The Science Behind Natural and Synthetic Root Growth Promoters
Though plants naturally produce their own root promoters, there are a variety of ways a motivated grower can stimulate root propagation safely and effectively. But which ones are most effective? Chris Bond explains how to separate the facts from the hype.
There are hundreds of products on the market claiming to stimulate, boost, enhance, or promote root growth, but with all the hype, how can you figure out what really works and what is a waste of your precious dollars?
There are products on the market that are designed to stimulate root growth, but you need to know what you are looking for before getting suckered by pretty packaging and unverifiable claims.
Why Use Root Promoters?
Most growers, from hobbyists to professionals, use rooting promoters as a matter of course when propagating plants. It may make one ponder the need, though, since plants have evolved to develop root systems on their own (otherwise they would have gone extinct or never developed to begin with).
While that is certainly true in nature, the propagation of plants introduces a wide range of variable and artificial conditions that a plant cutting’s natural counterpart may not ever encounter. The natural counterpart also has the benefit of numerous symbiotic relationships with beneficial flora and fauna; they are also not operating on anyone’s time table but their own.
In general, rooting promoters—be they hormones, stimulators, or other growth aids—are designed or gleaned from nature for their ability to aid in a plant’s need to develop a healthy root system quickly.
Other benefits from using root promoters, besides reducing the time it takes for a plant to initiate root development, are that cuttings treated with root growth stimulators typically have a higher survival rate than non-treated cuttings and that the number of roots produced per cutting is increased.
Another benefit, mostly for commercial propagators, is that the use of root promoters generally increases the uniformity of the cuttings’ root systems. This is important when keeping all container sizes uniform.
The Natural Root Promoters: Auxins
Plants naturally produce root promoters of different types called auxins. Auxins help to encourage or regulate plant growth through the processes of either cell enlargement or cell division. They can direct the growth of one area of a plant while restricting the growth to another, depending on where its resources are needed.
Any legitimate rooting hormone or root growth stimulator will have this in one form or another. For developing roots from propagated cuttings, look for a product containing the naturally occurring hormone indole butyric acid (IBA). This auxin helps the plants to develop new roots.
If you are transplanting a young plant that already has roots, but you want to stimulate more lateral roots, find a product containing the manufactured hormone naphthylacetic acid (NAA). Some rooting promoters on the market contain both IBA and NAA. Other real and synthetic plant auxins do exist, but they are not as efficient as they break down quickly and are of little long-term benefit to the plant.
Phosphorous is one of the nutrients needed by plants in large amounts, and one of its many functions is to aid in root development. Some root stimulators may contain phosphorous, but you can also use a separate phosphorous product on its own.
A word of caution about phosphorous: any amount not used by the plant ends up leaching out and often ends up in waterways. This causes unwanted and sometimes dangerous algae blooms. Make sure that you really need it and that your plants can use all the phosphorous that you give it. When in doubt, test the phosphorous levels of your media to see if your plants already have enough available to them.
The same can be said of nitrogen. Nitrogen fertilizers may be sufficient to help your plant develop new roots as the process of your plant adjusting to its new home can require additional nutrition. Be careful about over-fertilizing.
Regardless of which formulation you select for your rooting promoter, be careful not to contaminate or otherwise compromise the contents. Do not dip your cutting or plant into a bottle of root stimulator. This can introduce any number of unwanted pathogens that can then be spread to your other plants with each subsequent dip.
Prepare the desired amount separately, and keep the original container tightly closed when not in use. Do not save or reuse root stimulators that have already been in contact with your cuttings or transplants. Finally, read up on the differences between gel rooting hormones, powder rooting hormones, and liquid rooting hormones.
Though not a growth hormone, mycorrhizal fungi has been known for hundreds of years to benefit plants’ roots. The mutually beneficial relationship between this fungi and a plant’s root system is complex. In general, beneficial bacteria are stimulated, which in turn produce growth-enhancing chemicals within a plant.
The fungi feed on carbohydrates and proteins that are released by the roots of a plant, and the networks of beneficial fungi, which can go on for miles underground, can act as an extension for the roots’ ability to draw much-needed nutrients. Roots that have a complex mycorrhizal relationship are more resistant to root diseases, pests, and negative cultural conditions like drought or poor soil aeration.
Mycorrhizae has become widely available on the market. You can obtain it in liquid, granular, or powder form. It may also be embedded into soilless media. It should be observed, however, that the use of fungicides on your plant to prevent certain fungal diseases may negatively affect the beneficial fungi existing in your plant’s root network.
If you purchase a mycorrhizae product for root development and a fungicide, you may as well save your money and not buy the mycorrhizae until enough time has elapsed from your fungicide treatments.
You don’t have to look hard in the root promoting marketplace to find thiamine (vitamin B1), which is touted as either the ultimate root growth stimulator or as a cure-all for transplant shock. There are no verifiable studies to support these claims.
Plant research in the mid-20th century considered several auxins that were mixed with vitamin B1 and these compounds were indeed found to promote root growth. It has repeatedly been proven since that it was the auxins alone responsible for the root development, not the thiamine.
Vitamin B1 does have a place in plant development, however, but it is usually abundant enough in nature that additional doses are unnecessary. It is beneficial to add vitamin B1 to sterile media where there is no natural thiamine for the plant to use, but there are few other situations where it is actually needed.
Organic and Non-chemical Alternatives
If you are concerned about using artificial hormones or just aren’t sure which product to use, you can make your own organic root promoter by using willow tree branches and leaves. Fast-growing trees like willow produce large amounts of auxins. Willow tea, or willow water as it is called by some, is easily prepared with young, thin, willow branches.
To make a gallon of willow tea, cut four cups worth of thin branches with leaves into one to two-inch sections. Boil one gallon of water. Add the cut branches and leaves to the boiling water and let them sit until the water has cooled to room temperature. Sieve out the pieces and the auxin-rich water can be used immediately to water your cuttings.
Unused amounts can be frozen and thawed for later use. (If you cannot locate a willow tree to borrow a few branches from, you can make the same concoction with other rapid growers like poplars or silver maples.)
If you do not want to make your own organic root stimulator, you can find some on the market that have been approved for organic gardening by the Organic Materials Review Institute (OMRI). Look for the OMRI stamp on labels of root stimulators to ensure that what you are buying is in fact organic.
Another long-standing and proven natural product for aiding in root development for cuttings is honey. Regular, straight-from-the-pantry honey is a natural antiseptic. While it does not act as a rooting hormone, it protects your cuttings so that they can develop their roots without the risk of succumbing to disease. There is some indication that the sugars in honey are also beneficial for root development. Read More: The Various Forms of Natural and Synthetic Rooting Hormones.
What do herbicides have to do with root stimulators? Surprisingly, a lot. Unknown to many gardeners, several types of herbicides are actually nothing more than plant auxins. Common weed killers containing the compound 2,4-D work by stimulating plant growth. In the case of plants treated with herbicides containing these compounds, they cause the affected plant to essentially grow themselves to death, much like a cancer.
For root stimulation, these compounds are used in far weaker concentrations. This does not mean that you can use any weed killer in a diluted strength to put on your cuttings. Even at a highly weakened rate, they might inhibit other growth functions of your cuttings.
The point of mentioning herbicides here is to let you know you should not be alarmed if you recognize something typically used as an herbicide among the active ingredients on a bottle of root promoter. Lethality is a matter of dosage; even water is toxic if you consume enough of it.
Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional
Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.