How do I propagate plants by cuttings?
Propagation by cuttings relies on the production of a new root system from the cambium (water and nutrient conducting) tissue just below the epidermis or outer tissue (bark in woody plants) of the stem. Some plants (such as figs, Ficus species, form aerial roots and such species will readily strike roots when propagated by cuttings. However, others are much more difficult and require a few tricks of the trade.
The elements for successful cutting propagation are as follows:
Type of wood – Generally the softer the wood the better and quicker the cutting will strike. The three types are softwood (eg impatiens, brachyscome), semihardwood (eg callistemon, azalea) and hardwood (eg waratah, frangipani). A bit of research in books or internet will tell you the best type of wood and what time of year to get it for the species in question. Softwood strikes quickest but needs more stringent environmental (ie high humidity) conditions usually because it tends to wilt more easily.
To decide what is soft, semihard or hardwood, examine your intended plant. A softwood cutting comes from any new growth, and will look green and feel flexible and soft. You can encourage a plant to produce the right sort of growth by feeding and watering in the warm months of the year, and hopefully a few weeks later you’ll have some healthy fresh growth to use for your cuttings. A semihardwood cutting is one that has been on the plant long enough for the stem to begin to harden, but not long enough for it to become woody, which will then tip it into being a hardwood cutting. Look at the stem, and a semi or hardwood cutting will usually have a colour change where it is maturing. It will also feel less flexible.
Propagation environment – for leafy cuttings humidity can be created by greenhouse structures but also in miniature with recycled cherry tomato containers, clear plastic bottles cut in half, or small portable plastic containers designed for propagation of cuttings. Hardwood and semihardwood cuttings will often be OK under a tree or similar sheltered environment because the leaves are fully hardened with a waxy surface that new unfurling leaves in softwood cuttings have not yet formed. Removing the optimum number of leaves will also help minimise moisture stress.
Bottom heating of the pot also helps to stimulate root formation. Portable heat mat can be purchased through avenues such as gardening magazines. A hot water heater can also be a source of bottom heating ie put your pots on top of a water heater. Cuttings taken in the warmer months will be warm enough not to not need bottom heating.
The growing medium – Generally a good cutting propagation mix for cuttings has higher aeration than normal and this can be provided by mixing a general purpose potting mix 50/50 with a coarse material such as perlite or coarse washed river sand or even polystyrene foam beads.
Rooting hormones – are a group of substances known as auxins which regulate all sorts of activity in plant growth. Importantly for us they stimulate new root formation. Auxins are generally applied to the base of the cutting and are particularly effective where the tissue has been wounded. Auxins are often applied as powders but auxin gels and liquid formulations are also available that have other substances such as vitamins added that are known to further stimulate the rooting process. Rooting powders and gels are available at garden centres and large hardware shops with a gardening section. Usually they are labeled for suitability for soft cuttings, semi hard or for hardwood. The older the growth of the cutting, the more hormone strength will be needed.
Wounding of the base of woody cuttings – New roots form on cuttings in the tissue just below the outer ‘skin’ of the shoot. This is the tissue that conducts the water and sugars and is called the vascular cambium. When cuttings are taken of harder wood we can stimulate faster and better root formation by scraping off the surface tissue on the base of the cutting to expose the sappy conducting tissue where the new roots form.
Callusing – A layer of callus (similar to scab tissue in humans) tissue forms at the cut surface at the base of a cutting. It protects the inner tissue from disease and aids in water uptake prior to the new roots forming. This is usually a precursor to root formation. However, for some difficult to root plants (eg many grevilleas) we can get over callusing where a huge knob of callus tissue forms and seems to inhibit root formation. This can be scraped off if the cutting is taking too long.
Check out my videos on plant propagation-
Angus’s book, Let’s Propagate, has extensive information and pictures- buy now!