Secret ingredients: what’s really in your weed killer?

Even though I spend much of my time at work talking about the weed killer glyphosate, I actually like plants. My mom got me started with our vegetable garden when I was three or four. Years later we decided our yard needed more pizazz. One of our favorites in the new flower beds was soapwort, a plant Europeans introduced to North America. My dad and I found our soapwort while canoeing. When we brought it home, mom pointed out that soapwort had a lot of uses. Crushing the plant releases chemicals that lather up like soap. Hence the name and its use for washing hands and clothes. 

Soapwort’s Latin name, Saponaria officinalis, is a nod to the fact that it makes chemicals called saponins that are responsible for the lather. “Officinalis” refers to the fact that people use it medicinally. There are some reports that it helps with breathing problems although this comes with the caveat that large doses will damage people’s digestive systems. Saponins will kill red blood cells at high enough doses. Saponins can do all of this because they are surfactants.

Surfactants and soaps help dissolve fats in water. That, of course, is why we use them to clean clothes, dishes, and hands. While they remove dirt and grease, they also dissolve bacterial cell membranes. That’s why washing your hands prevents the spread of disease. Do the same with cells growing in a laboratory and the soap will kill them just like the bacteria on our hands. Drink a bottle of soap and you will get sick. Spray concentrated soapy water on insects, and they die. Same goes for plants. Soaps are sold as organic insecticides and herbicides. You’ll see them on product labels as “salts of fatty acids” but that’s just another way of saying, “soap”. 

Herbicides, including some that rely on glyphosate, also contain surfactants that allow weed control at lower application rates. Most glyphosate-based herbicide formulations, including the ones I use in my own yard, contain glyphosate, water, and surfactants. In herbicides, surfactants act like soap to spread glyphosate out evenly and cross the waxy surface of a weed’s leaves. Surfactants are present at about 2% in a ready to use herbicide such as the Roundup I use in my yard. Product labels list surfactants, as well as water, as “inert ingredients” or “other ingredients”. 

Anyone who follows the conversation about glyphosate and Roundup-branded herbicides is familiar with a certain pattern that these conversations follow – after establishing the low toxicity of glyphosate, you’ll be met with questions about the safety of formulated products. The presence of “inert ingredients” or “other ingredients”, the argument goes, results in much greater toxicity than from glyphosate alone. The argument continues that these inert ingredients are secret, never disclosed on product labels, and can produce a wide range of effects that no one could ever anticipate.

Ingredients that aren’t directly responsible for killing weeds or bugs or fungus are typically not listed on any pesticide product label other than as “inert ingredients” or “other ingredients”. It is also true that, in some situations, the “other ingredients” can produce more pronounced effects than glyphosate alone. As for the specific effects, however, they are easily anticipated. They’re the types of effects anyone would expect from something that acts like soap. In fact, regulators require safety data on surfactants to understand and manage any risks to humans or the environment.

In response to published reports that glyphosate-based herbicides can do things like cause genetic damage, disrupt hormone signaling, cause Autism-like symptoms in mice, or kill bees, we’ve published our own research on the safety of these products and written letters to scientific journals to put reported effects into context. Our studies show that the surfactants we use in our glyphosate-based herbicides do exactly what anyone would expect – they act like soap. 

So, if it’s true that the “secret ingredients” in glyphosate-based herbicides are just surfactants that act like soap, then how did we end up talking about this? A 1995 publication on risk communication by Professor William Leiss provides some context. In this publication, Dr. Leiss discusses how governments, industry, labor, and environmental organizations approach risk communication and briefly turns his attention to glyphosate. 

When you think about it, that approach is understandable. Glyphosate has low toxicity and its properties are well-known. Surfactants, on the other hand, can seem mysterious. Their official chemical names are nearly unpronounceable. Pivoting from glyphosate to surfactants can add fuel to a fire that cannot burn on its own. 

This doesn’t mean every new study claiming to show an unexpected effect of a formulated herbicide can be dismissed. Given what is known, however, when a new study proclaims “shocking conclusions about the secret ingredients in Roundup” it might be worth asking whether the results are what any reasonable person would expect from soap. Or even a humble plant, like soapwort.