Time to Wake Up Your Tuberous Begonias

By Larry Hodgson

The tuberous begonia (Begonia × tuberhybrida) is among the most popular summer bulbs. Its ability to produce beautiful, large, colorful flowers all summer long makes it a star. And it can do this even in fairly shady spots where so few other plants bloom well.

But to have it in bloom from the start of summer, you have to start this begonia indoors several weeks in advance. Usually at least 6–8 weeks before the last frost date. That can be as early as January or as late as May, depending on your local climate.

Regrowing or Fresh Purchase?

If you already have tubers from last year, just pull them out of their storage space, probably in the basement. Otherwise, begonia tubers are on sale in garden centers everywhere. Some even offer small tuberous begonia plants in plug trays. They’re already growing, ready to transplant into a 6-inch (15-cm) pot. Sometimes these little plants are even in bud!

The tuber resembles a flattened, fleshy brown bowl, as its upper surface is usually concave. It’s in this hollow that you already see, in earliest spring, the little white or pink shoots that will become the future flower-bearing stems. When you take them out of storage, the tubers are more or less in suspended animation. They’re waiting for you to plant and water them so they can continue their growth.

Simple Planting

Planting is certainly simple enough. Take a 6-inch (15-cm) pot and fill it about halfway with moist potting soil. Place the tuber in the center, on the potting soil. Just make sure the concave side, the ones with the shoots on it, is facing up. Now, pour in potting soil, barely covering the tuber. Water lightly. Just make sure you don’t leave any water sitting in the hollow of the tuber. That could lead to rot. Place the pot in a moderately well-lit location at about 70 to 75°F (21–24°C) to stimulate growth.

After a week or two, the shoots with lengthen and begin producing leaves. As they grow, gradually increase watering, just enough to keep the soil slightly moist at all times. If possible, also, lower the temperature at this point to around 60°F (15°C) at night. That isn’t absolutely necessary, but it does tend to produce a stockier, denser plant.

When the shoots are about 6 inches (15 cm) high, add more potting soil to the container. You can fill it up to ½ inches (1 cm) from the edge. The part of the stem that ends up covered in soil will then produce roots. This will result in a stronger plant that is less likely to break in the wind.

That the flowers of the tuberous begonia are edible? They offer a delicious tangy taste to salads, poke bowls and smoothies, not to mention giving your meal some very pretty colors!

Hardening Them Off

When outside temperatures warm up, you can begin to acclimatize your begonia to outdoor conditions. Take it outside on a fairly warm day (60°F/15°C or more), placing it in the shade at first. Bring the plant inside in the evening if the temperature drops below 60°F (15°C). Repeat daily, gradually increasing the light intensity each day for a week or so. Most tuberous begonias prefer growing in partial shade. Multiflora begonias (such as Non Stops), though, can tolerate full sun.

Transplanting and Care

When there is no more risk of frost and night temperatures remain reliably above 60°F (15°C), you can move them to their summer home. Transplant your tuberous begonia to the garden or to a planter. Plant hanging varieties in a hanging basket or window box.

During the summer, water and fertilize tuberous begonia as you would any annual.

And in the autumn, of course, don’t forget to bring the tuber back indoors after the first frost. You just have to keep it dry and dormant in your basement over the winter.

Other Bulbs to Start Indoors

It’s not just the tuberous begonias that sometimes need a head start indoors in the spring. The same kind of regime (planting indoors early in the spring) also applies to other frost-sensitive bulbs that you have kept stored in the basement during the winter:

• Alocasias,
• Callas,
• Cannas,
• Caladiums,
• Dahlias,
• Gladiolus,
• And many others.

It’s time to wake them up too for an early start to a long and gloriously flower-filled summer!