A plant growth regulator (PGR) is a natural or synthetic compound that changes how a plant grows. It can change height, branching, rooting or flowering time. It works through the plant’s own hormone system and does not feed the plant, so a PGR is not a fertilizer. What it does depends on the compound, the plant species and when it is applied.
Natural PGRs are plant hormones. Synthetic PGRs either copy those hormones or block them. In horticulture they are everyday tools. Uses range from rooting hormone on cuttings to growth retardants that keep potted flowers short and compact.
When cannabis growers and buyers say “PGR”, they usually mean one group: synthetic growth retardants such as paclobutrazol, daminozide and chlormequat chloride. Buyers worry about residues of these chemicals in so-called “PGR buds”. There is a real basis for that worry. However, many popular ways to “spot” PGR flower by eye are unreliable.
This article covers what each hormone group does and where PGRs are used legitimately. It also covers what is documented about the three retardants and how to protect yourself as a grower or a buyer.
Five groups of plant hormones
A horticulture textbook from the University of Minnesota (United States) lists five major groups of plant hormones: auxins, cytokinins, gibberellins, ethylene and abscisic acid. Oregon State University Extension is the farm advice service of a US university. It notes that most groups contain both natural hormones and synthetic substances.
| Group | Main job in the plant | Example in horticulture |
|---|---|---|
| Auxins | Make cells longer, start new roots, bend shoots toward light | Rooting hormone (IBA, NAA) on cuttings |
| Gibberellins | Make cells longer, so stems stretch | Adding gibberellin makes plants taller; blocking it makes them shorter |
| Cytokinins | Make cells divide, push out side branches | A synthetic cytokinin is used to increase branching |
| Abscisic acid (ABA) | Signals stress and closes the stomata (tiny pores in the leaf that let air and water vapour in and out) | Central to how a plant responds to drought and stress |
| Ethylene | Ages flowers, ripens fruit | Products that release ethylene are used on ornamental plants |
The table draws on two more US universities. The University of Arkansas explains that gibberellins control how much cells elongate. Adding gibberellin makes a plant tall and spindly. Reducing it makes the plant shorter, with thicker stems and smaller leaves.
Virginia Tech (Virginia Polytechnic Institute and State University) describes a synthetic cytokinin that stimulates side branching. It also names ethylene as the main hormone behind flower ageing and fruit ripening.
Auxin, gibberellin and ethylene: the three a cannabis grower meets most
- Auxin is the rooting hormone. When you dip a cutting in rooting gel or powder, you add auxin so the cut stem forms roots.
- Gibberellin drives stem stretch. Most commercial growth retardants work by cutting the plant’s own gibberellin production. That is why treated plants stay short.
- Ethylene controls the ageing of flowers and the ripening of fruit. The flower industry uses ethylene-releasing products on ornamental plants.
What to do: find the active ingredient on each root, growth or “bloom” product you use, and match it to the table above. Then you will know which hormone the product acts on and what change in the plant to expect.
Legitimate uses of PGRs
Rooting hormone on cuttings
Rooting hormone is the PGR most cannabis growers already use. Kansas State University (United States) notes that commercial nurseries use natural and synthetic auxins to start roots on cuttings. The common ones are indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA).
Auxins raise the share of cuttings that root and the number of roots on each cutting. Too much auxin can backfire. It can make the cut end form callus, a lumpy mass of tissue instead of roots, and actually hinder rooting.
What to do: use the rate on the label, not more. Root one tray at the label rate and a few cuttings with no hormone. Then count the rooted cuttings and the roots per cutting. The comparison shows whether the product really helps in your setup.
Translator’s note: rooting hormone helps a healthy cutting form roots. It does nothing against pests or diseases the cutting already carries. Cuttings from another farm can bring in mites, thrips or hop latent viroid (HLVd), a disease agent that stunts cannabis. They can also be the wrong strain. Keep new cuttings in a separate area and watch them before they join your main room, so a problem shows up before it spreads.
Growth retardants on ornamental plants
The University of Arkansas explains that most commercial growth retardants block the plant’s production of gibberellin. The result is short, sturdy potted flowers that travel well. Common active ingredients are daminozide, chlormequat chloride and paclobutrazol.
According to Arkansas, paclobutrazol is the most widely used growth retardant for greenhouse flower crops in the United States. The key word is “ornamental”. Virginia Tech’s guide lists these products as labelled for ornamental plants grown in containers in nurseries and greenhouses. They are tools for the flower industry, not for food crops.
