Weeds are plants that are out of place in a garden or landscape. Weeds are not always destructive, but almost all of them will ruin the uniform quality of the garden design. Herbicides are chemicals that reduce the impact of the weeds and kill them off. Herbicides come in several varieties according to when in the life of the weed you wish to apply the treatment.
Herbicides that only affect a certain range of plants are said to be selective, says the University of California Davis. Selective herbicides affect only the target weeds without damaging the turfgrass or other plants in the garden. Selective herbicides affect broadleaf weeds or grassy weeds according to the needs of the gardener.
Nonselective herbicides are used in sites where all vegetation requires removal. Nonselective kills almost all vegetation it comes in contact with. This herbicide variety should only be used when trying to clear an area of all weeds before planting a new garden or turf.
Herbicides injure plants in different ways. Contact herbicide damages a plant by causing a localized injury where the herbicide comes in contact with the plant. Contact herbicides are used against young weeds that have not built up a resistance to herbicide. Systemic herbicides move through the inside of the plant, blocking photosynthesis and breaking down food-holding cells.
Preemergent and Postemergent
Herbicides kill weeds at different times during the weed's life cycle. Preemergent herbicides are applied before weeds germinate and emerge from the soil. A preemergent herbicide is generally used on grassy weeds. Postemergent herbicides are used against established weeds.
According to the University of Missouri Extension, weeds generally become resistant to an herbicide after the same herbicide is used for several years in a row, between 4 and 10 years. Rotating the herbicides being used for control reduces the chance of herbicide resistance.