Herbicides are chemicals that prevent weeds, inhibit their growth or kill them outright. There are several types of herbicides, killing weeds at different stages of their life, as well as many different herbicide actions that destroy the weed's body and ability to produce new seed. There are almost as many different herbicide types as there are plant processes.
Selective Vs. Nonselective
Herbicides kill weeds according to whether the herbicide is chemically made up to be selective, where only specific plants are killed by the chemical, or nonselective, meaning all plants that come into contact with the chemical die. Although nonselective herbicides are formulated to kill specific weeds, misapplication may kill anything the chemical comes in touch with, says the University of Rhode Island.
Contact action herbicides kill parts of the weed the chemical actually comes in contact with. Herbicides that require contact are sprayed onto the plant and absorbed through the waxy layer of the leaf. Contact herbicides will kill only the area they touch and do not kill the weed roots, meaning the weed may grow back, says the University of Illinois Extension.
Systemic herbicides are absorbed by the roots or the foliage of the plant. Herbicides move throughout the plant and are further absorbed into the water around the plant cells say the Montana War on Weeds website. The herbicide moves into the cell and usually attacks enzymes which help the weed grow. The weed will die, and will not be able to grow back from the roots.
Pre-emergent herbicides kill weeds as they germinate from their seeds. To be effective, pre-emergent herbicides are applied directly to the soil and spread with a light application of water. The herbicide makes a barrier of poison that reduces the chance of weed germination, says the University of Illinois.
Post-emergent herbicides kill weeds once they are already established, either by contact or as a systemic poisoning. Post-emergent herbicides are applied either as selective or non-selective varieties. Toxins build up within the plant, eventually destroying the cells and killing the weed.