Research has found that small, non-lethal doses of glyphosate can stimulate plant growth and, in some weeds, substantially increase seed production.
Could this overlooked effect complicate efforts to control herbicide-resistant weeds?
Glyphosate is designed to kill unwanted plants. But what happens when a plant receives too little glyphosate to kill it?
The obvious assumption is that the herbicide simply fails to work. However, scientific research has identified another possible outcome: under certain conditions, low doses of glyphosate can actually stimulate plant growth and reproduction.
The phenomenon is known as hormesis, and researchers have been investigating it for nearly two decades.
Some studies have found that plants exposed to small amounts of glyphosate grow faster, produce more biomass or generate more seeds than plants receiving no glyphosate at all.
The findings raise questions about the unintended consequences of sublethal herbicide exposure, particularly where repeated spraying, spray drift or uneven application leaves some plants exposed to doses too low to kill them.
What Is Glyphosate Hormesis, and How Does It Work?
Hormesis describes a biological response in which a low dose of a substance produces a stimulatory effect, while higher doses produce inhibition or toxicity.
In plants, this means a chemical intended to suppress growth may, at sufficiently low concentrations, have the opposite effect.
In 2008, researchers led by Edivaldo Velini published a study in Pest Management Science examining the effects of low glyphosate doses on several plant species.
They found that glyphosate application rates ranging from 1.8 to 36 grams of acid equivalent per hectare stimulated growth in several species, including maize, conventional soybean, eucalyptus, pine and a broadleaf weed.
The response varied between species and plant tissues. Not every plant responded in the same way, and the doses producing stimulation differed.
Nevertheless, the findings demonstrated that glyphosate’s biological effects are not always a straightforward progression from no effect to plant death.
At sufficiently low doses, growth stimulation can occur.
Australian Study Finds Low-Dose Glyphosate Can Increase Weed Seed Production
One particularly striking finding came from research conducted at the University of Queensland.
In a study published in Weed Science in 2020, researchers Ahmadreza Mobli, Amar Matloob and Bhagirath Singh Chauhan investigated the effects of low glyphosate doses on common sowthistle (Sonchus oleraceus).
The researchers grew sowthistle plants in a screenhouse — a greenhouse-like structure enclosed with mesh rather than glass — and applied different doses of glyphosate while the plants were young.
At low application rates, the plants showed increased height and leaf production compared with untreated plants.
But the most significant finding concerned reproduction.
At a glyphosate dose of 5 grams of acid equivalent per hectare, seed production increased by 154% in the first experiment and 101% in the second, compared with plants receiving no glyphosate.
In other words, under those experimental conditions, a low dose of glyphosate more than doubled the number of seeds produced by the weed.
That finding is important because weed management is not simply about killing the plants visible today. It is also about preventing those plants from producing the seeds that establish future generations.
A weed that survives spraying and produces more seeds could potentially create a greater management problem in subsequent seasons.
However, these results came from controlled experiments. They do not establish that ordinary glyphosate spraying routinely increases weed seed production in agricultural fields or along roadsides.
Could Glyphosate Spray Drift Stimulate Weed Growth?
Spray drift is one potential route through which plants may encounter sublethal doses of glyphosate.
During herbicide application, some spray droplets can move beyond the intended treatment area. Plants at the margins of sprayed areas may receive substantially less herbicide than those directly targeted.
Uneven spray coverage, shielding by vegetation and other application factors can also result in plants receiving less than the intended dose.
A 2018 scientific review by Brito and colleagues examined glyphosate hormesis and noted that herbicide application rates are not uniform under field conditions.
The authors identified several responses reported in experimental studies, including increased growth, photosynthesis, seed production and changes in plant development.
However, the dose required to produce stimulation can vary considerably according to plant species, growth conditions and other factors.
This variability is an important limitation.
A dose that stimulates one plant may have no measurable effect on another or may cause injury instead.
There is also a difference between demonstrating growth stimulation under experimental conditions and establishing its ecological importance in real-world spraying programmes.
For example, a 2008 study by Nina Cedergreen examined whether growth stimulation from low glyphosate doses persisted over time. The researchers found that early increases in plant growth did not produce a statistically significant difference at harvest.
Not every initial growth response translates into increased reproduction or a lasting advantage.
Could Low-Dose Glyphosate Contribute to Herbicide Resistance?
The possible relationship between hormesis and herbicide resistance adds another dimension to the research.
Herbicide resistance develops when repeated chemical exposure favours plants capable of surviving treatments that kill susceptible individuals.
Over successive generations, those surviving plants may become more common within a weed population.
In New Zealand, herbicide resistance is already a documented agricultural concern, as we explored in our earlier investigation into herbicide resistance and changing weed-control practices.
Researchers have questioned whether growth stimulation at sublethal doses could influence this process.
A 2025 review published in Advances in Weed Science, authored by Stephen Duke, Regina Belz, Caio Carbonari and Edivaldo Velini, examined both the potential benefits and disadvantages of herbicide hormesis.
The authors suggested that hormesis could complicate weed management where resistant or more tolerant individuals receive doses that stimulate their growth or reproduction.
Under those circumstances, plants already better equipped to survive herbicide exposure might gain an additional competitive or reproductive advantage.
Hormesis does not necessarily create herbicide resistance, but it could give some surviving weeds an additional advantage.
The concern is that growth stimulation could, under some conditions, favour the survival and spread of plants possessing resistance-related traits.
