This is a plant cloning technique where bare plant parts are removed from the parent tree and induced to form roots and shoots which later grow to form new plants. The plant part that is removed is called a cutting. This cutting may be referred to by the location from which it is taken. Plants can be propagated from root cuttings, leaf cuttings or stem cuttings. Typically, stem cuttings of tree species are more difficult to root.
Cutting propagation is often the preferred method for plant propagation because it is the easiest and most cost effective way to produce a clone of a particular parent plant.
Stock plant management
The conditions frequently chosen to root stem cuttings are warm, moist and shaded or at least away from dry air movement. It is important to choose cuttings that are pest- and disease-free. If appropriate, use proper and prescribed pest management tools prior to cutting.
Stock plants should be at a stage of growth most likely to have root stem cuttings. Old or mature plants are often more difficult to root than young, vigorously growing plants. Therefore, established blocks of trees and shrubs used for parent plants, known as stock blocks, will need to be replanted every 5 to 10 years. The less mature a plant, the easier it is to root a cutting from it.
Stock plants should receive balanced nutrition, usually at a moderate level for average or more difficult to root plants. When applying fertilizer to stock plants, avoid too much available nitrogen or too great an imbalance of essential nutrients. Field grown blocks of stock plants are often fertilized only once a year. However, taking cuttings from field stock is relatively easier than from root container grown plants that may require many applications of fertilizer in a year.
Stability of components with minimal decomposition during propagation is necessary as changes in particle size also change the air and water balance in the substrate. The most common components used by professional propagators are combinations of pine bark, peat moss, coir, horticultural grade perlite and washed sand. Moistening components prior to mixing and filling propagation trays or benches is a very important step in preparation of substrates. Mixing dry components creates a mixture with low aeration and poor drainage characteristics as particles fit tightly together and seal capillary channels. The effects are long lasting and have definite influence on root development and growth.
Types of cuttings
Cutting types can be described by their origin such as leaf, root or stem cuttings. They can also be described by their location on the stem such as the cuttings from the ends of stems being called terminal or tip cuttings. Cuttings from further down the stem are called secondary cuttings. Terminal cuttings often root in higher percentages than those located further down the stem due to changes in natural hormone concentrations as well as hardiness.
The rooting environment
The desire to keep cuttings alive until they can root and support themselves has prompted plant propagators to develop many creative ways to provide an environment that favours rooting. Light, temperature and moisture are usually the environmental factors most often manipulated.
Light can keep a stem cutting from rooting by either being too bright or too dark. Inadequate light cause defoliation of cuttings and reduces rooting percentages. More frequently, cuttings are exposed to excessive light intensities that can cause heat damage during propagation. For this reason it is common to root plants under 25 to 70 per cent shade with 50 per cent being the most common shading provided for rooting stem cuttings.
The most effective environment for rooting different plants may vary somewhat. Most often, normal cool to warm greenhouse temperatures are maintained, depending on the needs of the individual plant, with cooler air temperatures at night than during the day. Shading is used to reduce heat build-up during bright, sunny days. Excessive high temperature causes excessive shoot growth in advance of root development and thus leads to increased water loss and death of cuttings.
High humidity around the leaves and stems of softwood and semi-hardwood cuttings keeps the cuttings from drying out and allows normal plant functions like photosynthesis and respiration to take place without the plant wilting while new roots are forming. Under most propagation systems, roots are formed in a medium that has the proper balance of moisture and air space to allow for the development of healthy roots.
Maintenance of high humidity, optimum temperature and light around the cutting is critical. High humidity may be provided by placing a flower pot, plastic bag or glass jar (figure 15a) over the cuttings or by misting the cuttings with water several times a day. Cuttings can also be placed in plastic trays covered with clear plastic stretched over wooden hoops or wire frame (figures 15b, 16a, b & c). The plastic will help keep the humidity high and reduce water loss from the cuttings. Shade and temperature can also be manipulated by using shade net or local materials such as palm fronds, leaves or grass over the cuttings.
Plants with thicker or waxy leaves suffer less from low humidity than thin-leaved plants. Plants in sealed containers may require a fungicide spray to deter harmful pathogens. If you need more elaborate facilities, you can construct a small hoop frame or use an intermittent mist system.
Procedures for rooting stem cuttings
Cuttings should generally consist of the current or past season’s growth. Avoid material with flower buds if possible. Remove any flowers and flower buds when preparing cuttings so that the energy can be used in producing new roots rather than flowers. Take cuttings from healthy, disease-free plants, preferably from the upper part of the plant.
The fertility status of the stock (parent) plant can influence rooting. Avoid taking cuttings from plants that show symptoms of mineral nutrient deficiency. Conversely, plants that have been fertilized heavily, particularly with nitrogen, may not root well. The stock plant should not be under moisture stress. In general, cuttings taken from young plants root better than cuttings taken from older, more mature plants.
Early morning is the best time to take cuttings, because the plant is fully turgid. It is important to keep the cuttings cool and moist until they are inserted in the rooting medium. An ice chest or dark plastic bag with wet paper towels may be used to store cuttings.
While terminal parts of the stem are best, a long shoot can be divided into several cuttings. Cuttings are generally 6cm to10cm long depending on the species. Use a sharp, thin-bladed pocket knife or sharp secateurs for the preparation of cutting. If necessary, dip the cutting tool in rubbing alcohol to prevent contamination or infections.
Remove the leaves from the lower one-third to onehalf of the cutting (figures 17 and 18). On large-leafed plants, the remaining leaves may be cut in half to reduce water loss and conserve space. Wounds may be created at the base of cuttings of difficult to root species.
Treating cuttings with root-promoting compounds can stimulate rooting of some plants that might otherwise be difficult to root. Prevent possible contamination or damage to the entire supply of rooting hormone by taking small quantities from storage whenever required. Any material that remains after treatment should be discarded and not returned to the original container. Be sure to tap the cuttings to remove excess hormone when using a powder formulation.
The rooting medium should be sterile, low in fertility and well-drained to provide sufficient aeration. It should also retain enough moisture so that watering does not have to be done too frequently. Materials commonly used are coarse sand, sawdust, a mixture of one part peat and one part perlite (by volume), or one part peat and one part sand (by volume). Media should be watered while being used.
Insert the cuttings ⅓ to ½ half their length into the medium. Maintain the vertical orientation of the stem (do not insert the cuttings upside down). Make sure the buds are pointed up. Space cuttings just far enough apart to allow all leaves to receive sunlight.
Water the container or set-up again after setting or inserting the cuttings. Cover the cuttings with plastic and place in shaded area to avoid exposure to direct sun. Keep the medium moist until the cuttings have rooted. Rooting will be improved if the cuttings are misted on a regular basis.
Newly rooted cuttings should not be transplanted directly into the landscape. They should instead be transplanted into pots. Growing them to a larger size before transplanting to a permanent location will increase the chances for survival. Handle rooted cuttings by the rootball or pot, not the stem. Otherwise, roots may break off due to the weight of the media on the roots.