Vegetative propagation: cuttings
Two methods – or more precisely two organs – are used for the vegetative propagation of begonias: the leaf or the stem. The growth form more or less dictates the plant’s ability to produce roots and, above all, new leaves or new stems. As a rule, rhizomatous begonias are propagated from leaves and upright forms from stems. There are of course exceptions: some rhizomatous (or similar) begonias can only be propagated from stems, and some upright forms are in fact “stemmed” rhizomatous begonias that can be propagated from leaves (mainly Asian species: Begonia diadema, Begonia deliciosa, nepalensis…).
Leaf cuttings
Many techniques are used: whole leaf, piece of leaf, square of leaf, with or without petiole*. Choose a leaf that is slightly mature, but not necessarily dried out. It must be healthy, with no sign of past or present disease. Your tool should be as sharp as possible (razor blade, grafting knife or sharpened secateurs), and you should make the cut in one quick, confident movement. Cut the leaf with a very short piece of petiole. Paradoxical as it may seem, the best rooting ability is concentrated in the 5 mm of petiole closest to the blade*. Be very careful that the cut end of the petiole is clean and neat, with no bruising. Let the wound dry for a few minutes – not long enough for the leaf to wilt, but enough for the cut to “heal” a little.
The potting mix matters: it must drain well so the cutting does not rot, yet hold enough moisture to keep it hydrated; be light enough not to compact and smother it, yet heavy enough to hold it firmly in place. The most classic mix, and the easiest to make at home, is equal parts peat and fine sand. Mix them thoroughly and break up any lumps. If you are feeling keen, sieve it.
Take a tray with a layer of fine crocks or expanded clay balls at the bottom for drainage. Spread your mix without pressing it down too much, leaving a small 2 cm “rim” at the top (in a few weeks this hollow will let you water more generously without overflowing, to moisten the young plant properly). Soak the tray from below rather than watering from above, to avoid crusting and compaction. Lay your leaf flat, pressing it against the surface of the substrate as much as possible. Bury the end of the petiole slightly, but do not firm too much, especially if your sand is not very fine, as you could damage the cutting. If your leaf is large, there is nothing to stop you nicking the forks of the veins on the underside, as you will often have read.
Water lightly, just enough to moisten the cutting and help the substrate settle against it. The leaf will root in two to four weeks, depending on the species or cultivar. Always be very careful with watering – it is the crucial point. Never let the substrate dry out, especially in the first two weeks, but do not flood it either: “neither too much nor too little”. No fertiliser, of course – even rooting hormones are unnecessary. Only repot when the root ball is about the same diameter as the leaf. Be very gentle with your young plant and its root system: it is still weak and breaks easily.
Repot into a substrate that is still fairly light but already more nutritious. This is an intermediate growing-on stage. The roots must still develop and take the shape of the pot before, a few weeks later, moving into a final, larger container that will stay the same until flowering. For now, plant in a mix of peat (2/5), fine sand or fine vermiculite* (2/5) and potting compost (1/5). Do not rush, of course. Choose a suitable pot and never go above ten to twelve centimetres. Again, do not firm it yourself, or only just enough to avoid air pockets. Simply water with a rose: the water will gradually settle the substrate around the roots.
Stem cuttings
This is the more classic method, the only variation being whether you use a section of stem or its tip.
Take your cuttings carefully, without cutting haphazardly. Cut cleanly, if necessary trimming the cut a second time. There must be no sign of bruising or torn tissue on the stem. As always, we insist on using the cleanest and sharpest tools possible (grafting knives or secateurs). If you use them often, sharpen them often.
The mix is the same as for leaf cuttings (1/2 fine sand – 1/2 peat). Mixed thoroughly, it can even be sieved, especially if your peat is a little coarse. Moisten it well before planting: it is important that the substrate has fully absorbed the water. Plan ahead, as a slightly dry substrate can take a while to re-wet.
On the whole, choose fairly stocky stem tips with the shortest possible internodes. The ideal cutting is about ten centimetres long at most. That said, everything is relative: the stem of some mature cane-like begonias can be twenty centimetres if the internode is that long. As long as the stem is not too soft, it can root perfectly well. “Dress” the cutting by removing, as cleanly as possible, the flowers, the buds of the first two nodes and the lowest leaves, keeping only two or three depending on their size. This limits evapotranspiration* and so preserves as many reserves as possible for your future “baby”. Always cut just below a node, of course, leaving only two or three millimetres.
You can root cuttings directly in small pots: one cutting per 7 cm pot or three per 9 cm pot. If you have a lot and are not sure of the result, start with a seed tray. Afterwards, repot only the best ones, discarding those that look too weak or have not rooted quickly enough.
Repot in the same way as leaf cuttings, only after six weeks or even eight to ten weeks, once the volume of roots is larger than the “crown” of the cutting. Do not wait if you see the new roots are cramped, coming out of the drainage hole or even forming a mat on the surface.
If you later want to keep plants low and bushy, pinch them out very early, from the second or third month, cutting just above a bud. Do not pinch above a flower stalk or flower scar. Speaking of flowers, get into the habit of removing them from your young cuttings, which tire quickly if they have to devote themselves to flowering with no reserves and a root system too weak to support any feeding.
Keep the cuttings or pots warm and enclosed, at least for the first two or three weeks. Maintain good humidity to help them through the early stages.
Finally, one last tip: do not overwater, or you risk seeing your young plants rot very quickly.
