As a human, I can easily comprehend plant reproduction from seed: like our own approach to procreation, it’s sexual reproduction. Many herbs also reproduce asexually (vegetatively) by producing structures such as offsets and bulbils that eventually become independent. But one kind of vegetative propagation is utterly foreign to my human condition: a complete plant can be regenerated from a severed piece of stem or root, a bud, a leaf or leaf part, even a small group of certain plant cells.
To me, this capability is awe-inspiring. Even more exciting is that a novice gardener like me with only a windowsill to work with can, with patience and understanding (and perhaps a little divine intervention), propagate many herbs from cut pieces of leafy stem. Many people have even rooted fresh-cut herb sprigs from the produce department at the supermarket.
How Herb Cuttings Root
When you cut off the end of a stem, you inflict a wound and cut off the water supply, and those two events determine what you and the cutting must do next if the cutting is to survive. Its job is to heal the wound and make new roots; yours is to provide a suitable environment-water, light, air, temperature, and soil-to keep it alive during that process.
Roots are most likely to develop at a stem wound that is in firm contact with moist soil. When a plant is wounded, hormones called auxins collect briefly around the wound and alter the nature of cell division in the cambium (inner “bark” of the stem) so that it begins to form embryonic root tissue. Meanwhile, callus material forms over the wounded area. If the wound is exposed to air, the callus dries and becomes hard; in contact with moist soil, the callus remains somewhat soft, and the burgeoning roots beneath it can emerge.
When to Take Stem Cuttings
Cuttings taken from herbs that are growing steadily (generally from spring through fall) have the best chance of rooting. The growth of many species slows down when temperatures drop and days get shorter, even if you bring them indoors, and cuttings taken then will root slowly if at all. However, you can take cuttings anytime from a plant that doesn’t slow down while wintering indoors. Cuttings with flowers forming on them are not likely to root strongly; if you decide to take cuttings when a plant is flowering, nip off the flowers to shift the cuttings’ energy into making roots.
Most herb stems darken as they harden, and the newer growth toward the tips will be a lighter color than that farther down. The succulent new growth is easiest and most likely to root; when the stems are just firm enough to snap instead of bending, they are ready to be taken as cuttings. Bay (Laurus nobilis) is a notable exception: take hardwood cuttings late in the season. Whatever the condition of the stem, it is wise to take cuttings only from the current year’s growth.
Plants that have been heavily fertilized to stimulate lush growth produce poor cuttings: it takes time for stems to store up the reserves needed for rooting. Choose plants, and stems of those plants, that are healthy and growing steadily. Cutting back the longer stems in late spring usually produces a large number of side shoots that will make excellent cuttings later. Meanwhile, you can root the tips you cut off-you may be overrun with new plants by season’s end.
Water plants well at least 12 hours before you plan to take cuttings. Cuttings from scented geraniums root more readily if you leave the parent plant unwatered until it wilts slightly, then water it thoroughly and take cuttings 12 to 24 hours later. It’s a good idea to take them late in the day and allow them to sit out in cool air overnight before “sticking” (planting). This is not true for other herbs, however.
Types of Stem Cuttings
A tip cutting is taken at the end of a stem and is distinguished by the growing point at its tip. When rooted, this cutting will grow primarily upward and, in most herbs, produce a single vertical stem if not pinched. This is the kind of cutting most commonly used for creating a topiary. The usual length of a tip cutting is 5 or 6 inches, and it can be taken from the main stem or from a larger stem that originated as a side shoot.
A sectional cutting is taken from the middle of the stem and thus has a cut on both ends. Sectional cuttings usually include several nodes (although one is enough to make a plant). Plants grown from sectional cuttings tend to be bushy because their growth occurs as side shoots.
A basal cutting is a whole side shoot, up to 6 inches long and usually unbranched, that is cut off close to the point where it emerges from the main stem. Because a basal cutting usually is younger than the main stem from which it grew, it likely will root more readily than a cutting from the main stem. Its growth habit is the same as that of a tip cutting.
