Wayne Handlos, Ph.D.
The most common method for propagating pelargoniums/geraniums is by cuttings. This is what in the past was called “taking slips”.
While it is possible to grow geraniums from seed, most highly regarded cultivars can only be reproduced consistently by cuttings. Open-pollinated flowers will not usually come true from seed in pelargoniums. So the only way to reproduce the plant you see is through vegetative means, i.e. taking cuttings and maintaining the clone.
Cuttings are taken from the ends of actively growing stems. The terminal two to four inches are cut from the parental plant with a sharp knife, razor blade or shears. The shoot should have at least two to four leaves. Cut the stem just below a node (the point where the leaf is attached to the stem) [link to anatomy]. Roots will only develop from the region of a node. Remove the leaves from the lowest one or two nodes. Bend the leaf until it breaks cleanly from the stem. (In some of the scented varieties of Pelargonium, the leaves may have to be bent carefully to the side so as not to strip tissue from and damage the stem.) Ideally the cutting should still have two to four large leaves.
The cutting is rooted in potting mix which is prepared by adding perlite or vermiculite to a commercially prepared potting mix. The mix should be about one-third to one-half perlite or vermiculite. It is easiest to root the cuttings in their own pots; a two inch pot is about the right size. Make a hole in the potting mix (a pencil works well) and insert the cutting so that at least one node is covered by the mix. Push the mix gently around the cutting to firm it in place. Add a label to the pot so you remember the name of the cultivar. (Use a pencil to write the label because many permanent inks fade in sunlight. Some inks are completely soluble in water.) Water the cutting gently and thoroughly so that water comes out the bottom of the pot. Keep the cutting out of direct, hot sun for a day or two. Keep it moist but not soggy. Some modern zonal varieties may root within two weeks (they are selected for their rooting ability), but some varieties may take much longer. Older, woody stems tend to root more slowly too and you may have to wait six weeks or more for a root system to develop.
If you are making cuttings from several different cultivars, disinfect your knife, blade or shears between plants. This will reduce the transmission of disease organisms and viruses from plant to plant. Be sure to wash old pots before reusing them.
Plants may benefit from bottom heat especially during the cooler days of the year. Propagating mats are available for this purpose at some garden centers.
When you see roots coming out of the bottom of your pot, the cutting is ready to be planted in regular potting mix. Invert the pot and gently tap it against a solid surface until the plant and soil fall from the pot. Carefully place the rooted cutting in a slightly larger pot, add soil, tap gently to settle the mix and water gently. You are on your way!