There seem to be two schools of thought about starting cuttings: some gardeners pot up their cuttings immediately. They cut off the stem, remove lower leaves and slip the cutting into moist potting soil without delay. Others claim better success when they take the cutting, then let it lie around for from a few hours, perhaps overnight, until a callus (layer of harder protective tissue) forms. Only then do they pot up the cutting in moist soil and allow the process of producing new roots to begin.
They both are. It just depends on what type of plant you are trying to root … and on your own preferences!
Thin or Soft Stemmed Cuttings
Cuttings of plants with thin or soft stems, especially those with thin leaves (coleus, abutilons, etc.) wilt quickly when the cut end is left exposed to the air. That’s a sign of severe stress and is not good for the cutting. Pot these up immediately … and cover them with a clear plastic mini-greenhouse (dome, plastic bag, etc.) without too much delay to prevent further water loss from the leaves. (Inside a mini-greenhouse, the atmospheric humidity will be close to 100%, reducing transpiration from the cutting’s leaves to almost nothing and thus allowing them to stay turgid.)
Also, keep the substrate of these plants slightly moist at all times until they root (new growth will then appear). Drought stress is never good for them.
Cuttings of plants with thick or woody stems, especially those plants considered succulents, don’t lose water nearly as quickly as softer-stemmed plants. Many have thick leaves that likewise hold onto their moisture; some have no leaves at all. You can actually drop such stems in an envelope and mail them to a friend with no further thought: they’re that tough. And they tend to rot if you don’t wait for the cutting to form a callus, a hard, dry “crust” at the base of the cutting. That can take anywhere from a few hours to a few weeks, even a few months in extreme cases.
In general, the thicker the stem, the longer it takes to seal itself off. Moderately thick-stemmed cuttings like crassulas, sanseverias and echeverias are ready to pot up in just a few days, even only overnight*. Likewise with cuttings of succulent leaves (many plants in the Crassulaceae family—Crassula, Echeveria, Kalanchoe, Sedum, etc.—, as well as species of Haworthia, Gasteria and Sansevieria, among others, root readily from leaf cuttings). There is only a very small wound on a leaf cutting, the spot where it was once attached to the stem, and it heals over quickly. Just leave such cuttings lying around on a tray in a dry spot, in light or in dark, until the cut end no longer looks moist before you pot them up.
*Alternate possibility: if you don’t want to wait to let these thinner-stemmed succulents callus over, pot them up immediately … into a totally dry rooting mix. Then leave the mix dry for a week. That will give them time to callus over out of sight in the growing mix. Then start to water … moderately. They won’t need much moisture until they’ve rooted.
Note that you don’t need to cover succulent cuttings with a mini-greenhouse while they’re rooting. They prefer the lesser humidity of open air.
When Stems Are Really Thick
Really thick-stemmed succulents can take ages to callus over. There are cactus and euphorbias, notably, with stems that can be 6 inches (15 cm) or more in width you’ll need to harvest with a saw and they can take weeks or months to fully callus over. They tend to be difficult to root and the chances of rot are quite high. To root these with as little loss as possible, try to take the cutting in the spring (always the best season for any cutting, with summer a close second) and do so by harvesting a secondary stem (branch), cutting where it joins the main stem: this leaves the narrowest possible wound, one that will callus over more rapidly. That said, you can cut the top of most cactus and euphorbias at their thickest point and the resulting cutting will still eventually callus over, giving you something you can tray rooting … after several months!
One note about euphorbias (genus Euphorbia). They produce copious amounts of white sap (latex) when you cut them. (This sap is highly irritating on most species, so wear gloves, and is also likely toxic, so don’t get it in your mouth or eyes.) To stop the “bleeding”, either dip the cutting in cold water or spray with cold water. That causes the sap to coagulate. Or expose it briefly to a candle or match flame to cauterize the wound.
Keeping Cuttings Upright
Typically, cuttings undergoing callusing are just left lying on their side and that’s fine … if you’ll be potting them up fairly soon, within a few weeks, for example. If the healing process is going to take months (again, mostly thick-stemmed cacti and euphorbias) and you leave them lying on their side in the same position all that time, the tip will start to grow upward—yes, even on a cutting with no roots! —, resulting in a plant with a bend in the stem when you stand it upright again for rooting. Either rotate the cutting regularly as it calluses over, giving it a quarter turn every few days, or find some way of keeping it upright while you wait for the callus to form. I’ve seen gardeners lean their cuttings against a wall, or tie them to stakes to hold them upright, but personally, I just drop them upright into a heavy drinking glass or other upright container. And wait.
Potting Up Callused Cuttings
The “quick-callusers” (i.e. most succulents with fairly narrow stems, like the typical crassulas, sanseverias and echeverias most people take cuttings of) root readily in regular potting or cactus soil. No rooting hormone is needed. One secret though: keep them distinctly on the dry side, barely watering, until you see signs of growth.
Thick-stemmed succulents (again, mostly cacti and euphorbias) are tougher to root and more subject to rot. I recommend applying rooting hormone before you set them in soil (it also happens to have a useful fungicidal effect)… and keeping the soil dry (i.e. don’t water at all!) until you see some growth, usually a sign rooting has started.
Also, keep all cuttings warm throughout the rooting period.
Some plants are in-betweeners: they’re neither true succulents nor true non-succulents. This includes the ever-popular pelargonium, aka geranium (Pelargonium), hoyas (Hoya) and a few others. You can treat these plants either way: some people pot up their semi-succulent cuttings immediately, others let them callus over for a short period, usually overnight. Both ways work.
Might I suggest rooting semi-succulents in an especially well-aerated mix, say perlite or sand? (Better aeration reduces losses due to rot.) And applying rooting hormone to the cut end? Plus keep the rooting medium only slightly moist. Semi-succulent cuttings just seem to be a bit touchy when it comes to watering and need a bit more attention than more typical cuttings.
So, whether you let cuttings callus over or not depends on the type of plant you want to reproduce … and how you garden. The choice is up to you!