A plant growth regulator (PGR) is a natural or synthetic compound that changes the way a plant grows, such as its height, branching, rooting or flowering time. It works through the plant’s own hormone system, not by feeding it, so a PGR is not a fertilizer. What it does depends on the compound, the plant species and when it is applied. The natural ones are plant hormones; the synthetic ones either copy those hormones or block them. In horticulture, PGRs are routine tools, from rooting hormone on cuttings to growth retardants that keep potted flowers compact. In cannabis talk, “PGR” usually means one group: synthetic growth retardants such as paclobutrazol, daminozide and chlormequat chloride. Consumers worry about residues of these in so-called “PGR buds”. There is some basis for that concern, but many popular ways to “spot” PGR flower are unreliable.
The five natural hormone groups
The University of Minnesota’s horticulture textbook lists five major groups of plant hormones: auxins, cytokinins, gibberellins, ethylene and abscisic acid. Oregon State University Extension notes that for the most part, each group contains both naturally occurring hormones and synthetic substances.
| Group | Main job | Example in horticulture |
|---|---|---|
| Auxins | Cell elongation, root formation, bending toward light | Rooting hormone (IBA, NAA) on cuttings |
| Gibberellins | Cell elongation, so stem stretch | Applied gibberellin makes plants taller; blocking it makes them shorter |
| Cytokinins | Cell division, side branching | A synthetic cytokinin used to increase branching |
| Abscisic acid (ABA) | Stress signalling, closing stomata | Central to drought and stress responses |
| Ethylene | Flower ageing, fruit ripening | Ethylene-releasing products used on ornamentals |
Sources for the table: the University of Arkansas explains that gibberellins control cell elongation, that applied gibberellins make a plant tall and spindly, and that reducing gibberellin makes it shorter, with thicker stems and smaller leaves. Virginia Tech describes a synthetic cytokinin that stimulates side branching, and names ethylene as the main hormone behind flower ageing and fruit ripening.
Legitimate uses
Rooting hormone. This is the PGR most cannabis growers already use. Kansas State University notes that natural and synthetic auxins are used by commercial nurseries to start root formation on cuttings. It lists indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) as the common ones. Auxins raise rooting percentage and the number of roots per cutting. Too much auxin, however, can promote callusing and actually hinder rooting.
Growth retardants on ornamentals. The University of Arkansas explains that most commercial growth retardants work by blocking gibberellin production inside the plant. This gives short, sturdy potted flowers that ship well. Common active ingredients are daminozide, chlormequat chloride and paclobutrazol. Arkansas calls paclobutrazol the most widely used growth retardant for greenhouse floriculture crops in the United States.
The key word is ornamentals. Virginia Tech’s guide lists these products as labelled for containerized ornamental plants in nurseries and greenhouses. They are flower-industry tools, not food-crop tools.
What is known about the synthetic retardants
Daminozide. This is the best-documented case. According to the US EPA, all food uses of daminozide were cancelled in 1989. The EPA’s review found that daminozide and its breakdown product UDMH caused tumours in multiple species of test animals. The EPA classified UDMH as a Group B2 “probable human carcinogen”, and daminozide with it. Today it has no registered food or feed uses; its remaining registration is for ornamentals in greenhouses and similar structures.
Paclobutrazol. It belongs to the triazole family of retardants and blocks gibberellin production. Virginia Tech’s guide tells growers not to reuse pots, trays or media previously treated with paclobutrazol, which suggests it stays active in the medium. A 2024 cannabis study in BMC Plant Biology notes that paclobutrazol may increase floral biomass and speed development, but that due to health concerns its use is prohibited in cannabis production.
Chlormequat chloride. The University of Arkansas notes that foliar sprays often cause a phytotoxic response (yellowing), usually hidden later by new leaves.
Why “PGR buds” worry consumers
The concern is residues. A 2013 study in the Journal of Toxicology reported that testing laboratories and authority laboratories have found the use of pesticides and plant growth regulators in medicinal cannabis cultivation to be quite prevalent. The same study applied several pesticides and paclobutrazol to cannabis and measured how much reached the smoke. Recoveries of residues were as high as 69.5%, depending on the smoking device and the compound.
That is the real, sourced basis for the concern: these compounds were developed for plants that are not eaten or smoked, some have a documented regulatory history, and residues can carry into smoke. It does not mean every dense bud is contaminated.
How people claim to spot PGR flower, and how reliable it is
| Claimed sign | Why it is unreliable |
|---|---|
| Rock-hard, unusually dense buds | Density also rises with light. A 2021 cannabis study found apical bud density increased linearly with light intensity. Genetics matter too. |
| Few visible trichomes, weak smell | Also caused by genetics, early or late harvest, and poor drying or curing. |
| Lots of dark orange or red pistils | Pistil colour changes as flowers mature and differs between varieties. |
| Harsh smoke, poor burn | Usually a drying, curing or moisture issue. |
| Very large buds for the plant size | Good light, 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 whose active ingredients are listed on the label.
Controlling height without growth retardants
Growers mostly reach for retardants to control stretch and pack more flower into a small space. There are safer ways:
- Training. Topping, bending and low-stress training spread the canopy and keep it level.
- Light. Adequate light intensity also improves bud density, as the 2021 study found.
- Genetics. Choose varieties that suit your ceiling height.
- Timing. Flip to flowering earlier if the plant is getting too tall for the space.
Common mistakes
- Treating every PGR as dangerous. Rooting hormone is a PGR, and a legitimate one.
- Treating every dense bud as a PGR bud.
- Using a “bloom” or “bulking” supplement whose active ingredients are not listed.
- Assuming a product is safe for a smoked crop because it is sold for flowers.