Chemicals that control and prevent weed growth are called herbicides. Herbicides that are used to prevent the germination of weed plants are categorized as pre-emergent herbicides, whereas herbicides that are used to kill growing weed plants are called post-emergent herbicides. Chemical herbicides can be further categorized as selective or nonselective based on whether or not they target only a specific subset of weed plants. The hundreds of specific chemicals used for weed control can be listed under large chemical groups based on their chemical composition.
Bipyridylium chemical herbicides consists of a group of post-emergent, nonselective weed herbicides that are sprayed directly on the weed foliage during periods of growth. This group of herbicide blocks the photosynthetic processes within the weed's leaves and causes the plant to wilt and eventually die.
Phenoxy acid chemical herbicides are a group of post-emergent, selective herbicides that are effective at damaging broadleaf perennial weeds. As a selective herbicide, phenoxy acid is sprayed directly on the weed growth and doesn't damage surrounding grassy plants. This chemical group works by permeating throughout the root system and eventually impairing the growth regulators within the weed.
Arsenical type herbicides are nonselective, post-emergent herbicides that act immediately on contact with weed foliage. Effective at controlling nutsedge and crabgrass, arsenical herbicides are fast-acting and cause the weed foliage to turn yellow within one week of application.
Dinitroaniline-based herbicides consist of a group of selective, pre-emergent herbicides that are sprayed on the lawn prior to weed growth to prevent future germination. This selective herbicide acts only on grassy lawn weeds and is typically applied during lawn irrigation. Dinitroaniline pre-emergent herbicides inhibit the germination of weeds by blocking the initial growth of the necessary root systems.