into producing showier male flowers.

When winter looms, it’s time to prepare and store the plants. Sanchez recommends the following five steps to save your potted tuberous begonia tubers:

Remove plant from pot before hard frosts occur. Cut back most of the top of the plant, leaving the ball of roots and soil intact.
Place in a dry, cool storage area (a basement or garage) and allow the tubers to cure for several weeks.
After curing, shake off the soil and remove the remaining stalks and roots. Any stalk or root left has the potential to rot and spread to the tuber. Put the tubers on screen trays or pack them in dry peat, sawdust, sand or other insulating material.
Store tubers in a dry, dark, cool (above freezing) area.
In the spring, start begonia plants by placing the tubers on damp potting soil in a warm environment. When roots and tops have started, plant them in pots in rich, well-drained potting soil. Bring outdoors when all danger of frost is past and place in a shady spot.