What to do: before you use any growth product, read the list of crops on its label. If cannabis is not listed, do not use the product on cannabis. The label decides what use is safe and permitted.
Translator’s note: in Thailand, paclobutrazol is not just a greenhouse chemical. Mango growers use it to make trees flower out of season, so it is easy to find. Because of that, some growers are tempted to try it on cannabis.
Translator’s note: use only plant-protection products registered with the Department of Agriculture (กรมวิชาการเกษตร), and only as the label says. A registration for mango does not cover cannabis.
The three synthetic retardants: what is documented
All three work against gibberellin, so the plant grows shorter, with thicker stems and smaller leaves. This is why growers find them attractive: a shorter plant fits a small room. Labs test for them because some have a documented regulatory history and because residues can reach the smoke. The details are below.
Daminozide
Daminozide is the best-documented case. The US Environmental Protection Agency (EPA) is the US government agency that registers pesticides. According to the EPA, all food uses of daminozide were cancelled in 1989.
The EPA’s review found that daminozide caused tumours in several species of test animals. So did UDMH (unsymmetrical dimethylhydrazine), which forms when daminozide breaks down. The EPA classified UDMH as a Group B2 “probable human carcinogen”. B2 is the EPA category used when the evidence of cancer comes from animal studies rather than from human data.
Today daminozide has no registered food or animal-feed uses. Its only remaining registration is for ornamental plants in greenhouses and similar structures.
Paclobutrazol
Paclobutrazol belongs to the triazole family of growth retardants, a chemical family that also includes several common fungicides. It blocks the plant’s production of gibberellin. Virginia Tech’s guide tells growers not to reuse pots, trays or growing media previously treated with paclobutrazol. That warning suggests the chemical stays active in the medium after the treated crop is gone.
Heliyon is a scientific journal. A 2024 study published there, on cannabis plantlets grown in laboratory culture, notes that paclobutrazol may increase floral biomass (the weight of flower) and speed up development. The same study notes that, because of health concerns, its use is prohibited in cannabis production.
What to do: do not reuse pots, trays, soil or coco of unknown history, such as second-hand stock from a nursery or an orchard. Paclobutrazol left in the medium could keep acting on your next crop without you knowing.
Chlormequat chloride
The University of Arkansas notes that sprays of chlormequat chloride on leaves often cause a phytotoxic response. This means the chemical itself damages the plant, seen as yellowing leaves. New leaves usually hide this damage later, so by harvest the sign may no longer be visible.
What to do for all three: check the active-ingredient line of every growth, bloom or “bulking” product for paclobutrazol, daminozide or chlormequat chloride. If you find one of them, or if the label lists no active ingredients, keep the product away from cannabis. That way, a buyer’s lab test will not find these residues in your flower.
Translator’s note: if you grow for a farm certified under GACP (Good Agricultural and Collection Practice, the quality standard for growing and handling medicinal plants), read the forum topic Does GACP Certification Mean PGR-Free?. It goes through the Thai standard line by line, so you can check what it actually requires.
Why “PGR buds” worry buyers
The concern is residues. A 2013 study in the Journal of Toxicology looked at medicinal cannabis cultivation. It reported that independent testing laboratories and government laboratories found the use of pesticides and PGRs to be quite prevalent.
In the same study, researchers applied several pesticides and paclobutrazol to cannabis. Then they measured how much reached the smoke. Up to 69.5% of the residue was recovered in the smoke, depending on the smoking device and the compound. In the worst case, more than two-thirds of the residue on the flower reached the person smoking it.
This is the real, sourced basis for the concern:
- These compounds were developed for plants that people do not eat or smoke.
- Some of them have a documented regulatory history, such as daminozide’s cancelled food uses.
- Residues can carry into smoke.
None of this means every dense bud is contaminated. Treat PGR residue as a question for a laboratory, not something you can judge by looking at a bud.
Popular ways to “spot” PGR flower, and how reliable they are
| Claimed sign | Why it is unreliable |
|---|---|
| Rock-hard, unusually dense buds | Density also rises with light. A 2021 cannabis study found that apical bud density (the top bud on each stem) rose steadily with light intensity. Genetics matter too. |
| Few visible trichomes (the tiny resin glands on the flower), weak smell | These can also come from genetics, harvesting too early or too late, and poor drying or curing. |
| Lots of dark orange or red pistils (the hair-like strands on the flower) | Pistil colour changes as flowers mature, and it differs between varieties. |
| Harsh smoke, poor burn | This is usually a drying, curing or moisture problem. |
| Very large buds for the plant size | Good light, plant training and genetics produce this too. |
None of these signs proves or rules out PGR use. The only reliable answer is a laboratory residue test. If you buy flower, ask for test results that include plant growth regulators. If you grow, the simplest protection is to use only products that list their active ingredients on the label.
