Ever since starting this blog, I’ve been more aware of what’s in the herbicides or insecticides that I might use. I’m sure most people just look at what the pesticide controls; I look at what the chemical(s) is (are) and then do some checking to see how it works.
I recently picked up some Weed B Gone Max to use on the lawn. I’ve stopped using the Scotts Step 2 Weed Control plus Lawn Food. When you use this product, you’re applying herbicide to the entire lawn. That makes sense if the lawn is really weedy but the weeds in my lawn are limited to the edges. The majority of the lawn is weed-free. I can’t justify applying herbicide where it isn’t needed.
As a result I’ve started using the Weed B Gone Max spray to apply directly to the weeds. When I looked at the active ingredients, I was a bit surprised. I expected the spray to contain 2,4-D (2,4-Dichlorophenoxyacetic acid) – this broadleaf herbicide has been around for ages. But the other chemicals, mecoprop-p and dicamba were new to me.
2,4-D, mecoprop-p and dicamba all have the same mode of action. These chemicals mimic the naturally occurring plant hormone indole-3-acetic acid (IAA or auxin). Auxin was the first plant hormone to be discovered and it helps to control and regulate the development and growth of a plant. When any of these three synthetic auxins are applied to a plant, they disrupt the normal growth patterns and cause the plant to twist, the leaves to cup and the stems to crack. Eventually this leads to the death of the plant.
Grass plants are tolerant of artificial auxins because they have specialized cells around their vascular tissue called schlerenchyma which prevent the vascular tubes from closing as the stems twist. The amount of schlerenchyma increase as the grasses grow; that’s why it’s not recommended to apply 2,4-D to a newly seeded lawn – the young grass plants don’t have enough schlerenchyma cells to withstand the herbicide’s action.
Artificial auxins are absorbed through the leaves of a plant. That’s why when you’re using a granular formula like Scotts Step 2, you have to apply it when the grass is wet so it will stick to the leaves of the weeds, otherwise it won’t work. The effects of the herbicide can be seen in about a day but it can take up to two weeks to kill the plant.
But if all three of these artificial auxins work in the same way, why are there three of them in Weed B Gone Max and not just one? Come to find out, some weeds are more affected by one artificial auxin than another. 2,4-D is great on dandelion and plantains but not chickweed and ground ivy. Mecorpop-p controls chickweed, clovers and ground ivy and dicamba is effective for controlling chickweed as well but doesn’t control plantains. By combining these three herbicides, overall weed control is better.
So that’s why all three herbicides are in Weed B Gone Max – any one artificial auxin wouldn’t kill all of the weeds but the three together will take care of most weed problems in turf. When I use pesticides, I usually prefer to use just one but in this case I can make an exception. I know what these herbicides do, I understand their mode of action and the combination of three makes sense. But I’m still not going to spray it indiscriminately on the lawn. Spot application makes sure that the least amount of herbicide is used in the most effective way.