Preen Weed Control Firstly, this product must be handled correctly as it is environmentally toxic and even harmful to human health. Even though this is a post-emergent granular weedicide, it does not contain highly toxic chemicals found in similar weed control products.Click to see full answer. Simply so, is preen safe?The main chemical in products such as Preen Garden Weed Preventer is trifluralin, which can irritate eyes and skin. It is harmful to fish and other aquatic life as well, and it should not be used near waterways, ponds, storm sewers or even driveway drains. is preen carcinogenic? Preen Lawn Weed Treatments This chemical is in many “weed and feed” products and is also used on crop fields. It is also a suspected endocrine disruptor and has been declared a possible carcinogen. In respect to this, is preen as bad as RoundUp? Preen’s active weed-killing agent is trifluralin. The formula is patent-protected. The manufacturer claims that like RoundUp it doesn’t hang around in the soil and it isn’t harmful to wildlife. People often ask me if Preen is really just corn gluten meal, with some chemicals added for good measure.Is preen safe around kids?Preen is a popular and highly effective garden weed preventer. Be sure seedlings are 2 to 3 inches tall before applying the natural weed preventer, and always follow label instructions. Your gardens will be safe for kids and pets immediately after application.