Preen’s main product line does pose environmental risks, particularly to fish and aquatic life. The active ingredient in most Preen weed preventers is trifluralin, a pre-emergent herbicide that is highly toxic to aquatic organisms at very low concentrations and persists in soil for months. The EPA has identified potential risks of concern for mammals, birds, fish, aquatic invertebrates, and both aquatic and terrestrial plants. That said, the degree of environmental harm depends heavily on how and where you apply it.
What’s Actually in Preen
The standard Preen Garden Weed Preventer contains trifluralin, a synthetic herbicide that stops weed seeds from germinating. Preen also sells an Extended Control formula containing a different chemical called dithiopyr, and a Natural version made with corn gluten meal. Each of these has a very different environmental profile, so it matters which product you’re using.
Trifluralin is the ingredient that draws the most environmental concern. The EPA classifies it as a “Group C, Possible Human Carcinogen,” and its ecological risks are well documented. Corn gluten meal, by contrast, is essentially benign. The EPA states it is not harmful to humans, other non-target organisms, or the environment, and no toxic effects have been identified in mammals, birds, or fish.
Serious Risks to Fish and Aquatic Life
Trifluralin is exceptionally toxic to fish, even at concentrations measured in parts per billion. Rainbow trout can die within 96 hours at concentrations as low as 10 micrograms per liter. Bluegill sunfish are similarly sensitive, with lethal thresholds starting around 8.4 micrograms per liter. To put that in perspective, a single microgram per liter is one part per billion, an almost undetectable amount of chemical in a huge volume of water.
The damage goes beyond outright kills. Fathead minnows exposed to just 5.1 micrograms per liter over a long period died partway through the study. Survivors spawned 100 days later than unexposed fish. Sheepshead minnows developed spinal deformities and changes to their pituitary glands at concentrations between 1 and 5 micrograms per liter. Atlantic salmon exposed to trifluralin for just 11 hours became more susceptible to fungal infections over the following year, and some developed vertebral deformations.
Freshwater invertebrates like water fleas and snails are also vulnerable, with lethal concentrations starting around 50 micrograms per liter. Amphibians fare no better: Fowler’s toads showed lethal effects at 110 to 200 micrograms per liter. If trifluralin washes off your garden beds and into a storm drain, creek, or pond, even small amounts can cause real harm to aquatic ecosystems.
How Long Trifluralin Stays in Soil
Trifluralin is moderately to highly persistent. Under controlled lab conditions, its half-life ranges from 81 to 356 days. In real-world field studies across the U.S. and Europe, the average time for half the chemical to break down was 164 days, roughly five and a half months. In colder climates, degradation slows considerably. Some field measurements showed it took over 1,000 days for 90% of the chemical to disappear.
Sunlight speeds up breakdown significantly. Trifluralin exposed to light on sandy loam soil has a half-life of about 41 days, compared to 66 days in the dark. Volatility is another major route of dissipation: when applied to the soil surface without incorporation, 41 to 68% of the chemical can evaporate within 24 hours. This is why product labels require you to work it into the soil or water it in within 24 hours of application. That requirement exists partly to keep trifluralin from drifting into the air and landing on non-target areas.
Effects on Soil Health
The picture for soil organisms is more mixed. Earthworms are relatively tolerant. Lethal concentrations in lab tests exceeded 1,000 milligrams per kilogram of soil, far above what you’d apply in a garden. However, earthworms did lose body weight at the lowest concentration tested (95 mg/kg), suggesting sublethal stress is possible even at lower levels.
Soil microbes show a range of responses. At typical application rates, soil respiration (a measure of overall microbial activity) dropped to about 75% of normal levels and stayed suppressed over a five-week observation period. At higher concentrations, microbial activity initially crashed to 25% of normal before partially recovering to about 60%. Some bacteria and fungi showed no inhibition at all, and certain soil microorganisms can actually break trifluralin down and use it as a food source. Certain types of soil algae are sensitive, though. Genera like Chlamydomonas and Ulothrix declined in treated soil, while others like Chlorella were unaffected.
How Likely It Is to Reach Water
Trifluralin binds tightly to organic matter in soil, which limits its ability to leach into groundwater. The EPA considers it unlikely to contaminate groundwater under normal conditions. In practice, monitoring surveys support this: trifluralin was not detected in drinking water supplies across dozens of municipalities in Manitoba and Alberta, and was absent from irrigation water samples in Saskatchewan.
That said, it does show up occasionally. Trace levels (in the nanogram-per-liter range) were found in several municipal water supplies in Saskatchewan in 1987. One well in southern Ontario tested at 41 micrograms per liter, well above the concentrations known to harm fish. Surface water detections, while rare, have occurred at trace levels in Manitoba. The main risk isn’t deep leaching but surface runoff, especially during heavy rain shortly after application. Sandy soils with low organic content are the most vulnerable, adsorbing only about 10% of the chemical compared to organic-rich soils.
Preen Natural: A Safer Alternative
Preen Natural uses corn gluten meal instead of trifluralin. It’s a non-volatile powder that tends to stay where you put it, with no identified toxicity to mammals, birds, or fish. Corn gluten meal is a byproduct of corn processing and functions as a mild pre-emergent by inhibiting root development in germinating seeds. It also adds nitrogen to the soil, which means over-application could theoretically contribute to nutrient runoff, but at recommended rates the EPA considers it safe for the environment.
The tradeoff is effectiveness. Corn gluten meal is significantly less reliable as a weed preventer than trifluralin, particularly in wet conditions. But if your priority is keeping chemicals out of nearby waterways, it’s the Preen product with essentially zero ecological risk.
Reducing Environmental Harm
If you choose to use standard Preen, a few practices make a meaningful difference. Water it in or rake it into the soil immediately after applying, both to activate the product and to prevent it from volatilizing or washing away. Stick to the recommended application rate on the label. Heaping it on doesn’t improve weed control and increases the amount available to run off.
Leave a buffer zone between treated beds and any waterway, storm drain, or low spot where water collects. Even a strip of untreated lawn or mulch can intercept runoff before it reaches sensitive areas. Avoid applying before heavy rain is forecast, since a downpour shortly after application is the scenario most likely to carry trifluralin into water. And if your garden borders a pond, stream, or wetland, the corn gluten meal version or manual weeding are far safer choices given how toxic trifluralin is to aquatic life at trace concentrations.