Growing tuberous begonias

Judith Adam

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Male and female blooms on tuberous begonias. Photo courtesy of
Male and female blooms on tuberous begonias. Photo courtesy of
Male and female blooms on tuberous begonias. Photo courtesy of

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!

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7 thoughts on “Growing tuberous begonias”

  1. You mention crocosmia. When did you write this? Where can crocosmia be purchased? In what month is it available? When is the best time to plant?

  2. I believe the only Begonia that could be propagated in this fashion is Begonis grandis. It makes small “bulblet” like growths in the leaf axils. They can be collected in the fall and planted indoors to start small new tubers.
    Regular tuberous begonias will not have offsets. Large tubers may be cut into a couple of pieces if there are sufficient eyes. The cut surfaces should be allowed to dry for a few days to callous. A dusting of garden sulphur would likely prevent potential rot. There used to be a fungicide called Bulb-dust, but I’m not sure it is still available.

  3. Hi Gail (March 14),

    To store the tubers over winter, allow the first light frost in autumn to ruin the begonia foliage. Then take it out of the pot, and gently brush off all soil from the tuber and its roots. At this stage, leave the upper stems intact on the tuber, and lay the plant on newspaper in a warm place to dry for several days. When the tuber is sufficiently dry, the upper stems should detach easily. If they seem still anchored on the tuber, let the plant dry for a few more days until the main stem will easily come off.

    When the tuber is thoroughly dry and detached from the stems, examine each for any areas of rot. If you find a soft spot, it can be carefully excised with a pen knife and the cut flesh dusted with powdered sulfur. Wrap each tuber in a single sheet of newspaper (or small paper bag), and put them into a cardboard box with a lid, like a shoe box (no plastic bags). Keep the box in a cool (slightly lower than room temperature) and dark place until starting the tubers again in spring.

    I’m going to do it myself!

    — Judith

  4. I read in book – that I can’t recall offhand – that if you take the peat mix the packaged begonias come in and dampen it a little; then put the bags in the refrigerator vegetable drawer or some other cool dark place, it will promote the scales that have broken off to set bulbets that can be planted. With care they are supposed to be ready to flower within three (3) years.

    Though I’ve grown begonias in the past, I never collected the bulbs to save them at the end of the year. After I considered the cost of buying more begonias this year, I’m trying out the chilling for bulbets for the first time and also have committed to collecting and storing the bulbs in the fall.

    Thanks for a great article!


  5. Last year, I discovered that tuberous begonias also grow well from seeds started under the lights.
    How does one save the tubers over the winter?

  6. One note that might be added for the benefit of those planning to set tuberous begonias in an outdoor bed. Starting them too early can result in very lanky growth which poses difficulty in the necessary hardening off process. Most home gardeners don’t have the greenhouse facility to ensure short stocky growth. There is also quite a difference in various areas of the country relative to frost-free dates. Here in our part of Nova Scotia, we can rarely plant begonias out until the first week of June, so a start towards the end of April will produce plants that will withstand the move better.
    One drawback to this is a slightly later bloom time, but that again is a trade-off with strong plants. Considering that begonias will bloom well into the autumn in areas where early frost isn’t a hazard, that is quite acceptable.


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