A heel cutting is a basal cutting with some of the main stem tissue attached. It will often root more readily than a basal cutting because of the broad exposure of cambial tissue. Heel cuttings may sometimes be conveniently taken while stripping side shoots from the lower end of a tip or sectional cutting.
How to Take Cuttings
To deter fungi and other soil-borne organisms, clean tools well and then sterilize by dipping them in alcohol. Pruning shears tend to bruise the stem tissue, and this can prevent or impede callus formation and encourage rot. If you use pruners or scissors for taking cuttings, cut the stems a little below the lowest desired cutting point, then recut them with a sterilized razor blade or sharp knife.
The lower cut of each cutting should be made at an angle to maximize the amount of cambial tissue exposed to the soil. Many gardeners advise cutting just below a node-a visible growing point or joint on the stem where leaves and side branches may emerge. However, roots will form equally readily wherever cambial tissue comes in contact with moist soil, regardless of node location.
When you take a sectional cutting, make the upper cut straight across the stem just above a node, both to minimize the wound and to help identify the orientation of cuttings that have few or no leaves. You can maximize the number of plants grown from a single stock plant by taking a series of sectional cuttings, each with only one node. Nodes without visible growth can root and grow, but those that are showing some side-shoot growth will be easier to orient correctly in the rooting medium.
To take a heel cutting, grasp a side shoot where it meets the main stem and pull downward so that a strip of main-stem tissue peels off with the shoot. If the main stem does not peel readily, make narrow cuts in the stem a little above and below the side shoot with a sharp blade. Such a wound on the lower end of a stem cutting will encourage rooting; it is not likely to damage an established plant as long as the wound is shallow and clean and the outer stem tissue around the wound has not lifted up. Taking two heel cuttings directly opposite one another may weaken the stem, however.
Stripping & wounding stems. Remove all leaves and side shoots from the lower two-thirds of the stem. This helps prevent stem rot and creates a small wound at each node, which encourages rooting. If the stem is young and succulent, stripping off leaves exposes the cambium sufficiently to encourage rooting. If the stem has hardened even slightly, scrape it on two sides with a sharp blade to create narrow, shallow wounds over the length of the stem that will be buried. This exposes more cambial tissue and increases your chances of success.
Rooting compounds. Rooting compounds (rooting hormones) are synthetic auxins that imitate the plant’s hormonal response to wounding and thus encourage development of embryonic roots. A powdered form containing talc is commonly available in garden centers. Most formulations contain a small amount of fungicide to deter disease during rooting. Rooting compounds should be used with caution as they can irritate human skin, and inhalation can cause respiratory arrest. Though very succulent stems may not require treatment with a rooting compound, it can speed rooting in woodier stems and may be essential to successful rooting of some species.
Rooting compound is applied to the stripped stem just before sticking. To avoid contaminating the compound in the container, shake a small amount onto a piece of paper, dip the lower part of the stem into it, and then knock off (don’t blow off) the excess. Be conservative with rooting compound: more is not better. Auxins promote the formation of embryonic roots, but high levels afterward will inhibit further root development.
You can make a less toxic rooting compound by soaking the stems of new-growth willow or forsythia for 48 hours in water and using the resulting tea to wet the rooting medium or soak stem cuttings briefly before sticking.
Choosing and Preparing Containers
Cuttings can be rooted in almost any container provided that it has drainage holes. If you’re rooting a large number of cuttings, a flat of 2- or 3-inch cells, such as a seed-starting flat, is preferable, but if you have enough space, you can save a transplanting step by rooting the cuttings in individual pots.
Rooting medium. The rooting medium must maintain adequate moisture and air around the stem, supporting it without restricting the emergence of new roots. By far the most common and versatile rooting media are soillike mixes, many of which contain no actual soil. Common ingredients include garden soil, sand, perlite, vermiculite, peat moss, and pine or hardwood bark. Commercial blends are available at garden centers, and every propagator has a favorite recipe for a homemade mix. Expense, availability, sterility, and weight are factors you may wish to consider in choosing a rooting medium.
