Cloning, either for large-scale plant multiplication or just to create a duplicate or two of a favorite plant, has long been a process carried out by horticulturalists. The process, for most growers, has moved beyond lopping off a plant stem and heeling it into the ground in the hope new roots will eventually form.
Nowadays, with an understanding of the effects of rooting hormone compounds, stimulants, temperature, oxygen and humidity control, stock plant conditioning and many other factors, cloning has become a science with many tools and techniques to take advantage of.
The enjoyment of the cloning process comes partially from creating new plants identical to the parent, but also in discovering exactly what works for different species and perfecting a technique that is successful each and every time.
Tips for Plant Cloning: From Simple to Advanced
One of the most attractive aspects of vegetative (asexual) propagation is that the materials, processes and techniques can be as simple or as advanced as the grower dictates. Beginners often give cloning a go with very basic methods—placing a cut stem into a jar of water on a windowsill or using a tray of coarse propagation sand in a warm spot is how many of us first started out with easy to root cuttings.
Advancements made to this method often involve heated propagation mats or pads to provide the warmth that stimulates root formation; automated misting of leaves to maintain turgor; dedicated cloning and nursery areas in the indoor garden; the use of cloning gels, liquids and powders; and the use of aeroponic cloning units.
More sophisticated and advanced cloning methods can, these days, be carried out on a small scale and include micro propagation (tissue culture), air layering, budding and grafting, all of which can offer advantages for certain plant species.
Why Should You Clone Your Favorite Plants?
Aside from the obvious advantage of producing new plants from old, cloning has other distinct advantages over propagation via seed. The new cloned plants will have the same genetic characteristics as the parent plant, whereas seed-grown plants can vary considerably from the parent, depending on a number of factors, including how they were bred. So where genetic diversity is not desirable, or where certain plants don’t set seed, vegetative propagation methods are used for multiplication.
Another advantage is the avoidance of a long or difficult seed germination phase and/or extended juvenile period—vegetative propagation avoids the sometimes long non-flowering juvenile phase in some plants, speeding up production as compared to propagation from seed.
Stock to be cloned can be selected so that only male or female plants are produced and also provides the possibility of combining two or more clones in the same plant by grafting or budding. Overall, cloning is known to be a highly economic method of propagating large numbers of the same plant in a relatively short time frame.
Cloning Preparation: Stock Plant Conditioning
The first step in successful cloning starts long before any cuttings are taken. Stock or mother plants need to be in good health to provide tissue that will withstand the stresses of the cloning process. This includes freedom from any pests and diseases that will carry over to the new plants. Mites and mildews in particular are an issue as contamination from stock plants often causes losses in the propagation phase or even once the new plants are well established.
Another vital aspect is plant nutrition; with hydroponic methods we can ensure stock plants for propagation receive all the essential elements required for healthy foliage, while at the same time not inducing any overly soft vegetative growth which often roots poorly. Attention to EC and sufficient levels of boron are vital for stock plant conditioning.
Boron levels in cloning tissue have been shown to influence the rate of root formation, while EC can be used to manipulate and strengthen stock plants before cutting materials are taken. Gradually increasing the EC of the nutrient solution puts slight osmotic stress onto the plant, making it more resistant to water loss and desiccation by lowering the moisture content in the foliage and increasing the dray matter percentage.
Increased EC and high light levels assist the plants in storing more carbohydrates, which in turn helps with rapid root formation and maintaining the cutting in a healthy condition once removed from the mother plant.
Once stock plants have been pre-conditioned for cloning, the next step is the process of selecting, preparing and treating stem cuttings. Cutting material needs to be taken from growth that is not overly soft and has had time to mature and harden slightly; this is best carried out early in the day when stock plants are the most turgid. Stems should be cut from the mother plant with a sharp knife that has been sterilized to prevent diseases from infecting the exposed tissue.
Most plant species benefit from cuttings being prepared from fresh material immediately after cutting; however, hardier cuttings can be stored wrapped in plastic for a few days before preparation.
Once cut, the foliage from the lower two thirds of the stem is removed to prevent excessive moisture loss and the cut stem is treated with root promotion products before being positioned into the propagation unit or system. Hardier plant material benefits from a small degree of wounding on the lower stem portion before being treated with cloning gel, power or solution.
