Paclobutrazol: Poison, Potential or Panacea?

By Andrew Schell
Published: October 1, 2015 | Last updated: May 3, 2021 04:03:04
Key Takeaways

Paclobutrazol is a plant growth regulator (PGR) that is present in a number of popular products on the market and is used by a significant percentage of growers in the indoor gardening industry. But, is it safe?

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Paclobutrazol, chlomequat, and daminozide are similar plant growth regulators (PGRs) that are present in a number of popular products on the market and are used by a significant percentage of growers in the indoor gardening industry. But, are they safe?


I have delved into the archives to summarize the performance data and general consensus in the scientific community of these three PGRs. I will focus mainly on paclobutrazol, the most common of the three plant growth retardants.

Upon proper application, these three PGRs inhibit a natural plant hormone called gibberellic acid, which is responsible for cell elongation and cell division. This in turn limits plant height and internodal length. Fruits and flowers come out more compact and dense, a highly desired trait with certain plant species. They also initiate earlier flowering and fruit set.


As a triazole fungicide, paclobutrazol increases a plant’s resistance to fungi including molds, powdery mildew and rusts. It also increases a plant’s resistance to bacteria and drought. The chemical has been shown to increase yield in apples and enhance mini-tuber production in potatoes.

Paclobutrazol is also used by golf courses to ensure a uniform, compact putting green surface. Labor is reduced in agricultural operations when using PGRs because less topping, pinching and pruning is needed.

Plant growth regulators work systemically and plants uptake them readily through their roots. Because they are transported through the xylem, they have been shown to be less effective when applied as a foliar spray. Grow media that has been exposed to PGRs should not be reused because residues from the chemicals persist and can affect subsequent crops.


Plant growth regulators definitely have a negative effect on the microbial community in the soil. Testing on mango orchards in Brazil showed that environmentally relevant concentrations of the chemicals actually killed beneficial microbe populations by up to 60%.

The surviving microbes and their associated enzymes showed lower activity rates. The stasis (balance) of the microflora in the root zone is hindered, which lowers the overall fertility of the medium and can negatively affect the nutrient balance and uptake.


While there is plenty of research in the scientific literature pertaining to these chemicals and their effects on plants, there is little experimental data outlining the effects of paclobutrazol, chlomequat and daminozide on fish, mammals, humans and the environment at large. In scorpion fish, paclobutrazol accumulates in toxic levels in the brain while lowering sperm count and important enzyme activity.

Fish exposed to paclobutrazol show decreased levels of amino acids in their brain and liver. They also have a reduced capacity to detoxify chemicals because the antioxidant activity in the brain is reduced.

Recent experiments have shown that paclobutrazol has negative effects on the reproductive systems and development of rats. Survival rates of litters were significantly lowered. Research papers note altered locomotion and fetal malformations in rat subjects. If something is toxic to a rat, it is most likely toxic to a human, although on a lower scale.

The genetic makeup of rats is 99% similar to humans, and as fellow mammals they share the same organs and biochemical pathways. Studies in Denmark reflect reproduction and developmental problems in pigs that were fed cereals and grains that were treated with chlormequat at normal doses.

As far as environmental impact, the paclobutrazol molecule has low mobility and low volatility so it is tough to remove from soils. It is attracted to organic matter and tightly binds to it. Paclobutrazol can persist in a contaminated medium for years before breaking down. It is stable at a large range of temperatures so it can handle fluctuations without degrading.

Agricultural products that contain paclobutrazol, chlormequat and diminozide contain the industrial or technical grades of the chemicals. This requires the solutions to be only 95% pure while the remaining 5% can contain heavy metals, stabilizers, preservatives and other toxic adulterants.

The chemicals are dissolved in a dimethyl sulfoxide (DMSO) solution before bottling, which is listed as a Schedule 4 poison in Australia. In the United States, all three of these PGRs are illegal to use on plants grown for human consumption.

They can still legally be used on ornamental plants, although they are completely banned for sale in California and Oregon. They have been totally banned in most European Union countries, with the main exception being the United Kingdom.

Paclobutrazol and chlormequat are not listed as human carcinogens, but this is probably because they have not been tested as such. Daminozide is listed as carcinogenic.

There are more environmentally friendly ways to limit vertical plant growth. A unique chelated form of iron, Eddha Fe, can be applied in the mid-flowering stage to trigger the production of autumn hormones, namely ethylene. This process will inhibit vertical growth naturally, albeit not as drastically as observed with the PGRs.

Another option is to selectively breed cultivars that display the desired phenotypes, rather than using chemicals to manipulate the existing varieties. A closer look at potential alternatives to these chemicals must be examined.

There are now a number of laboratories that offer testing for the residues of these chemicals on fruits and flowers. A consumer can now know for sure whether or not a product has been treated with these (and many other) unwanted substances.

Until more research is performed on the potential hazards of these PGRs on humans and the environment, we should not want them anywhere near our groundwater, our streams and rivers, our gardens or our families.


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Written by Andrew Schell

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Andrew Schell has 15 years of technical gardening experience, from warehouse operations in Los Angeles to the full-sun fields of Humboldt County. With a biochemistry degree from Humboldt State University and a plethora of lab experience, Andrew lends his perception of plant processes and nutrients from the molecular level. He is the International Sales Manager for House and Garden Nutrients.

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