Garden soil drains too slowly to use alone and is not sterile. You may sterilize it by spreading a thin layer on a cookie sheet and baking it for an hour in an oven set at 350°F. (Anyone who has ever done this can attest to the terrible odor that results.) Garden soils differ so widely that an actual recipe for a blend is practically impossible.
Coarse builders’ sand is a component of traditional rooting mixtures. It’s inexpensive but too heavy for most commercial use. Used by itself, it promotes the growth of coarse, straight roots that are difficult to transplant, but in blends with other ingredients, roots are well branched and more easily transplanted. It’s best to sterilize sand as you would soil.
Perlite is a white, porous volcanic material, readily available in garden centers, which lightens soil and improves aeration and drainage. Perlite generally does not hold enough moisture nor provide enough support to be used alone as a rooting medium, but it is an excellent addition to blends. Equal parts of perlite and sand is a good formula. If more than one grade of perlite is available, choose the coarser. When mixing perlite with other materials, avoid breathing the dust; moistening it before mixing will reduce that problem.
Vermiculite, an expanded clay, holds too much moisture and too little air when used alone, but works well in combination with perlite and other ingredients. Choose a coarse grade; fine ones tend to be too soggy.
Peat moss is a lightweight, resilient, essentially nonnutritive organic material which provides good support for cuttings. It is difficult to wet initially (hot water helps), but once wet, it holds moisture without becoming waterlogged. Many professional growers prefer a rooting medium consisting of equal parts peat moss and perlite.
Bark is lightweight and porous and is a popular ingredient of rooting media in the southeastern United States, where it is available and inexpensive.
Turface is a clay product used in building soil texture for golf courses. Though not yet a retail product, it is becoming popular as a rooting medium and can sometimes be obtained through a local golf course.
Growers disagree whether fertilizer should be added to a rooting medium. A healthy stem cutting contains within it, or can generate through photosynthesis, the nutrition and energy necessary for forming roots. Although research has demonstrated slight improvements in rooting when a carefully measured amount of fertilizer was included in the rooting medium, the advantage is far outweighed by the ease with which you can kill the cuttings. The best time to add fertilizer is after the roots have formed and the plant begins to grow visibly.
Fill the container at least three-quarters full of rooting medium, then set it in a pan of water until the top of the mix appears moist. Let the container drain for ten minutes.
Stem cuttings must be inserted (“stuck”) so that the lower parts are in firm contact with the moist rooting medium. (Sectional cuttings must be right side up!) Poke a hole slightly larger than the stem in the moist medium, insert the stem (covered with rooting compound if you are using it), and firm the medium around it. The lower end of the stem must be in firm contact with rooting medium, not the bottom of the container.
Tip cuttings and sectional cuttings that consist of several nodes are planted as outlined above. Single-node cuttings should be set at an angle with the emerging shoot exposed and vertical. Those without visible growth should be set at a similar angle with the node just below the surface of the medium.
Place each kind of herb cutting in a separate flat as they are likely to root at different rates. Label the containers as to name and planting date.
Rooting in water. Although some herbs-including many of the mints-root readily in water, the roots tend to be brittle, adapted to extracting nutrients and oxygen from water. When transplanted to soil, the plant undergoes shock as the roots adapt to growing in the new environment. If you do root cuttings in water, keep them in the water only until roots are visible, then pot them up in a suitable rooting medium to complete the process.
Maintaining the Cuttings
Creating new roots is extremely stressful for the cut stem, and by the time roots have formed, the foliage may look bedraggled. If any leaves or other parts of the cutting dry out or rot during rooting, remove them immediately. Cut off any flowers that may form.
Water. Cuttings need a constant supply of moisture. One approach is to hand-mist the cuttings every time you walk past, perhaps two or three times an hour for the first few days. (Most cuttings can survive the night unmisted.) The rooting medium must be kept moist at all times, and the cuttings should be protected from drafts.
However, not all of us can arrange to be walking past our cuttings with a spray bottle two or three times an hour. Another approach is to maintain a humid atmosphere around the cuttings with a plastic tent arranged so that it doesn’t touch the leaves. Rigid covers such as sweater boxes need no special support, but floppy food storage or dry cleaners’ bags should be held above the cuttings with a wire frame or other improvised support. Commercial propagating boxes are available in case you’d rather use your chopsticks for eating instead of for propping up drooping plastic.
