Raising Begonias: Dust Fine Seed, Starting Tubers and Rooting a Single Leaf

Sowing dust fine seed on the surface

Begonia seed is close to dust and behaves like it. A packet holds thousands, a breath scatters them, and they are almost invisible on damp compost. The seed also needs light to germinate, so it must never be covered with even a dusting of compost. Fill a small tray with fine seed-starting mix, level it, firm it flat with a board, and water it thoroughly from below by standing the tray in an inch of water until the surface darkens. Do all the wetting before you sow, because watering from above afterwards washes the seed down into the compost and out of the light where it will not germinate.

Mix the seed with a teaspoon of dry silver sand before scattering it. The sand shows you where the seed has landed and stops you emptying a third of the packet into one corner. Sow far more thinly than feels reasonable; overcrowded begonia seedlings are too small to separate and damp off as a block. Pelleted seed, coated into small handleable balls, is worth the price for small quantities because it can be spaced individually, and it must still sit on the surface.

Cover the tray with a clear lid or a bag to hold humidity, and hold it at 70 to 75F (21 to 24C) in bright indirect light. Germination takes two to three weeks, sometimes longer, and it is uneven, so do not clear a tray that has only a few seedlings up at day fourteen. Take the cover off as soon as a reasonable number are through, because still saturated air over congested seedlings is exactly what damping off wants.

The reason most people do not grow begonias from seed is the lead time. From sowing to a plant ready to go outside is sixteen to twenty weeks, so the sowing has to be made in the depths of winter under lights. Everything about the plant after that is easy; it is the first two months that need the equipment.

Transplanting seedlings and growing on the seedlings

Begonia seedlings sit as green pinheads for weeks and appear to be doing nothing. That is normal; they are making roots before they make a top, and they accelerate visibly once the first true leaves open. Resist raising the temperature or feeding them heavily during that stall, because warmth without matching light produces long pale stems that never make a decent plant. Light is what to add.

Transplant when the first true leaves are large enough to hold, which is four to six weeks after germination. At that size a plant label or the tip of a pencil is the right tool. Lift each seedling from below with the label, hold it by a seed leaf and never by the stem, and set it into a plug or a small pot at the same depth it was growing. A crushed stem is fatal even though the seedling looks fine for two more days.

After transplanting seedlings, drop the temperature to around 60F (16C) and give the brightest position available. Begin a quarter strength liquid feed two weeks later, once the roots have taken hold, and repeat weekly. Pot up into three inch (7.5 cm) pots before the roots circle the plug. Cool, bright and steadily fed is what makes the short stocky plant that fills out later; warm, dim and hungry makes a stretched one that never catches up.

Note the sowing date and the date you first see green, then the date you transplant. Begonia germination is slow enough that without those written down you cannot tell a tray that needs more patience from one that has failed, and people throw away good trays every year for exactly that reason.

Rooting a leaf, which is far easier than seed

The foliage begonias are increased from leaves, and this is the propagation method the genus is famous for. A single mature leaf will produce several complete new plants, because begonia leaf tissue can regenerate a whole plant from the point where a main vein is cut. It works on the rhizomatous and rex type foliage begonias reliably, less well on the fibrous and tuberous types, which are better done from shoot tip cuttings.

There are two ways to do it. The first is to take a whole healthy mature leaf, turn it upside down, and cut across each of the strong main veins with a clean blade, making a nick about a quarter of an inch (6 mm) long every inch or so. Lay the leaf right side up flat on damp seed-starting mix and pin it down with small stones or bent wire so that every cut vein is pressed firmly against the surface. Within four to eight weeks a small plantlet appears at each cut.

The second way is easier to manage and gives fewer but sturdier plants. Cut the leaf into wedges, each wedge a triangle with a piece of a main vein running to its point. Push each wedge point down into damp compost so a third of it is buried. Plantlets form at the buried vein end. Either way the requirement is the same: bright indirect light, compost that is damp but not wet, and humid air, which means a propagator lid or a clear bag held off the leaf by sticks. Around 70F (21C) is right.

Pot the plantlets individually once they have two or three leaves of their own and are big enough to handle, which is usually two to three months from starting. Leave the parent leaf attached until it yellows; it is still feeding them.

1. Take a mature, unblemished leaf Choose a fully grown leaf from a clean healthy plant with no sign of mildew. Cut it with the stalk attached using a clean sharp blade.
2. Cut across the main veins or into wedges Either nick every strong vein on the underside at intervals of about an inch (2.5 cm), or cut wedges each containing a piece of a main vein.
3. Set it on damp compost with firm contact Lay the whole leaf flat and pin it down, or push the wedges in point first. The cut vein must touch damp compost, or nothing forms.
4. Keep it humid and out of direct sun A propagator lid or a bag held clear of the leaf, bright indirect light, around 70F (21C). Vent it daily so the surface does not stay saturated.
5. Pot up when the plantlets have two or three leaves Lift each plantlet with a little compost and pot it individually. Leave the parent leaf in place until it yellows on its own.

Stem cuttings, and dividing what you already have

For the fibrous bedding types and the cane stemmed ones, a shoot tip cutting is quicker and more reliable than a leaf. Take four inch (10 cm) tips from non flowering shoots, strip the lower leaves, and push them into damp gritty potting mix. They root in two to three weeks at 65 to 70F (18 to 21C). Begonia cuttings root readily in plain water too, though the roots that form there are brittle and the plants take longer to settle after potting, so compost is the better route if you have the choice. This is the way to carry a favorite bedding plant through winter on a windowsill without keeping the whole plant.

Rhizomatous foliage begonias are also divided. In spring, when growth restarts, tip the plant out and cut the creeping rhizome into sections with a clean knife, making sure each piece has at least one growing point and some roots. Lay each section flat on fresh compost with the rhizome sitting on the surface rather than buried, and keep it barely damp until new leaves appear. Burying a rhizome is the usual reason a divided plant rots instead of growing.

Tubers can be divided as well, but only at the point in late winter when the shoot buds are visible and before they have extended. Cut the tuber into pieces with a clean knife, each piece carrying at least one shoot bud, and leave the cut surfaces to dry and callus in the air for two days before potting them up. Dust the cuts with sulfur if you have it. A tuber cut and potted straight into damp compost rots at the wound far more often than not.