I’ve been a bit hesitant posting about Roundup, the non-selective herbicide. In some circles the word “Roundup” has become a shibboleth dividing the world into those who are environmentally conscious, opposed to big business and against genetically modified crops and those who aren’t. Personally, I’m somewhere in the middle on this whole debate. But in this post I just want to talk about what Roundup is, how it works and what’s in the bottles of Roundup lining the shelves of garden centers.
What got me thinking about Roundup was some of the commercials that I’d seen on TV for this product. One said that one kind of Roundup would kill weeds within a few hours. There’s also a commercial for a Roundup formulation that will continue to prevent weeds for up to three months.
Back when I was in college, Roundup was a fairly new and highly touted product. In a weed science class (yes, there are weed science classes!) we learned about how it worked and what it does. What I’d learned in that class was that Roundup doesn’t kill weeds instantly and it offers no extended control. So what’s up with those commercials that I’d seen?
The active ingredient in Roundup (at least it used to be) is glyphosate. This chemical is called a non-selective or broad spectrum herbicide because it doesn’t distinguish between different kinds of plants. Basically, if it’s green, glyphosate will kill it.
The way glyphosate works is interesting. When sprayed on a plant, the chemical is absorbed and tends to accumulate in the meristem of the plant. Meristem is a kind of tissue in plants that’s undifferentiated and can produce all kinds of different cells and tissue. You could call it the stem cells of a plant. The growing tip of a plant has apical meristem which produces leaves, stems and allows the plant to grow. Root meristem is located at the ends of roots and produces the various tissues that make up the root. Meristem is some of the most active tissue, constantly dividing and differentiating. While accumulating in the meristem, glyphosate also finds its way to the chloroplasts of the cells where photosynthesis happens.
When glyphosate is present in a plant, it attaches to an enzyme called 5-enolpyruvylshikimate-3-phospate or EPSP synthase. This enzyme is involved in a key step in the production of the amino acids phenylalanine, tyrosine and tryptophane. When glyphosate is attached to the EPSP synthase, the enzyme stops working and the plant can’t produce these three amino acids. Without them, the plant can’t make proteins containing these amino acids, the building of cell wall is disrupted, hormone production is interrupted and energy transport is stymied. While this biochemical disruption affects the whole plant, it’s especially harmful in the chloroplasts and the actively growing meristem. The plant begins to starve and dies within a week or two.
The other thing about glyphosate is that it doesn’t remain effective outside of the plant for very long. The chemical binds to the soil and is broken down by microorganisms. You can notice this if you mix up some Roundup in a sprayer that isn’t clean. After a few days, it becomes less effective because the glyphosate binds to the dirt in the sprayer and can’t be absorbed by the plant.
I know this is a lot of technical information, but all of this was in my mind while seeing those Roundup commercials. Since glyphosate interrupts amino acid production, it can’t kill a weed in a few hours – it takes at least a week. And since it binds to soil, it’s not a long acting herbicide – it only affects the weeds that are sprayed with it.
So this sent me on a search to the Scotts web site and I have to say that I was amazed. Back in the day, Roundup was glyphosate, period. But that’s not the case any longer. The fast acting Roundup has glyphosate but also diquat, a fast acting herbicide. Extended control Roundup also has glyphosate but added to it is Imazapic, an herbicide that prevent the germination of seeds. There’s also a Roundup for poison ivy that has glyphosate and Triclopyr, an herbicide for woody plants. If you’re looking for just glyphosate, you have to do some hunting; according to the site, only one Roundup product for use by homeowners (Roundup Super Concentrate) is just glyphosate.
Why are there all of these different formulations? I have my guesses! The only one that makes sense to me is the poison ivy product – woody weeds are hard to kill and the addition of a woody weed herbicide would likely make the product more effective. The others seem to me to be little more than attempts to appeal to suburban gardeners. People want weeds to die instantly; glyphosate takes a little time. So by adding diquat to the mix, people get faster, visible results. The extended control appeals to people who want to spray once and be done.
The other thing that has probably led to all of these formulations is that the patent on glyphosate expired in the year 2000. Now any company can use glyphosate; these various formulations help to keep Roundup competitive.
As I’ve said before, I’m OK with conscious and limited use of chemicals in the yard. But I like to know what I’m using and like to use the least possible. I find glyphosate works well for killing weeds in places were they can’t easily be controlled with cultivation. But I don’t want to use a mixture of chemicals if one will do the job. I don’t need instant results when I spray weeds. I also prefer to spray as needed and not use an extended control product that stays active in the soil for months.
Before I learned all of this, I thought Roundup was glyphosate. Now I know that’s not the case. I have some Roundup in the garage that has glyphosate and diquat – I’ll use what I have but from this point on, I’ll be reading the labels before I buy any more Roundup. I want to know what’s in the bottle before I use it.