Taking stem cuttings of coleus and philodendrons is simple. Just insert the stems into a pot of moist potting mix, cover them with a small plastic bag or dome to create a greenhouse effect, and in a few days or weeks the roots will begin to form. You can even root them in water, although the success rate in the long run is significantly lower. (Read Starting Cuttings in Water: Not Such a Good Idea to understand why.)
With cacti and succulents, however, it’s a whole different story. Cuttings placed directly in water are as likely to rot as they are to take root. And even when they do root in water and you think you’ve succeeded, so you plant them in a pot of soil, then they rot! And if conditions aren’t perfect, even succulent cuttings planted directly into a pot of moist potting soil—the method which works so well with non-succulent plants!—also often rot. It can be very discouraging!
Let the Wound Callus Over
It’s the open wound on the cutting—the gaping hole where you cut the stem free of the mother plant—that is the usual source of rot. It’s soft and moist: a real treat for nasty fungi and bacteria. And the thicker the stem, the larger the wound … and the more likely rot will set in! Well, many succulents have exceptionally thick stems.
However, if you allow the wound to heal over first—that is, get it to form a callus—, the risk of rot is nearly eliminated. And succulent collectors have found a way to do exactly that.
They cut the stem free of the mother plant, but don’t pot it up nor place in water right away. Instead, they lay it on its side with its wound exposed to the air. Then they wait for the wound to callus over before potting it up. That can take a few days, a few weeks, even months in the case of really thick-stemmed cuttings. But it does eventually happen. Once the callus covers the wound and is woody and hard, the risk of rot if pretty much zero. Problem solved!
Well, partly solved. Because this can cause a complication.
If you leave the cutting on its side long enough for the wound to heal, not just a few days or weeks, but months, the tip will start to bend upwards. Because even a non-rooted succulent cutting is alive and will try to grow. This gives you a new plant with a permanent kink in its stem.
This is mostly a problem with slow-to-callus cuttings like euphorbias and cacti.
Use Air Rooting to Keepin a Slow-Healing Stem From Curving
There is, however, a way to succeed with succulent slow-to-callus stem cuttings while keeping them beautifully straight and symmetrical. You have to hold the stem upright as the callus forms and air root it. Yes, root it in the air rather than soil or water. And here’s how to do just that:
Find a terra cotta pot. (You’ll want some weight if you want the cutting to remain upright and most plastic pots are just too light.) Then turn it upside down and insert the cutting into the drainage hole. That will hold the stem upright. If the stem is too thin, you many need to place several cuttings in the hole so that they don’t slip into the pot. Or stuff a piece of rag or tissue paper between the stem and the drainage hole to hold it in place.
With thick cuttings, you have the opposite problem. You often need to enlarge the hole to make it big enough to the cutting to fit into. This is easy to do. Gently break off pieces around the hole’s perimeter with a hammer and screwdriver. Then place the pot supporting the cutting in a well-lit place … and ignore it for a while!
Out of sight under the pot, the cut surface will callus over and air roots will form on the part of the stem kept in the dark. (Yep, even if there is no soil!) And all this time, the upper part of the future plant will receive adequate light and will start to grow slowly. When cutting has completely callused over and started to produce roots, remove the it from its holder and pot it up. Just use the same type of potting soil mixture as for the adult plant, usually a very aerated potting soil. One designed specifically for cacti and succulents, for example, would be fine.