Translator’s note: in Thailand, cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. It may be sold only with a PT 33 (ภ.ท.33) prescription, which covers at most 30 days. Advertising and online sale of flower are prohibited.
Translator’s note: as a buyer, ask the licensed shop that fills your prescription for the certificate of analysis, the lab report for that batch. Check whether its list of tested compounds includes plant growth regulators. If it does not, that certificate tells you nothing about PGRs.
Translator’s note: as a grower, ask the laboratory before you send a sample which compounds its residue panel covers. Do not assume a pesticide panel includes paclobutrazol, daminozide or chlormequat chloride. Knowing the list in advance means you pay for the test you actually need.
Controlling height without growth retardants
Growers mostly use retardants to control stretch, the fast height gain after the switch to flowering. They also want to fit more flower into a small space. There are safer ways to do both:
- Training. Topping, bending and low-stress training (tying branches down without cutting them) spread the canopy and keep it level.
- Light. Adequate light intensity also improves bud density, as the 2021 study found.
- Genetics. Choose varieties whose final height suits your ceiling.
- Timing. Switch to flowering earlier if the plant is getting too tall for the space.
Translator’s note: many growers in Thailand and Southeast Asia work in small tents or converted rooms with low ceilings. Measure plant height on the day you switch to flowering, and again when the plants stop getting taller. Write both figures down for each variety. That record tells you how early to switch next time, with no retardant needed. Try any new variety with a few plants first, so an unexpectedly tall strain does not fill a whole room.
Common mistakes
- Treating every PGR as dangerous. Rooting hormone is a PGR, and a legitimate one.
- Treating every dense bud as a PGR bud. Light and genetics also make buds dense.
- Using a “bloom” or “bulking” supplement whose active ingredients are not listed. If you cannot see what is in it, you cannot know which hormone it acts on or what residue it leaves.
- Assuming a product is safe for a smoked crop because it is sold for flowers. Flower-industry products are made for plants that nobody eats or smokes.
Translator’s note: to check whether a fertiliser or supplement is legally on sale in Thailand, read the forum topic Verifying a Fertiliser Is Legally in Thailand. It explains the registration and what the label must show, so you can reject unlabelled products before you buy.
Three things to do this week
- Check every label. Look at each root, growth, bloom and bulking product you own. Set aside anything containing paclobutrazol, daminozide or chlormequat chloride, and anything with no active ingredients listed. This removes the most likely source of PGR residue from your grow.
- Ask for the right test. When you buy flower or send your own for testing, confirm that PGRs are on the list of tested compounds. Only a test that includes them can answer the PGR question.
- Record stretch per variety. Note the height at the switch to flowering and the final height. This gives you a way to control height without chemicals in the next run.
If you have had flower tested for PGRs in Thailand, or have a question about a product label, post it below this topic. Include the product name and the active ingredients from the label so other growers can check their own products.
References
- University of Minnesota (United States): horticulture textbook, chapter on the five major groups of plant hormones.
- Oregon State University Extension (United States): natural and synthetic substances in each plant hormone group.
- University of Arkansas (United States): gibberellins and cell elongation; growth retardants that block gibberellin production; paclobutrazol in US greenhouse floriculture; phytotoxicity of chlormequat chloride sprays.
- Virginia Tech (United States): guide to plant growth regulators labelled for containerised ornamental plants in nurseries and greenhouses; synthetic cytokinin for branching; ethylene; reuse of paclobutrazol-treated pots, trays and media.
- Kansas State University (United States): auxins (IAA, IBA, NAA) for rooting cuttings; callusing at high auxin rates.
- US Environmental Protection Agency (EPA): daminozide, cancellation of food uses in 1989, UDMH and Group B2 classification, remaining ornamental registration.
- Ajdanian et al. (2024). Investigating the impact of paclobutrazol and tannic acid on floral development of in vitro-grown cannabis plantlets. Heliyon 10(17): e36768.
- Sullivan N, Elzinga S, Raber JC (2013). Determination of pesticide residues in cannabis smoke. Journal of Toxicology 2013: 378168.
- Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12: 646020.