How do herbicides (organic, if such exist, or non-organic) work?
South Bend, Ind.
Herbicides are considered a subcategory of pesticides, for all you confused by my last pesticide comments. Herbicides kill plants with a vast array of ingenious torture and maiming techniques. (Maiming a plant isn’t quite like maiming an animal: plants can grow back broken limbs, and it’s hard to deny a plant its food source unless you expend the effort to get the plant out of the ground entirely.) Effective herbicides work at a cellular level to disrupt the functioning of the life-support system of the plant.
Chemical herbicides may: block enzyme/amino-acid production, block photosynthesis, disrupt cell membranes, stop or accelerate cell division, destroy chlorophyll, stop protein synthesis. A few examples of herbicides allowed under the organic rules are acetic acid (which is basically a component of vinegar) and mixtures containing clove oil, to burn the plant; and some types of soap, to dry the plant. All these result in a mutant plant which can no longer function and dies. Wikipedia will give you a good, more technical description of these actions.
Contact herbicides must be sprayed on the actual plant in order to do their work, while systemic herbicides can be sprayed onto the plant or soil and later be innocently taken up by the plant into itself as part of its nutritional regimen.
Herbicides are popular because they work well at the outset. They do the job and kill target plants. It can hardly be necessary to add that they can also kill and maim non-target plants and animals, and can have undesirable long-term impacts. Hence, we would like to reduce the use of herbicides, particularly on the homeowner front.