The Science of Natural and Synthetic PGRs (Plant Growth Regulators) and Their Impact on Plant and Human Health

As the name suggests, Plant Growth Regulators [PGRs] are a group of plant hormones that influence the way that plants grow. This could mean increasing or decreasing growth rate, influencing branching, promoting elongation of growth, or increasing the number of flowers (Went et al. 1978).  PGR compounds include Auxins, Gibberellins, Cytokinins, Ethylene, and Abscisic Acid, as well as several others (How Hormones and Growth Regulators Affect Your Plants, 2012).  

PGR’s are found naturally and play critical roles in plant development. 

Below is a table of phytohormones that are used as PGR’s, l their sources and their effects on plants:

PGR’s and human health

Research indicates that some plant hormones have no adverse effects on human health.  Some are OMRI-listed certified organic products that can be used for organic crops (Organization, and Organic Materials Review Institute, 2001).  Others are extremely harmful, including many dangerous synthetic PGRs that are readily available to the general public at garden supply stores.  For many, we simply do not know the effect on human health, as adequate research has yet to be performed. 

An example of a common synthetic PGR is the rooting hormone indole-3-butyric acid. Indole-3-butyric acid is a synthetic hormone made to mimic the natural PGR compounds auxins. According to the EPA, it is thought to be safe for humans and other animals because it has no known harmful effects.  Additionally, indole-3-butyric acid does not remain in the environment for very long. According to the EPA, indole-3-butyric acid can be safely used for cloning plants, including for food crops (EPA) 

Gibberellins are a naturally occurring plant hormone that are used to induce cell growth and elongation, they also are sprayed on foods to increase the shelf life of store-bought vegetables and fruits. Gibberellins have no acute toxic effects so they are often assumed safe, although recent research has shown a correlation with several negative long-term effects (Xu et al. 2019).

On the other hand, some are known carcinogens.  For example, Daminozide is a synthetic PGR which is a known carcinogen.  It remains in plant tissues for long periods, often present in fruits and flowers at harvest and when consumed as food. It was banned for use on food crops but is labeled for use in ornamental plants to stop stem elongation and promote flowering. Though it is only supposed to be used in ornamental it was snuck into the cannabis market under the brands including “Phosphoload” and “Flower Dragon”. These products were labeled as containing “humatic isolates” but secretly also contained the dangerous daminozide which was left off the label.  

(Americans for Safe Access, 2011).

Many other PGRs available to consumers do not have enough data on the potential effects on animals and the environment, or what they are metabolized into, so it is best to avoid using them on plants that will be consumed. 

How do I avoid using dangerous PGRs in my garden unintentionally?

Make sure you know what is in the fertilizer you are using. Blending your own fertilizer using verified organic and natural ingredients that are certified as food-grade is the best way to ensure your crops will be safe for human consumption. Otherwise, be sure to check if there have been recalls in the past or if the company has previously been caught including things like PGRs or pesticides illegally. Check the ingredients listed on the packaging, General Hydroponics’ Bush Load, for example, contains the potentially harmful PGR, Paclo, which you can see on the label.  Beware though, some companies are dishonest about what they put in their products. Over the years several fertilizers have been recalled for containing PGRs or substances regulated as pesticides. Even companies that claim to never contain PGRs have been caught including them secretly in their fertilizers (Mitchell, 2017).

There are many organic fertilizer ingredients that provide naturally occurring plant hormones and are deemed to be safer than synthetic concentrated hormones. Kelp meal and willow bark extracts have both been used for the natural hormones that they contain. Willow bark can be used as a rooting hormone because it contains the compound Salicylic acid, which stimulates root growth in plant cuttings (Shamsul et al. 2013). Kelp meal contains cytokinin, auxin, and gibberellins which have been shown to increase fruit yields.  In a study on the effects of kelp, kelp outperformed concentrated extracts of the equivalent hormones(Temple et al. 1989). 

Final conclusion

PGRs are chemicals used to influence plant growth. They cover a lot of different chemicals, some natural and some synthetic. They are safe to use if the guidelines for use are followed, but some can be dangerous and should never be used on plants meant for consumption. The best way to avoid harmful PGRs in your own garden is by purchasing fertilizers with all ingredients listed by reputable companies or mixing your own fertilizer with safe ingredients.

Potentially harmful PGRs 

• Paclobutrazol (Palco)
• Daminozide
• Uniconazole
• Chlormequat chloride
• Fertilizers labeled - “not for use on plants intended for human consumption”

Sources of Natural PGRs that have No Known Harmful Effects

• Alfalfa meal
• Aloe Vera
• Barley
• Birch Leaves
• Cornmeal
• Coconut Water
• Kelp 
• Spirulina
• Wheat
• Willow bark

References

Went, F W, and Kenneth Vivian Thimann. Phytohormones. New York, Allanheld, Osmun/Universe Books, 1978.

Support, E. W. (2021, February 10). How hormones and growth regulators affect your plants. OSU Extension Service. Retrieved February 17, 2022, from https://extension.oregonstate.edu/gardening/techniques/how-hormones-growth-regulators-affect-your-plants 

Xu, Chun-Shuang, et al. “The in Vitro Effects of Gibberellin on Human Sperm Motility.” Aging, vol. 11, no. 10, 22 May 2019, pp. 3080–3093, 10.18632/aging.101963. Accessed 17 Feb. 2022.

California Certified Organic Farmers (Organization, and Organic Materials Review Institute. OMRI Generic Materials List, with the National Organic Program Final Rule Listings. Eugene, Or, Organic Materials Review Institute, 2001.

US Environmental Protection Agency. Indole-3-Butyric Acid (046701) Fact Sheet.

September 20, Americans for Safe Access on, and 2011. “Long-Banned Alar (Daminozide) Shows up on Hydroponic Store Shelves before Being Removed Again.” Americans for Safe Access, 20 Sept. 2011, www.safeaccessnow.org/long_banned_alar_daminozide_shows_up_on_hydroponic_store_shelves_before_being_removed_again. Accessed 17 Feb. 2022.

Mitchell, Dale. Issuance of Statewide Stop Sale, Use, or Removal Order and Notice of Statewide Detainment, Seizure or Emargo. 24 Aug. 2017, aapco.files.wordpress.com/2017/08/ssurod-advanced-nutrients-180114-fert2017-34.pdf.

Temple, W. D., and A. A. Bomke. “Effects of Kelp (Macrocystis Integrifolia AndEcklonia Maxima) Foliar Applications on Bean Crop Growth.” Plant and Soil, vol. 117, no. 1, June 1989, pp. 85–92, 10.1007/bf02206260. Accessed 11 Mar. 2020.

Shamsul Hayat, et al. Salicylic Acid : Plant Growth and Development. Dordrecht, Springer, 2013.

OSU Extension Service “How hormones and growth regulators affect your plants.” (2012, July https://extension.oregonstate.edu/gardening/techniques/how-hormones-growth-regulators-affect-your-plants

Humboldt Seeds. “How do Plant Growth Regulators (PGRs) work on my weed” - (2017, March 30). Humboldt Seed Organization. Retrieved February 18, 2022, from https://www.humboldtseeds.net/en/blog/plant-growth-regulators-weed/

Ries, S. K., Werth, V., Sweeley, C. C., & Leavitt, R. A. (1977). “Triacontanol: a new naturally occurring plant growth regulator.” Science, 195(4284), 1339-1341. 10.1126/science.195.4284.1339