A 2026 scientific review examining thirty years of glyphosate-resistant crops and weeds also discusses this possibility. The researchers suggest that certain glyphosate doses may stimulate the growth of resistant weeds, potentially enhancing their survival and reproduction. However, they emphasise that further investigation is needed, particularly under real-world field conditions.
In particular, many hormesis experiments have not been conducted under field conditions or examined the resistance mechanisms of the plants involved.
The hypothesis is scientifically plausible, but its practical importance has not yet been established.
What Does Glyphosate Hormesis Mean for New Zealand?
Glyphosate is used in New Zealand agriculture, horticulture and vegetation management, including the maintenance of roadsides, parks, reserves and other public spaces.
No More Glyphosate NZ has previously investigated whether repeated herbicide spraying along fence lines and roadsides could contribute to resistance. The research into glyphosate hormesis introduces another potential consequence of sublethal exposure that deserves closer examination.
Where glyphosate is applied repeatedly, what happens to plants that survive exposure to small amounts of the herbicide?
Some may be naturally tolerant. Others may possess resistance traits. Some may simply escape receiving a lethal dose.
The research suggests that, for certain species and under particular conditions, sublethal exposure may do more than leave a plant alive. It may stimulate growth or seed production.
However, we have not established that glyphosate-induced hormesis is occurring in New Zealand roadside vegetation or that it is contributing to local herbicide resistance.
That distinction matters.
The international studies provide a reason to investigate the question, not grounds to assume the same outcomes are occurring here.
It would be useful to establish whether New Zealand researchers have examined hormesis in locally important weed species, particularly those exposed to repeated glyphosate applications.
Why the Effects of Low-Dose Glyphosate Need Further Research
Glyphosate-induced hormesis does not mean that spraying generally makes weeds grow faster.
At effective doses, glyphosate remains capable of controlling susceptible plants. At lower doses, the response depends on the species, its physiological condition, the environment and the amount of herbicide received.
But the scientific findings challenge the assumption that a non-lethal herbicide dose necessarily has no meaningful consequences.
In some experiments, low-dose exposure has stimulated plant growth. In others, it has increased seed production. Researchers are also examining whether these responses could influence the selection and spread of herbicide-resistant weeds.
The implications extend beyond the immediate effectiveness of an individual spraying operation.
Weed control decisions affect which plants survive, how they reproduce and which characteristics become more common in future generations.
This also connects with a broader question we have explored: how repeated attempts to control biological systems can favour adaptation, and why building more resilient farming systems may ultimately prove more effective than relying on increasingly complex chemical treatments.
As herbicide resistance becomes an increasingly important challenge, understanding these less obvious biological responses may prove valuable.
The question is not whether glyphosate always encourages weed growth. It clearly does not. The question is whether some of the plants that survive low-dose exposure are being given an unintended advantage — and whether existing weed management practices adequately account for that possibility.
That is a question worth investigating.
Scientific references
The possibility that a herbicide could stimulate the very plants it is intended to kill may seem counterintuitive, but it is not a new or unsupported idea. Researchers have been documenting and investigating glyphosate-induced hormesis for nearly two decades.
The studies below provide the scientific foundation for this article. They also demonstrate why the distinction between laboratory findings, controlled growing conditions and real-world spraying practices matters. What has been observed experimentally is important, but so too are the questions that remain unanswered.
Velini, E.D. et al. (2008). Glyphosate applied at low doses can stimulate plant growth. Pest Management Science, 64(4), 489–496.
https://doi.org/10.1002/ps.1562
Mobli, A., Matloob, A. & Chauhan, B.S. (2020). Glyphosate-induced hormesis: impact on seedling growth and reproductive potential of common sowthistle (Sonchus oleraceus). Weed Science, 68(6), 605–611.
https://doi.org/10.1017/wsc.2020.77
Brito, I.P. et al. (2018). Hormetic effects of glyphosate on plants. Pest Management Science, 74(5), 1064–1070.
https://doi.org/10.1002/ps.4523
Duke, S.O. et al. (2025). Understanding herbicide hormesis: Evaluating its positive and negative aspects with emphasis on glyphosate. Advances in Weed Science, 43.
https://doi.org/10.51694/AdvWeedSci/2025;43:00006
Alcántara-de la Cruz, R. et al. (2026). Thirty years of glyphosate-resistant crops and weeds: Current situation and future prospects. Pest Management Science, 82(7), 5987–6009.
https://doi.org/10.1002/ps.70742
Cederlund, H. (2017). Effects of spray drift of glyphosate on nontarget terrestrial plants — A critical review. Environmental Toxicology and Chemistry, 36(11), 2879–2886.
https://doi.org/10.1002/etc.3925
Cedergreen, N. (2008). Is the growth stimulation by low doses of glyphosate sustained over time? Environmental Pollution, 156(3), 1099–1104.
https://doi.org/10.1016/j.envpol.2008.04.016
Scientific research does not always provide immediate or straightforward answers. Sometimes its greatest value lies in revealing consequences that were not anticipated when a chemical or technology was first introduced.
The research into glyphosate hormesis raises precisely that possibility. If low-dose exposure can stimulate growth or seed production in certain weeds, how often might this occur outside controlled experiments, and what could it mean for long-term weed management?
These are not reasons to jump to conclusions. They are reasons to keep asking questions — and to ensure those questions are investigated independently.
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