If you’ve been to a garden centre this past week to look at the packaged summer bulbs, you might have noticed a small open bin of begonia tubers. The tubers are sensitive to humidity and they’re packed in an open mixture of peat and straw to prevent them from developing mildew fungus. These lumpy, brown potato-like forms will grow to produce large double blooms on upright or hanging plants in many colours and styles, some with rose form flowers and others with frilled and bi-colour petals. I bought two each of white and apricot, double rose form.
The tubers have a concave top with a scar where last year’s stem was cut, and a rounded bottom with dried root remnants. The best way to select healthy tubers is to feel their weight. Those that feel weighty and solid have good moisture content and are full of energy, just ready to grow. A comparatively lightweight tuber may have been subject to too much heat or exposure in storage, and contains less stored energy. There may be small pink buds on the top, and those are the beginnings of this year’s leaves. If buds are present, handle the tuber carefully so that they remain in place.
Begonias are easy to grow under plant lights or near a bright window, starting in mid to late March. They like lots of air circulation and temperatures between 15° and 24°C. Select a pot two to three inches (5 to 8 cm) wider than the tubers and fill it two-thirds full of soilless mix. Set the tuber into the pot with the rounded bottom resting on the soil, and fill with more mix to cover the tuber. Water the begonias just enough to keep the soil moist, but not noticeably wet. In two to three weeks you’ll notice the soil surface crack, and soon small leaves will emerge. When roots fill the pots, transplant them into six- to 10-inch (15- to 25-cm) pots, using more soilless mix. You might want to mix in some time-release fertilizer granules, formulated for container plants (check the package for how much to use, and for how long it’s active). You can also use a liquid fertilizer with a higher middle number (similar to 5-15-5) to promote flower bud formation, applied every three to four weeks through the growing season.
Begonias grown outdoors in summer should be protected from hot sun, and prefer a cool location with morning light. Light breezes keep air circulating around the plants and help to prevent mildew. Remove the flowers when petal edges are brown, rather than allow the aging flowers to shatter and fall into the pot, where they will rot on the soil surface.
Begonias are monoecious, with (double) male and (single) female flowers on the same plant. Each double male flower has two single female flowers (sometimes referred to as pseudo flowers), one on each side of the double blossom. The double male flower is always in the middle, and will appear larger. Removing the smaller female flowers (similar to disbudding chrysanthemums) prevents seed formation, and directs more energy to enlarging the remaining double male flower. Do this as early as possible when the flowers are still in the bud stage, using a small, sharp pair of pointed scissors (such as embroidery scissors). You might want to let the first set of male and female buds grow a bit larger before attempting to disbud the set, just to get a good look at what their arrangement is. Thereafter, you can continue disbudding through the summer as more flowers are produced. But even if you don’t bother with the disbudding process, you’re going to really enjoy growing begonias!