Herbicides are selective or non-selective poison that kills weeds. Different herbicides work on different parts of plant physiology in various ways. Some are translocated by the plant's vascular system, some affect the plant on contact, and some remain in the soil and damage subsequent crops. Other herbicides travel as a vapor, infecting neighboring plants.
British and American scientists in the 1940s discovered auxinic herbicides, which are among the most widely used classification of weed killers. Auxinic herbicides kill broadleaf weeds in cereal crops such as wheat, corn, barley and soybeans. It is applied after weeds grow, entering the plant vascular system and infusing it with poison. Toxicity to soil varies with individual types of auxinic herbicide. Trade names include Remedy, Grandstand and Attain.
Non-selective herbicides destroy or prevent plant life regardless of species. Selective herbicides are toxic to broadleaf weeds or grass species. Broadleaf herbicides are often used on lawns because they do not affect grass. The toxicity of all herbicides to plant physiology depends on concentration and when it is applied. Pre-emergent herbicides kill weeds as they germinate, post-emergent herbicides kill young plants.
Contact and Systemic Herbicides
Contact herbicides such as Roundup kill growing plant tissue when they come in contact. Growth stops, plants become straw-colored, turn brown and die. Contact herbicides also kill nearby ornamental plants and crops. Systemic herbicides are absorbed by plant roots and leaves, entering the plant vascular system. Translocation occurs upward from roots. Some types of systemic herbicides are designed to remain in the soil and kill emerging weeds.
Excessive use of herbicides can injure the target plant and surrounding plants. Herbicides that are carried by water may be absorbed by root systems nearby. Symptoms of herbicide poisoning include yellow and blackened leaves; twisted and deformed leaves; spotted, speckled leaves. Grasses become stunted and purplish from excessive herbicide exposure. Some plants show symptoms on older leaves such as red or purple veins on leaf margins or stretched and "cupped" leaves.
Seedling and Cell Growth
Herbicides applied to soil to inhibit weed growth may also injury crop plants such as corn. Emerging shoots appear thick, short and red or purple in color. Shoots may not emerge from the ground, or develop stunted leaves that do not unfurl. Abnormal seedlings may result from herbicide exposure and not germinate, according to a report on herbicide modes of action by the Ontario Ministry of Agriculture website.