The following article by Barbara Berg appeared in the July/August 1996 issue of the Begonian, official magazine of the American Begonia Society
Most begonias can be easily propagated if a few simple rules are followed. The materials required are inexpensive.
Use the ready-mixed potting media available in plant stores such as Jiffy Mix. Sure-fire Mix. ProMix and Reddi Earth; mix your own of perlite, vermiculite and peat or sphagnum moss, or use sand, sphagnum moss, perlite or vermiculite alone. A little experimentation will help you find the one which works best for your conditions. I use sphagnum moss, (long fiber, not milled) for some tender begonias, or a prepared mix for most varieties and species.
The important requirement of your propagating equipment is that it be able to comfortably hold the media, the cutting, provide the needed moisture, drainage and hold humidity in its environment if necessary for the more tender varieties. Sweater boxes, pots with sheet plastic covers or Wardian-type cases are all appropriate.
With a few exceptions most begonias will propagate from any type cutting. The most important exceptions are that cane and semperflorens are not satisfactorily rooted from leaf cuttings. Semps are most successfully propagated from well branched stem cuttings. If you do not use branched cuttings you will end up with a ~totem poles which wilt never be a satisfactory plant.
Using clean scissors or a knife cut about three to six inches of stem or branch of your plant depending on Its growth habit. Remove flowers, buds, and any lower leaves which would be in the rooting medium. Dip the stem in rooting hormone (Rootone) and insert from one-half to two inches into the damp not wet) medium. You may or may not need to cover them depending on conditions in the area in which your plants are grown.
Rhizomes are modified root stems of varying sizes. With a clean knife cut the rhizome in pieces with one or more Isaves, dip in rooting hormone and place the pieces at a slant in the medium. In some cases rhizomes with no leaves can be laid flat on the mix and rooting will occur. Some plants can be divided with rooted rhizomes cut from the parent plant and potted directly. With all rhizomes be careful of over watering and causing the rhizome to rot.
Mallet cuttings are taken from the plant to include a leaf or branch and a portion of the main stem on each side of the cutting. Dip the cutting in the rooting hormone and insert in the medium with the leaf or branch upright. Canes and branching begonias propagate very satisfactorily using this type cutting.
Using single leaves with a short stem (an incn or less) dip the stem end in the rooting hormone and insert the tip Into the medium. Larger leaves can be trimmed to smaller size to make them easier to handle. Rex begonia leaves can be cut in multiple places across the leaf veins and laid flat on the medium to produce plantlets at each cut.
Leaves can be cut into wedge shaped pieces with the center of each wedge having a sizeable vein, dipped in the rooting hormone and inserted at an angle into the medium. Rexes are rooted commercially in this manner and with a minimum of care it is almost fool-proof.
POTTING THE NEW PLANT
When the new plants appear and have an adequate root growth, transplant them to an appropriate size pot. Over-potting will rapidly kill your plants. A 2 to 3 inch pot will most probably be adequate and re-potting will be necessary in about six weeks or so. Pot the plant in successively larger pots (one inch at a time) each time the root ball is the full shape of the pot, or until the plant is the size which makes both you and it comfortable.
Credits / references:
Begonian, official magazine of the American Begonia Society.
Rooting Cuttings: The Art of Proper Cuttings, by Brad Thompson