If you stored begonia tubers (bulbs) indoors last fall, it’s time to think about waking them up. Most gardeners start them indoors around the end of March or in April so that they’re ready to bloom when it’s time to plant them outdoors for the summer.
This is also the season for buying new begonia tubers. You’ll find them in just about any garden center in the spring, of course, but also on the Internet. Try CalBegonias in the USA, Botanus in Canada and Blackmore and Langdon’s in Britain.
Off On the Right Foot
Tuberous begonias (Begonia x tuberosa, B. boliviensis, etc.) come from fairly moderate climates, neither hot nor cold, and don’t much like wildly changing temperatures, especially when they are sprouting. Since almost all non-tropical climates have very variable temperatures in spring, it’s always best to start these tubers indoors where you can better control their conditions. Plus, that gives you the afore-mentioned head start on the season!
Start tuberous begonias indoors about 6 to 8 weeks before you expect to be able to plant them outdoors, that is, when there is no longer any risk of frost. Depending on your local climate, that could be as early as April, but in most areas north of the Mason-Dixon line, gardeners can’t really be sure of warm weather much before mid- to late May or even June, so consider that the date you should use when determining when to start your tubers.
If you simply brought last year’s tubers indoors in their pot and the pot is stored somewhere dark and cool at the moment, just bring it into a warm, moderately lit spot and start watering. Gentle heat, about 70 to 75˚F (21 to 24 ° C), will help wake them up. Water very lightly at first, until you see strong green growth, a sign the tuber has grown new roots.
If you stored the tuber loose or if you just bought a new one, fill a 6-inch (15-cm) pot to about half its height with barely moist potting soil and place the tuber in the center, on top of the potting mix. Make sure you place it with the concave side facing upwards. It’s likely you’ll see a few pinkish buds at this time, showing that is indeed the side that needs to face upwards.
Next barely cover covering the tuber with potting mix, then set the pot in a well-lit spot and water lightly. Don’t let any water accumulate in the depression on top of the tuber, at least not yet. That could lead to rot.
Normal indoor temperatures of 70 to 75˚F (21 to 24 ° C) are just fine for waking up a begonia. Water as needed so the soil never dries out completely.
When the plant begins to put out strong growth and healthy leaves, gradually increase the watering rate, as it will dry out more quickly. Remember though that you still want to keep the potting mix barely moist and certainly not soaking wet. It’s also wise to put the plant in a cooler spot at this point, as low as 50 to 60˚F (5 to 10˚C): that will give a denser, more attractive plant.
This is also the time to start giving the pot a quarter turn every time you water, always in the same direction. That will help give you a more symmetrical plant that doesn’t lean towards the sun.
Taking Cuttings: An Option to Consider
You don’t have to take cuttings from your tuberous begonia. You can just let your plant grow without interfering… but if you want more plants, the time is right when the stems are about 6 inches (15 cm) tall. That’s because each tuber normally produces several stems, yet you really only need one for good bloom. Removing and rooting the extra stems will give you plants that will bloom the first summer and, in the fall, there will already be a tuber at the base of each one that you can store over the winter. So taking cuttings allows you to increase your collection rapidly.
To start a cutting, simply pull on a stem and you’ll find it will readily break off its base, where it sprouts from the tuber. Insert it into a small pot of slightly moist potting soil: no rooting hormone is required. Cover the pot with a transparent plastic dome to help maintain high humidity and place it in a warm, well-lit spot, protected from direct sunlight.
After 2 to 4 weeks, you’ll see new leaves appear, a sign the young plant is well-rooted. So just remove the dome and replant into a 6-inch (15 cm) pot to give it room to grow. That’s all it takes to produce a new plant!
At the same time as the stems are ready to root, that is, when they’re about 6 inches (15 cm) tall, it’s also time to add more soil to the pot. Fill it to about 1/2 to 1 inch (1-2 cm) of the brim, thus covering the lower stem. Burying the stem in this way encourages it to produce roots and gives a stronger plant, less likely to snap in the wind.
As outdoor temperatures become warmer and nights remain above 50˚F (10˚C), you can start acclimating your begonia to outdoor conditions. Put it outside on a warm day, in the shade at first, then bring the plant back in the evening if the night is going to be cool. Repeat this daily, letting the plant spend the night outdoors when temperatures allow and gradually increasing the light it gets. Tuberous begonias are essentially part shade plants: they like a bit of sunlight each day and shade the rest of the time. Multiflora begonias, like the ‘Non Stop’ series, are more sun tolerant and when properly acclimated, will grow in full sun in cool summer climates.
Outdoors for the Summer
When there is no longer any risk of frost, you can plant your tuberous begonia outdoors in the summer or just place its pot outside. Trailing varieties will of course look best planted in hanging baskets or flower boxes.
During the summer, water and fertilize your begonia just like any garden plant. And in the fall, of course, don’t forget to bring the tuber back indoors after the first frost.
Other Tender Bulbs to Start Early
The tuberous begonia is not the only tender bulb worth starting indoors in spring. The same kind of regime (planting indoors in a pot to give the plant a headstart on the season) can also be applied to other bulbs that you stored indoors over the winter: alocasias cannas, callas, caladiums, dahlias, gladioli, etc. It’s also time to pot up pelargoniums (geraniums) you stored dormant. Here’s an article on getting them too off on a good foot: Time to Wake Up Dormant Plants.
Starting tender bulbs indoors is one of the gardener’s rites of spring. Enjoy!