PGRs Weed: Are Plant Growth Regulators Safe for Consumers?

pgr weed
By Nick Congleton Updated March 8th

Fact-checked by Deb Tharp

Throughout the history of agriculture, farmers have always looked for ways to increase crop yields. In modern times, that often means turning to chemicals and genetic engineering to give crops an edge over pests and increasingly unpredictable environmental conditions. Cannabis is no different. 

Plant growth regulators, or PGR weed, is the latest attempt made by cannabis growers to maximize the amount of cannabis they can grow using a host of chemicals to modify the plant’s growth cycles.

Of course, it doesn’t take a lot of research to understand why chemicals on products that people consume are troubling. There have been plenty of instances where a chemical that was thought to be “safe” actually caused a disease or illness. 

For medical cannabis patients, such concerns are magnified. When you’re struggling with an already serious medical condition, the notion that PGR cannabis could be creating additional problems that you aren’t aware of is more than a little troubling. Many medical marijuana patients turn to cannabis for a natural alternative to pharmaceuticals, leading them to question whether they’re simply swapping out one science experiment for another.

What is PGR Weed?

pgr weed

PGR stands for plant growth regulators. These are chemicals that alter a plant’s growth rate. They can either inhibit growth, causing the plant to remain underdeveloped, or they can expedite growth, making the plant grow more rapidly. 

Plants grow through the process of cell division. When a plant’s cells have received enough nutrients, they divide, creating more cells. Then, those cells elongate, allowing them to take in more nutrients and form the structures of the plant, like leaves and roots.

PGRs work by either interrupting those processes or speeding them up. Some growth regulators work on cell division, while others work on the elongation phase of growth. Depending on the goal, some plants are treated with one or both types of PGRs.

When considering PGRs in weed, you’re going to be looking at chemicals that stimulate growth, rather than hinder it. Cannabis growers are looking to use PGRs to increase cannabis crop yield and do so more quickly. As demand for cannabis grows, cultivators need to produce more plants more quickly. PGRs are a way for them to stay on top of the demand. 

PGRs also can be used to create uniform crops. For indoor grow options, this means maximized space and less maintenance.

How is PGR Weed Made?

PRG weed is created by exposing cannabis plants to certain chemicals which modify their growth. These chemicals can either be sprayed onto the plant or fed through the roots in a fertilizer or nutrient mix. Either way, the chemicals get into the plant’s cells and alter either its cell division rhythms, the elongation of its cells, or both. 

Typical PGRs are:

  • Ancymidol
  • Chlormequat chloride
  • Daminozide
  • Ethephon
  • Flurprimidol
  • Paclobutrazol
  • Uniconazole

The PGRs used in cannabis cultivation are daminozide and paclobutrazol.

PGRs are banned for use on food crops in many countries and states, including California. Unfortunately, that doesn’t stop some cultivators from using them for cannabis. The laws haven’t quite caught up.

Common Nutrients Used in PGR Weed

Most growers don’t add the PGRs to their cannabis plants separately. Instead, the PGRs come premixed in the fertilizers they use. In fact, some fertilizers available to home growers have PGRs mixed in, and it was recently uncovered that some aren’t even listed on the label, violating California law.

If you’re looking to avoid PGRs, avoid the following fertilizers:

  • Bushmaster
  • Flower Dragon
  • Gravity
  • PhosphoLoad
  • TopLoad
  • BushLoad
  • Rox
  • Boonta Bud
  • Mega Bud

PGR Weed: Possible Risks and Health Concerns

As with most chemicals, there are safety risks. The risks for PGRs are fairly well known, which is why they’re banned for most crops. In fact, daminozide has been banned for food crops in the US since 1989, yet it still shows up in unregulated cannabis. It's just one more reason why cannabis consumers should purchase from the regulated market in their state: because they can monitor for unwanted chemicals and pull contaminated products off the shelves.

Each chemical is different, and the risks they pose vary. For the main two that are used in cannabis cultivation, there has been a reasonable amount of research done, highlighting the health concerns they pose.


Paclobutrazol is one of the more common PGRs in both cannabis and the wider agricultural world. It’s actually still used in growing apples, mangos, avocados, cabbage, cauliflower, and broccoli in the US. In cannabis, fertilizers like Bushmaster, Gravity, Flower Dragon, and TopLoad all use paclobutrazol, though its use in cannabis production has been banned in California and several other states.

Despite its common use, there’s mounting evidence that paclobutrazol may be toxic. A study conducted by the EPA on mice demonstrated that it can be toxic to the liver and cause damage at high doses. 

Other studies have shown that paclobutrazol causes liver toxicity, it can cause developmental defects in mammals. Other species experienced other toxic effects.


Daminozide was banned in 1989 for causing cancer. It remains banned by the EPA because of its carcinogenic properties. Once again, the laws for cannabis growers are lagging behind (though more and more states are catching up): daminozide is banned in California, Colorado, and many other states.

How to Spot PGR Weed

There’s no 100% certain way to tell if cannabis is PGR weed, outside of chemical testing, but there are several telltale signs to look out for.

PGR weed characteristics chart
  1. The buds will be extremely dense. Instead of healthy, leafy looking buds, you’ll have super dense nugs where the plant remains tightly curled. You can feel the difference in weight, and it will feel spongy and thick.
  2. The plant will have fewer or sick looking trichomes. The trichomes are the white-ish clear crystal-like structures covering the outside of the bud. They produce both the terpenes and cannabinoids in the bud. There will be significantly fewer trichomes on PGR cannabis or they will be a sickly yellow-brown color.
  3. Buds will have a lot more red or brown hairs. While it’s normal for cannabis buds to have some red or brown hairs, PGR buds have a lot more of them. 
  4. PGR cannabis won’t smell much. PGR cannabis has fewer trichomes. Fewer trichomes equates to fewer terpenes and less of the iconic cannabis smell. If the bud doesn’t have a distinct smell, even when ground, there’s a good chance it’s PGR weed.

Learn more about finding the best bud with our 9 Simple Ways to Tell Good vs Bad Cannabis.


There hasn’t been a study done on the effects of PGR cannabis on humans. There’s no definitive scientific conclusion just yet. However, there’s enough evidence surrounding plant growth regulators to say fairly confidently that the risks to consumers shouldn’t be ignored. There have been enough studies done to show that PGRs are harmful when consumed, and that they may cause liver damage and cancer. It doesn’t take much of a stretch to believe that inhaling these chemicals can be harmful. 

Until the laws catch up, it’s best to consult with your budtender and try to find reliable natural strains to use regularly. And if you grow your own cannabis, avoid fertilizers that contain PGRs. Opt for organic alternatives instead.

The information in this article and any included images or charts are for educational purposes only. This information is neither a substitute for, nor does it replace, professional legal advice or medical advice, diagnosis, or treatment. If you have any concerns or questions about laws, regulations, or your health, you should always consult with an attorney, physician or other licensed professional.

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