For small-leaved or other tolerant cuttings (such as rosemary) that don’t require extremely high humidity, and for cuttings being rooted in a humid climate, many gardeners simply lay pieces of tissue loosely over the tops of the cuttings (resting on supports, not on the leaves) to form a tent. But most herb cuttings need a moister environment during the first days of rooting.
Temperature. When rooting containers are placed on a windowsill or elsewhere indoors, soil temperatures tend to be high during the day and low at night, which encourages fungal problems. Soil temperatures day and night ideally should be kept above 65°F. Most herb cuttings root well at soil temperatures as high as 80°F, using bottom heat to promote rapid root growth. The tops should be kept 10° to 20°F cooler.
Light. Cuttings need bright light for photosynthesis, which fuels the processes of regeneration, but direct sunlight will overheat and dry the foliage (or cook it if under plastic), which means certain death. A north window or artificial lighting can provide adequate light without high air temperature. For details on the types and uses of artificial light, see “Growing Scented Geraniums Indoors” by Judy Lewis (The Herb Companion, December 1992/January 1993).
Transition from Cutting to Plant
As soon as roots have formed, which may take two to six weeks or longer, the cuttings will begin to produce new leaves, and those that are rooting on a windowsill may turn toward the light. For me, the excitement of seeing these signs of renewed life is the payoff for all the attention I’ve invested.
Now is the time to fertilize the plants lightly-one quarter or less of the manufacturer’s recommended dilution for house plants-with a balanced (20-20-20) fertilizer. Fertilize a little less often than usual for that particular herb, gradually increasing the dose to normal.
Cuttings that have rooted in small cells should be transplanted as soon as the roots can hold soil: when you lift up the stem, the soil comes up with it. (If the rooting medium is sand, don’t pull up on the stem; instead, dig it up carefully with a plant label or other small tool.) Roots may also be visible at the drainage holes. If the plants are left too long in small containers, the roots will begin to grow around the sides, which can stunt growth even after transplanting. When repotting the new plants, leave the rooting medium on the roots to avoid injuring the fine root hairs. Expose completely enclosed cuttings to air flow gradually over the course of a week; slow down if they begin to wilt.
Consider the alternatives
Rooting stem cuttings is fun, and you can take a lot of cuttings from one plant and still keep the plant. But rooting cuttings also can be tricky and time-consuming, and some herbs are so easy to propagate from root divisions that it seems like a waste of time to try to start them from cuttings. These include yarrows, artemisias, dead nettle, bee balm, catmint, oregano, lamb’s-ears, and tansy. All you need is a shovel to carve off a corner of an established patch of these herbs and you have a start that is ready to go and grow as soon as you plant it in your garden. It makes sense to expend your patience and care on rooting cuttings of those species that are difficult or impossible to increase by division.
— David Merrill, managing editor of The Herb Companion, has grand plans for filling a corner of his yard with scented geraniums from cuttings now rooting on his windowsill(s).
A few commercial products may be useful in propagating your herbs from cuttings.
Especially in early spring, many garden centers sell seed-starting trays that also serve well for rooting small cuttings. The tray holds a set of 11/2- or 2-inch seed pots and is covered by a transparent plastic lid. The tray can be filled with water to moisten the rooting medium in the pots and to maintain humidity in the closed tray. The lid can be propped open to allow some air flow if condensation on the lid indicates that the atmosphere inside is too moist.
Bottom heat warms and encourages the stems to put out roots but lets the leaves remain cool and moist. Heating mats are a simple and convenient way of delivering bottom heat, but a less expensive method is to add electrical heating cables or tape to the bottom of a propagation tray. Cables or tape must be buried in sand to prevent spot warming or burning of the plastic tray and pots, or they may be buried deeply in potting mix and the cuttings planted above them. Soil-heating devices usually are equipped with thermostats; the less expensive models are preset to a recommended temperature. Heated, enclosed propagators can be used indoors or in a greenhouse or cold frame.