This involves removing a small area of the outer stem tissue at the base of the cutting to expose more area from which roots can easily emerge. The development of new roots (adventitious roots) takes place just outside and between vascular bundles inside the stem and usually occurs after the production of callus.
Once the clones have been prepared and treated, the cut stem may be placed into a wide range of propagation mediums from the traditional sand/coconut fiber mixtures, fine perlite, perlite/vermiculite combinations, rockwool cubes, specific synthetic propagation materials and others.
Cuttings of some species may be rooted directly in aerated water while many indoor growers prefer customized cloning units that use aeroponic misting to maintain both high moisture and oxygen content around the cut stem surface. The misted bases of the cuttings in these units are exposed to fresh supplies of dissolved oxygen on a very regular or sometimes continual basis, meaning dissolved oxygen is never in short supply.
Whichever method is used, the process of new root formation will be most rapid when sufficient warmth is provided to speed the rate of new cell development, while humidity is maintained around the foliage to delay desiccation. Providing either bottom heat or a warmed nutrient solution in cloning units speeds up the rate of root formation on the cuttings of many species.
There is some variation between ideal levels for different plants, so it pays to check this out when propagating for the first time. For many plants, the ideal temperature conditions consist of a warm base and slightly cooler top—this gives root formation processes a boost, but at the same time slows respiration and loss of carbohydrate reserves in the cutting’s foliage.
This differential between air and stem base temperatures is easy to achieve when using heating pads or mats to only warm the growing substrate, while retaining air temperature at normal levels.
Root Formation Promotion
Aside from the auxin plant growth regulators (IBA and NAA) contained in cloning gels, powers and solutions that stimulate and speed up the rate of root development, other compounds are known to have a synergistic effect on this process.
One of these synergists is boron, which helps stimulate root formation in at least some plants, and the use of boron in combination with the auxin IBA can increase the rooting percentage, number and length of roots. With hydroponic cutting propagation, boron can easily be incorporated into the growing medium or mist solution, or if it is not already applied, via rooting compounds.
Thiamine (vitamin B1) is another root promotion substance in cuttings of many species—studies have shown thiamine assists with promotion of rooting in terms of speed and number of roots formed.
Salicylic acid is another root formation synergist, particularly of soft wood and semi hardwood cuttings, which has been scientifically proven to be beneficial for a number of species, although the mode of action of these compounds is still not fully understood.
While synthetic rooting promotion products are well proven and highly effective in promoting root formation on cuttings, some studies have found that inoculation with plant growth promoting rhizobacteria (PGPR) can have a similar effect.
Certain species of beneficial bacteria (strains of Bacillus in particular) have been shown to induce root formation in stem cuttings. It is thought these bacteria may work though a number of different mechanisms, one of which is the ability of these bacteria to produce a natural plant growth promoting phytohormone, IAA (indole-3-acetic acid). This means that use of these bacteria to provide additional auxins to increase the success rate of propagation via cuttings could be used by organic growers.
Another organic additive that may assist root formation are humic or fulvic acids, both of which are common supplements used by hydroponic growers. These compounds have been found to promote root initiation in cuttings, indicating they have auxin-like activity.
Plant Cloning Troubleshooting
Cloning failures can be common in the early stages of learning this skill. Mistakes may originate from the use of overly soft cloning material that desiccates before roots are able to be formed, the presence of rots and pathogens that destroy the cutting rapidly, or an overall lack of any root formation.
The cloning process is a race between desiccation of the cutting foliage and the formation of new roots. If the rooting process is too slow due to unfavorable conditions, desiccation can occur before the clone is able to take up water and nutrients from a new root system. Slow root formation also increases the likelihood of pathogens and rotting occurring, while excessive leaf drop may also result in death of the cutting.
Less common problems may originate from the over or under use of rooting compound products—using a rooting compound that is too concentrated for soft tissue cuttings can damage the stem and prevent root formation, while a lack of use can result in a prolonged period before roots form or the production of only a few, sparse roots.
Insect pests such as fungus gnats, which are attracted to the humid conditions inside propagation units, may also cause failure in clones. The fungus gnat adult fly lays eggs in growing medium and the resulting small, cream-colored larvae feed on cut stem bases and new roots of cuttings.
Diseases such as Pythium, powdery mildew and Botrytis (grey mold) also thrive in humid environments, so only taking cloning material from disease-free stock plants and maintaining a high level of hygiene are the most effective methods of preventing pathogen problems in the propagation phase.