Most of us got our first trial at plant propagation by sticking a sprig of mint or philodendron vine in a glass of water. After a few weeks, we had roots! Then they were ready to be planted in soil.
Plants that root in water
There are a number of plants which can be propagated in this exceptionally easy way: coleus, dieffenbachias, begonias, brugmansias, tomato plants, bee balm, many types of salvia, dracenas, african violets and more. These plants resist rotting, absorb water directly into a freshly-cut surface, and may have latent root buds along the stem. Many come from the mint or nightshade families, or are tropical houseplants, selected from species that tolerate wetness and are adapted to rooting quickly to spread in their highly-competitive native habitats.
Cacti and succulents
On the other end of the spectrum are succulents, many of which would rot in water but carry enough moisture in their fleshy leaves or stems that they can root in soil without special care. Most sedums, aloes, aeoniums, jade plants or cacti will easily root after air-drying for a day or two and being inserted directly into soil. They can last a long time on stored water so the cuttings can live for weeks or months without roots. Some types of succulents will generate new plants from a single fallen leaf.
Everything else
The largest number of plants fall into a difficult middle category: they’re both vulnerable to rot in pure water and unable to store enough to live without their roots unless they have some sort of life support. Some species seal wounds quickly and won’t take in water through a cut stem in a vase. Others are adapted to dry climates and haven’t developed defenses against bacteria and fungi that live in water, or can’t transport oxygen to submerged tissues so they suffocate and die below the water line. To get these cuttings to root, you need to control their environment more carefully.
Industrial nurseries use elaborate systems for this: tight temperature controls, chemical formulas, sterilized equipment and sprayers that mist the plants every couple minutes to keep leaves consistently wet. They’ll even create new plants by tissue culture, an advanced propagation process starting with small clusters of cells added to a sterile petri dish. For the home gardener, this is too elaborate to be practical, so some of the most challenging plants are beyond our reach. But we can still propagate a long list of plants at a moderate skill level with a fairly simple setup.
A simple nursery setup for cuttings
I propagate most of my cuttings in a basic “greenhouse” made with clear plastic containers and plastic wrap. The clear container lets me see the young roots forming, and the plastic wrap will trap humidity to achieve a 95-100 percent humidity level that prevents cuttings from losing moisture. I fill it part way with a rooting media (potting media, sand, perlite or vermiculite). Most plants are OK with potting media but some of the plants that rot more easily need an inorganic substrate.
There’s no easy rule to determine whether a plant is easy to propagate by cuttings or how to select the best part of the plant to use; plants are too diverse. I recommend looking up the species or variety to learn its propagation methods. Government databases, universities or botanical societies have straightforward online databases to search. You should find whether or not propagation from cuttings is possible, the best time of year to take them, whether the best tissue comes from the stem tip or the base of the stem, whether to select young or old stems, you’ll see how long the cuttings should be, whether the rooting media should be kept very moist or on the dry side, and other variables that will impact whether your cuttings succeed.
Taking the cuttings
Generally, if I’m lacking more detailed info about a species, I will take a full stem from the donor plant if that’s available without doing too much damage to it. It’s best to do this in the morning when the stems are hydrated after a cool night.
I’ll take the single stem and cut it lengthwise into several individual cuttings. That way, one cutting will be made of old basal stem, which is hormonally prepared to root in many species, one cutting will be a young stem tip, which in some species forms the most vigorous roots, and gradations between them. I make sure each segment has at least three nodes (the place where a stem connects to a leaf and new shoots can emerge), and with each segment I’ll remove the lowest leaf and keep the top two.
Inserting the cuttings in the rooting media
It can be tricky coming up with the right moisture level for rooting media, since different plants prefer different amounts of moisture. A good middle ground is to wet the media thoroughly and let it drain so it is saturated but not super-saturated.
I stick the cuttings so that the node corresponding to the lowest leaf is right on the surface of the rooting media. That places one node at the bottom of the cutting under the media, one internode is buried the media, and one node is partially covered. Two leaves are able to photosynthesize, but the top leaf is completely clear of moist rooting media in case the one resting on the media rots. The batch contains variety of tissue types and positions from which roots may grow.
Rooting hormone powder or gel is optional. It certainly helps plants that are harder to root from cuttings, while fast-rooting plants produce a lot of the same hormone internally and won’t derive much benefit from what you add.
If the stem is rigid, you can stick the cutting directly into the media, but in most cases it helps to dig a hole with a pencil or toothpick, insert the cutting and pat down the media around it.
Maintaining the cuttings
After the cuttings are struck, I cover the container with plastic and poke small holes in it to enable a little bit of gas exchange. It can be placed on a windowsill that doesn’t get too much direct sun, or, more successfully, under bright white fluorescent lights.
A note on plant lights: Artificial light for plants doesn’t need to be special, but it has to produce light without too much heat, so do not use incandescent bulbs. The closer the light appears to natural sunlight, the better. Newer LED setups might combine red and blue light, which should work. Otherwise, plain white light is best. (DO NOT use blacklights or UV lights; plants don’t use ultraviolet light to photosynthesize. They use the same wavelenghts produced in the largest amount by the sun, which are also what your eyes use to see and it will appear white.)
Cuttings under artificial lights need to be on a timer: plants require a period of darkness every day to undergo a full metabolic cycle, but will root faster if there is more light than darkness. I use a 16-8 photoperiod, or 16 hours on, 8 hours off, unless I learn otherwise about a particular species.
Moving to the next phase
Different plant species take varying lengths of time to root, so you’ll have to keep an eye on your cuttings by looking through the container. Sometimes I lift a cutting or two out to check, although I do this with caution because it can break small roots off or delay their growth. I usually move the plants to the next phase when the roots are a centimeter to an inch long.
When a cutting is mature enough to transfer, it’s better to do it as soon as possible because the very high humidity environment under plastic wrap can leave plants vulnerable to forming mold or developing diseases. But the plant will be shocked and die if it is moved directly into dry air or hot sun, and needs some intermediate steps. Initially, you can transfer it into small pots in potting mix and left in the same environment with the same light cycle that it rooted in.
From there, you can gradually harden your plants off. Watch closely for wilting or signs of severe drought stress such as leaves crisping up around the edges—if that happens, you’ll want to slow down the hardening-off process or cover the plants with a clear plastic bag to raise the humidity again.
If all goes well, root systems will quickly grow. When you see roots coming out from the bottom of the container, you can transfer the plants outdoors in shade on a mild day. Avoid hot sun! Over time, you can move the plants to a spot that gets early morning sun. You may want to pot them up into larger pots if you plan on leaving them in hot sun or overwintering them in containers. Otherwise, continue to introduce them to brighter light and eventually you’ll be able to plant them out in your garden!