How Herbicides Work
There are two basic types of herbicides: contact and systemic. Contact herbicides work by burning the foilage of the plant, and are non-selective, damaging or killing the plant. Contact herbicides do not kill the roots, so some plants come back, i.e. perennials. Systemic herbicides are both selective and non-selective and work by translocating via phleom (the cells in a plant that conduct nutrients), killing the root. Selective herbicides kill only certain types of plants; non-selective kill almost everything. Proper application of an herbicide is vital (see our flyer on "Why Herbicides Fail"). This flyer assumes that steps have been taken for a correct application.
All herbicides are absorbed through the foilage of the target plant. Some herbicides enter through the cuticle of a plant (the outer surface of the skin or a plant), others through the stomata (minute openings in the leaves). With contact herbicides, the foilage cells are killed, turning the plant brown. Systemics, however, are transported through the leaves and into the plant's nutrient system.
Once your apply a systemic herbicide, it must move through the plant to the site of action. This can be the roots or any other part of the plant that is disrupted by the herbicide (depending on chemical and mode of action). It is important to know that the healthier a plant is, the better the kill. If the plant leaves are damaged, the plant is drought stressed, or growing in shade, you may not get the effect you are after.
Disruption of Metabolism
The final stage of death occurs when the absorbed herbicide disrupts metabolism and causes the plant to die. Plant metabolism is at its highest when plants are young, healthy, and actively growing. This slows down as weeds grow older. This is why larger, older weeds are harder to kill than younger ones.