A cutting is a piece of the stem of a plant which, under suitable conditions, will initiate root growth. Propagation by cuttings has a number of advantages over seed propagation, the most important of which is the reproduction of plants of identical form and flower colour as the parent. This cannot be guaranteed with seed and, for this reason, named cultivars should not be propagated from seed and, if they are, they MUST NOT be distributed under the cultivar name.
While many plants are easily grown from cuttings, some can be quite difficult. Beginners are advised to start with species that are known to give reliable results. These include Callistemon, Correa, Melaleuca, Prostanthera and Westringia to name just a few.
Cuttings should preferably be taken from firm, current season's growth. Both hard, woody material and soft, 'floppy' growth are generally less likely to be successful. The suitability of the material can be tested by bending the stem through about 60 degrees. If it springs back to its original position, it should be suitable, but if it breaks or remains limp, it is best avoided. Sometimes, however, the only material available will be less than ideal and it need not be rejected because of this - successful cuttings can sometimes occur from the most unlikely looking material. When trying to propagate a particularly valuable species, any cutting is better than no cutting!
Cuttings can be taken at any time of year but root formation is usually very slow in the colder months unless supplemental heating is available - see Advanced Techniques for further information on this.
The main requirements are a clean, sharp knife or blade and a "cold frame" to maintain a humid environment while roots are developing. As shown in the diagram, the 'frame' could consist simply of a plastic bag tied around a pot of cuttings. For larger quantities, a small frame could be built or a large, perspex-covered tub could be used.
Cuttings can be placed into either individual small tubes or a number of cuttings of the same species can be placed into a single 'community pot'. A potting mix consisting of 75-80% washed river sand and 20 - 25% artificial peat moss has been found to be suitable as a general purpose mix for cuttings. In time most growers develop their own mix "recipe" to suit their individual needs.
Cuttings are generally about 75 mm long although this will vary depending on the physical size of the stem and leaves of the plant.
If the material is suitable, two or three cuttings can be made from the same piece of stem. The lower end of the cutting is usually cut directly below a leaf/stem junction (a node) as this is claimed to promote more reliable root formation. Whether cutting below a node is absolutely essential is debatable but it is a good rule to follow if possible. The bottom half to two thirds of the leaves are stripped off if this can be done without tearing the bark, otherwise the leaves should be carefully cut off. Large leafed species should have the area of the remaining leaves reduced by about a third.
The lower 15 to 20 mm of the cutting can be 'wounded' to encourage root formation over a larger area; this is done by removing a small sliver of bark with a sharp knife, as shown in the diagram.
Root initiation can often be assisted by use of certain hormone preparations. The most common is Indole butryic acid (IBA) although Naphthalene acetic acid (NAA) is also used, sometimes in combination with IBA. These hormones are commonly available in a talc-based, powder form and are applied to the base of the cutting, as shown in the accompanying photo. The 'medium' strength is recommended for most applications. Liquid or gel preparations are also available and many experienced propagators regard these as being more effective than the powdered form.
|Applying a talc-based hormone powder
to the base of a cutting.
Whatever the form of the hormone used, it should be appreciated that they have a limited shelf life. To extend the useful life of the products they should be stored in a cool, dark place (such as a refrigerator) when not being used. It is also recommended that a small quantity of the hormone be taken from its container for each propagation session and, to avoid contamination, any residue should not be returned to the original container.
To prevent damage to the ends of the cuttings, it is preferable that they not be pushed into the potting mix. A hole can be prepared for each cutting using a skewer or something similar, and each cutting should be placed so that it touches the bottom of the hole. The potting mix should then be firmed around it.
After all cuttings have been 'set' they need to be watered and placed in the propagating frame in a shaded position. The frame should be kept closed to maintain humidity and should also be kept as full as possible for the same reason. If insufficient cuttings are available, empty areas can be taken up with open containers of water.
While root formation is taking place, the aim is to keep the potting mix moist (not wet) and the foliage of the cuttings cool. In hot weather a light spray, sufficient only to wet the foliage, can be applied in the morning and evening and a more thorough hosing given as needed. The frequency of hosings depends on the circumstances but is unlikely to be more than twice a week in hot weather. The foliage spray should be minimized for cuttings with dense, hairy leaves (eg flannel flowers) as rotting may occur. If fungal problems develop, any affected cuttings need to be removed from the frame and discarded. If the problem persists, the use of a fungicide spray may be unavoidable.
|Left: A batch of prepared cuttings ...note the wire frame which prevents the plastic bag from collapsing. Right: Roots protruding from the drainage holes indicate a group of well struck cuttings!|
Many cuttings will 'strike' (ie form roots) in 4-8 weeks in warm weather. Some species, however, may take much longer...up to 12 months or more in certain cases. If a cutting remains green and healthy it can be left in the frame for as long as needed (or until the propagator runs out of patience!).
Some plant species do, however, develop heavy callusing around the end of the cutting which either prevents root formation or only allows for week and fragile roots. Some grevilleas are in this category. If this occurs (and this can only be determined by removing the cuttings from the mix), the callus can be carefully trimmed back to healthy wood and the cuttings re-set in the mix.
After 4-8 weeks, root formation may be observed through the drainage holes in the pots. If no roots are visible through the drainage holes, root development may be checked by gently tapping out the soil mass from the pots.
|Left: Separating struck cuttings. Right: Filling potting mix gently around the roots of the cutting.|
Once roots have formed, separation of cuttings for potting on needs to be done carefully to minimize root damage. Of course, the possibility of damage is reduced if single cuttings had been placed initially in individual small tubes rather than community pots.
This is a most essential step. Many struck cuttings are lost in the few weeks after removal from the propagating frame due to inadequate hardening. This can be carried out using a similar frame to that used to strike the cuttings. The hardening frame, however, is best kept closed during the daytime and propped open at night. After two weeks of this treatment, struck cuttings should be sufficiently hardened to be removed from the frame and placed in a shaded position for another two weeks. They can then be moved into the open.
Cuttings in community pots may not all form roots at the same time. In this situation, the soil mass should be gently broken up, struck cuttings transplanted into individual small pots for hardening and those without roots re-set and placed back into the propagating frame.
After 1-3 months, the hardened cuttings will be ready to transplant into larger (125 - 150 mm) pots. Plants will be ready for the garden when good root development has occurred. This can be checked by carefully tapping the plant out of the pot. The roots of the plants should not be allowed to become "pot-bound" (ie. develop into a tight, coiled mass). If this does occur, some root pruning will be needed before planting to ensure that the roots of the plants can easily spread into the surrounding soil. Certain groups of plants such as Boronia and Prostanthera are prone to become pot-bound fairly quickly.