By Luke Sumpter

Plant growth regulators (PGRs) are chemical inputs used to improve bud size and yield. They essentially work by hacking plant hormones to accelerate processes that fuel growth, and suppress ones that deter or diminish it. However, these molecules are also known to damage the environment, and might pose a threat to human health. Learn more about these controversial growth regulators, and how to identify PGR weed.

What Are PGRs in Weed?

PGRs are simple molecules that—you guessed it—aim to regulate plant growth and development. Several of these chemicals occur naturally in plants, where they help to drive important physiological processes. However, humans have successfully engineered synthetic PGRs that are applied to increase harvests and extend the postharvest life and storage quality of fruits.

Naturally, commercial cannabis cultivators have developed an interest in these chemicals. They work well on other fruit-bearing crops, so it stands to reason that PGRs could help to bolster bud size, increase harvest weight, and ultimately improve profit margins. But are they good for consumers?

Pesticides and herbicides are recognised as notorious contaminants in legal cannabis markets, especially those in North America. Studies have assessed cannabis smoke for pesticide residues, with researchers describing contamination as “alarmingly high[1]” and significantly concerning. Now, scientists are starting to explore the phenomenon of PGR residue in buds[2].

Although PGRs help to maximise volume, consumers are showing increased demand for organic, sustainable, and artisan cannabis products free from synthetic inputs, and grown on a smaller scale. Some research suggests that water pipes and bongs help to filter out[3] some nasty chemicals, but the presence of any contaminants doesn’t sit well with many cannabis users.

But aren’t PGRs applied to food crops? They are indeed. For example, farmers apply different kinds of these chemicals to improve the shapes of apples and to stop fruits from dropping too early. But many commercially grown foods also contain pesticide residues. Just because an agriculture input enhances growth and productivity doesn’t mean it's safe to ingest.

Below, you’ll get to know PGRs on a deeper level. After finding out how they work in plants, you’ll discover if they pose any danger, as well as how to identify weed grown with PGRs.

What Are PGRs in Weed?

The Role of PGRs in Plants

PGRs are plant hormones. Just like animal hormones help to regulate physiological events, plant hormones drive growth, flowering, fruiting, and other processes. However, in animals, hormones enter systemic circulation to exert effects via distant organs and glands. In plants, hormones tend to produce effects right where they’re synthesised. Thus, these chemicals work to create isolated effects—but they also work together to produce a range of responses.

The main PGRs found naturally in plants include:

Abscisic acid
This PGR works to close stomata (the small pores on the surface of leaves) during drought conditions. Because these holes regulate the movement of gases in and out of leaves, closing them allows plants to hold on to water when the resource becomes scarce. Abscisic acid also plays a role in abscission—the natural detachment of dead leaves and fruits from branches.
This plant hormone contributes to the senescence of leaves and flowers. Have you ever picked a tomato and watched it turn red on the windowsill? You can thank ethylene for facilitating the ripening process off the branch.
This chemical plays a key role in germination. As germination begins, enzymes free up gibberellin into its active form. The chemical then breaks the seed’s dormancy. The molecule also regulates sexual expression and plays a role in hermaphroditism in numerous species.
This PGR plays several crucial roles, including the formation of root cells. Auxin also underpins phototropism—the process that causes plants to grow toward light.
These chemicals help to drive cell division and fight off senescence.

Why Do Growers Use PGRs on Cannabis?

Ultimately, cultivators apply PGRs to cannabis to produce bigger and thicker buds. Although they may be visually appealing, these aesthetics come at a cost.

Applying PGRs to plants essentially hacks their endogenous hormones. Naturally occurring hormones work partly by breaking down or activating DNA transcription proteins. These proteins either activate or suppress genes that promote growth, and hormones maintain control over them. The introduction of synthetic exogenous hormones allows growers to disrupt this biological process. By adding hormones that activate target transcription proteins, growers can intentionally ramp up the number of proteins that promote growth, and suppress those that slam on the brakes.

This tinkering leads to larger and thicker buds that look much more appealing to the naked eye. The activation of growth-promoting genes causes cells to uptake more water, which forces them to expand. Auxin also plays a role in increasing the size of plant cell walls.

