Nothing beats multiplying your own plants. Whether it’s from seed or vegetative cuttings, perfecting propagation can not only give a succession of healthy young planting stock, but also an immense sense of satisfaction.
While propagation of most commonly grown hydroponic plants is usually a straightforward process, understanding the physiology behind striking cuttings in particular is worth taking a closer look at, as technology, product development and new research are constantly providing more tools for us to work with.
Propagation From Cuttings
Cuttings from a stock plant, or mother plant, used to make new plants are called clones because they have the same genetic characteristics as the parent plant. Seed-grown plants, on the other hand, can vary considerably from the parent plant, depending on a number of factors including how they were bred. When genetic diversity is not desirable, or when certain plants don’t set seed, vegetative propagation methods are used. The process of new root formation on cut stems has been well documented for a wide range of plant species, and hydroponics and indoor gardening provides us with an environment in which successful propagation can be carried out by everyone. These days even “bench top” tissue culture processes are used by indoor gardeners to propagate more challenging species such as orchids.
The Basics of Using Cuttings
Most gardeners know the basic process of taking cuttings. A section of stem with leaves is taken from a suitable plant, then the lower stem is stripped and firmly planted into a free-draining medium. Rooting gel, powder or liquid formulations may be used to speed up the root formation process and the cutting is then misted or covered to prevent excess water loss. Bottom heat may be provided, depending on the species, and then the wait for root formation begins.
There are a large number of variables that can determine the success or failure of vegetative propagation, from the condition of the mother/stock plants, to the rooting compounds applied, temperature, light, carbohydrate content, oxygen levels, substrate or misting system and a number of “root promotion” synergists. Obtaining new plants from cuttings is a delicate process. The stem, cut off from its supply of water from the roots, can desiccate and die before new roots have formed. The cut stem can act as an entry point for a number of fungal and bacterial pathogens that can rot the cutting before it strikes, and sometime roots just fail to form for any number of unknown reasons. Elimination of all of these factors is difficult, but knowing exactly what conditions are required for rapid root formation goes a long way towards successful propagation.
The condition of the stock or mother plant plays a major role in determining how well or quickly cuttings will strike. Many inexperienced gardeners often overlook this vital step and take cuttings from plants that are either too tender, soft or unhealthy.
With the advanced nutrition hydroponic gardening provides, we can ensure stock plants for propagation receive all the essential elements required for healthy foliage. Control of electrical conductivity (EC) is one way of strengthening stock plants before taking cuttings. Gradually increasing EC in the nutrient solution puts slight osmotic stress onto the plant, making it more resistant to desiccation, lowering the moisture content in the foliage and increasing the dry matter percentage.
The EC should not be increased to the point where plant wilting occurs, but provided the plant experiences good growing conditions with sufficient light and correct temperatures, increased EC assists the plants in storing more carbohydrates, which in turn helps with rapid root formation and maintaining the cutting in good condition once removed from the mother plant. Along with EC control, optimal or slightly increased light levels also help harden the plant material for propagation.
Nutrients for stock plants in preparation for propagation should avoid the use of aggressive vegetative formulations that are high in nitrogen. Nutrients with a more balanced N-K ratio help avoid any excessively soft growth. Studies have also found that a good level of boron nutrition in stock plants assists with root initiation after cuttings have been taken. Plants being given a complete nutrient solution will have sufficient boron for this purpose.
Depending on the species being propagated, there are also certain growth stages or times of year when many plants will strike roots from cuttings more rapidly. Some plants, like winter dormant fruit trees, are propagated when dormant through winter, while many others can have cuttings taken year round provided fresh new growth has had sufficient time to harden up.
Some species should not be propagated when they are flowering or have flower buds formed, as even if the flower buds are removed, cuttings don’t initiate new roots rapidly. It’s likely these types of plants have some sort of internal control or physiologic condition associated with flower bud formation that inhibits root formation on cut stems. Generally, it’s best to select stock plants for propagation either before or after flowering has occurred.
Once stock plants have been pre-conditioned for propagation, the next step is selecting, preparing and treating the stem cuttings. Cutting material needs to be taken from growth that is not overly soft and has had time to mature and harden slightly. Stems should be cut from the mother plant with a sharp, sterilized knife to prevent diseases from infecting the exposed tissue.
Most plant species benefit from cuttings being prepared from fresh material immediately after cutting, but 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.
Rooting Compounds and Synergists
Root promotion products for use on cuttings are widely available in gels, liquids and powders of varying strengths. These products contain growth regulators that stimulate the production of adventitious roots on stem and leaf cuttings and give greater success rates and more numerous roots formed than untreated cuttings. Auxins (IBA and NAA) are plant growth regulators that stimulate root development by inducing root initials that differentiate from cells of the young secondary phloem, cambium and pith tissue of the cutting material. IBA is most commonly used in these products, as it is relatively stable and doesn’t break down due to enzymes within the plant.
Root promotion products may also contain a range of other ingredients and synergists proven over many decades to assist with propagation. One of these synergists is boron, which helps stimulate root formation in at least some plants. The use of boron in combination with the auxin IBA can increase the rooting percentage, and the number and length of roots. With hydroponic cutting propagation, boron can easily be incorporated into the growing medium, mist solution or rooting compounds. Thiamine (vitamin B1) is another root promotion substance used in the cuttings of many species to help with promotion of rooting in terms of speed and number of roots formed.
Microbial and Organic Additives
While synthetic rooting promotion products are well proven and highly effective in promoting root formation on cuttings, some studies have found similar effects by inoculating cuttings with plant growth-promoting rhizobacteria (PGPR). Certain species of beneficial bacteria, strains of bacillus in particular, 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 to produce a natural plant growth-promoting phytohormone, indole-3-acetic acid (IAA). This means use of these bacteria to provide additional auxin could be used by organic growers. Other organic additives that may assist root formation are humic and fulvic acids. These humic acid compounds promote root initiation in cuttings, indicating they have auxin-like activity.
Optimal Conditions for Root Formation
Good levels of oxygen around the cut stem base is a requirement for callus and root initiation. Propagators typically use a slightly coarse, free-draining substrate such as sharp sand mixed with vermiculite or peat for striking cuttings. However, research shows the size or number of air pores in the cutting’s substrate doesn’t necessarily help provide enough oxygen for the most optimal rate of root formation. It is the dissolved oxygen concentration directly at the interface of the cutting’s stem surface and the water that is important. This is one of the main reasons why hydroponic cloning mist chambers can provide better results than solid propagation substrates for many plant species. The misted base of the cutting is exposed to fresh supplies of dissolved oxygen on a regular basis, meaning dissolved oxygen is never in short supply.
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, but each species is different. For many plants, the ideal temperature conditions consist of a warm base and slightly cooler top to give the root formation processes a boost while slowing 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 heat pads or mats to only warm the growing substrate, while retaining air temperature at normal levels.
Leaf Retention and Cutting Care
A major part of successful root formation is maintaining the correct environment around the top of the cutting. Leaves may be reduced in size to lower the rate of water loss, but leaving a certain amount of foliage assists root formation in non-dormant plants. Many plants benefit from misting to prevent desiccation, particularly in the first few days after removal from the mother plant.
Some plants may rot if this is used excessively and are better just covered with light, breathable horticultural cloth. With many plant species, it is likely that additional root formation growth regulators or other beneficial compounds are produced in the stem and leaves, which further assist the cutting to form roots at the base, so some leaf retention is recommended for many plants.
Producing clones by taking and striking cuttings in hydroponics is a great way to produce new, healthy planting stock genetically identical to carefully selected parent plants.