Pelargoniums root readily
Zonal pelargoniums are easily propagated by rooting cuttings. Our great-grandmothers probably did this by sticking cut stems into the soil beside the parent plant but results are more certain with more careful procedures.
When the weather is warm, cuttings will give a high rate of success particularly if some simple precautions are taken. Select vigorous maturing shoots, preferably ones that show a slight pink tinge to the green of the stem. With a clean, sharp blade cut a piece with three or four sections of stem, making the cut just above a leaf so as to leave no stump to die back on the donor plant.
Cleanliness improves success
Like surgery on people, results are best if everything is clean and as close to sterile as possible. The cutting is stripped of all but a couple of the uppermost leaves and of the stipules that flank them. The stem is then trimmed cleanly just below the lowest joint to leave a cutting three or four sections long.
For maximum success each cutting is completely immersed in a suitable antiseptic (We usually use “No Damp”, a proprietary treatment against damping off of seedlings, diluted as directed. “No Damp” contains 8-hydroxyquinolinium benzoate.) Excess liquid is shaken off and the cut end only is touched into rooting compound. It is important to emphasize that only the cut end is treated as zonal pelargoniums are sensitive to these compounds and an overdose can kill the cutting. For this reason we prefer a rooting powder in a brand that comes in three strengths, using the weakest, designated for soft wood. (One-strength brands may be diluted two to four times by mixing with talcum power. Gel forms are usually too strong and difficult to dilute.)
Some people prefer not to use a rooting compound and have a high degree of success.
Containers for cuttings should be clean and new or well rinsed after a soak in disinfectant, either a household variety or diluted bleach. (Clay pots will require extensive soaking to remove the disinfectant.) They are then filled with non-infected, porous medium. This may consist of clean peat mixed with an equal volume of perlite, pumice or sand. A good potting mix will work but added perlite helps improve it.
Inserting the cuttings
When the pot is full, it may be watered lightly with diluted “No Damp” and allowed to drain fully. Again, an overdose can inhibit rooting and clean water may be preferred. Cuttings are inserted in holes made with a dibber or chopstick to about half their depth and gently firmed in with the base of the cutting in contact with the medium at the bottom of the hole. The cuttings are then placed in a moderately warm place out of direct sunlight. Under these conditions zonal pelargonium cuttings should root within 10 to 14 days and normally require no further watering during this time. Cuttings will rot if over-watered. It is inadvisable to spray the leaves except in the driest atmospheres or to cover the cuttings in plastic. It may do no great harm if the cuttings are carefully lifted to examine them during rooting but it should be remembered that the new roots can easily be broken. Without having to lift the cuttings, rooting will be indicated by renewed growth of the unexpanded leaves.
Regal pelargonium cuttings often take much longer to root, though they may first callus heavily. Once callused they will usually root eventually.
During rooting any dead or dying leaves should be carefully picked off. Plants should be potted up before the roots become heavily intertwined. When potting, the tops should be stopped by pinching out the smallest growing tip to promote branching. For best growth the cutting may be started in individual small pots, thus avoiding damage in transplanting.
Avoid over-watering
Over-watering kills more pelargoniums than any other mistreatment and they do best in an open, well drained medium. Except in the coldest weather, water the pots well, remove any water that drains into the saucer and do not re-water until the medium is almost dry again.
While cuttings for the next season are usually taken in the latter part of August, before the cooler and damper nights begin, they may be taken at any time provided the medium is kept slightly warmed, for example by heating cable, and the atmosphere is kept free of dampness. Cold, dull conditions and short days make rooting difficult in winter.
Common causes of failure are: Too much or too strong rooting compound; too cold or too wet conditions; too high humidity created by covering the cuttings; infected cuttings or potting medium; medium allowed to dry too much.
Ian C. Gillam 2008