However, despite bigger buds, playing around with plant hormones comes with a serious downside to cannabis growers; compared to normal flowers, PGR weed features lower levels of cannabinoids and terpenes. As such, these buds are much less tasty than those that don’t undergo treatment, and produce less in the way of psychoactive effects.

Why Do Growers Use PGRs on Cannabis?

Are PGRs Dangerous?

Ongoing studies are exploring the potential impact of PGRs on human health. While PGRs are approved for agricultural use in some countries, they are banned in others. For example, European farmers are able to use daminozide as a pesticide, but it’s deemed a probable carcinogen in the United States and remains illegal to apply to crops.

Check out some of the most common synthetic PGRs below, and their potential dangers:

Chlormequat chloride
This small and simple molecule helps to bolster yield[4] in grains, tomatoes, and peppers, among other plants. However, exposure to the chemical can irritate the lungs and cause nausea in the short term, and can damage the liver[5] over long periods of exposure.
Approved for use in a host of European countries, this PGR is used to enhance the growth of tree fruits such as peaches, cherries, prunes, and pears. Despite its efficacy, the Pesticide Properties DataBase, published by the University of Hertfordshire, lists daminozide as a probable human carcinogen[6].
This chemical actually works as a plant growth suppressor. By inhibiting the natural PGR gibberellin, it helps to reduce internodal spacing and create stouter plants that fruit earlier. However, the molecule has been shown to produce reproductive and developmental issues in animal models.
  • How PGRs Impact the Environment

PGRs aren’t only a potential threat to cannabis consumers. The application of these chemicals on an industrial scale means, just like pesticides and other inputs, they end up in the environment.

Poor soil practices that increase runoff and erosion mean these chemicals make their way into the groundwater, rivers, and the wider ecosystem. The full impact of this pollution remains unknown, but research shows that PGRs can impact organ development[7] in some species of fish and exert toxic effects[8] on the reproductive systems of animals.

How to Identify PGR Weed

You can find PGR weed basically anywhere, especially in countries where cannabis is illegal and unregulated. But how can you tell these contaminated buds apart from untreated ones? Luckily, there are several tell-tale signs; but before we proceed, you should know that not all PGRs are toxic. Some natural PGRs, such as kelp and chitosan, create buds that look similar to synthetically treated ones, without adding a toxic burden. Therefore, you need to consider your source and how much you trust them when you come across buds with these traits:

  • Dense buds: PGR weed is extremely dense. Although some growers brag about how dense their flowers are, PGR buds look unnaturally thick. They feel heavier in the hand, and they’re slightly harder to break apart with the fingers. Weed that looks unusually thick ends up this way due to unnatural hormone hacking.
  • Reduced aroma: PGRs may create thick and large flowers, but they negatively affect the terpene profile. These flowers are much less aromatic than their natural counterparts. Even when pressing them up against your nose, you’ll detect light scents of earthiness at best.
  • Dark colouration: They’re very dark, almost brown, in appearance. PGR weed also has much less of a sheen because of reduced trichome count (the sparkly glands that produce cannabinoids and terpenes).

PGRs: A Reason to Grow Your Own or Store Wisely

There’s a whole lot of weed out there, and a big portion of it contains PGRs. But this doesn’t have to be a bad thing. While we don’t recommend smoking PGR weed, due to the lack of safety data, these contaminants make weed users think twice before buying cannabis, ultimately pushing them toward growing organically at home or purchasing naturally cultivated buds from small businesses in legal markets.

The more we educate ourselves about the potential dangers of the cannabis supply chain, the more we can strive to make better decisions.

External Resources:
  1. Determination of Pesticide Residues in Cannabis Smoke
  2. Pesticides in cannabis: A review of analytical and toxicological considerations
  3. Determination of Pesticide Residues in Cannabis Smoke
  4. Chlormequat chloride | C5H13Cl2N - PubChem
  5. Chlormequat Chloride
  6. Daminozide
  7. Toxic Effects of Paclobutrazol on Developing Organs at Different Exposure Times in Zebrafish - PubMed
  8. Toxicological characteristics of plant growth regulators and their impact on male reproductive health - PubMed
This content is for educational purposes only. The information provided is derived from research gathered from external sources.

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