What Makes a Weed Killer Kill the Weeds?

Weed killers destroy plants on contact or through their vascular system.

Several terms are used to describe the types and actions of weed killers and these descriptions may overlap in single products. Different weed killers work on different types of plants or different components of a plant. Weed killers are powerful chemicals that can produce harm when label instructions are not followed closely. Some weed killers cause physical damage to non-targeted plants, to the user or those that come in contact with their residue or vapors.

Contact

Contact weed killers have chemical compounds that destroy plant tissue as it touches the sprayed material. The plant does not absorb the chemicals or translocate them through its vascular system. Young herbaceous (non-woody) plants are the most susceptible to all types of weed killers. Selective contact herbicides are designed to kill a specific group of plants, such as the broadleaf weeds commonly found in lawn turf. Other herbicide formulas use chemicals that specifically destroy or stunt grassy plants.

Selective

Non-selective herbicides kill all plants they come in contact with regardless of species. They are formulated as contact or systemic poisons. Some systemic weed killer formulas act as a synthetic growth hormone, mimicking the plant's natural auxins. They are absorbed and translocate, even down to the root system of the plant. The plant responds with twisting growth and downward curvature of stems and leaves. Weed killers that remain in soil particles for a long time can remain toxic when they come in contact with people or other plant and animal life.

Pre-emergent

Weed killers in this class of herbicides prevent plant cell division which inhibits the growth of roots after seed germination. They are selective chemical formulas designed for specific types of annual grassy weeds. Pre-emergent weed killers need water to create the chemical reaction which destroys the germination process. Some types of pre-emergent weed killers remain chemically active in the soil for several months.

Foliar

The most widely used weed killer is foliar sprayed glyphosate with its systemic action on plants. It is effective on perennial weeds because of its capability to move quickly through the plant's vascular system. It inhibits new growth by interfering with aromatic amino acid synthesis. Visible signs of yellowing and wilting occur within two to four days on annual weeds and seven to 10 days on perennial weeds. Glyphosate formulas have been identified by research at the University of Caen, France, as causing reproductive problems by interfering with human cell formation.

References

• PubMed: Glyphosate Formulations Induce Apoptosis
• University of Rhode Island Extension: How Herbicides Work

Resources

• University of Arizona Cooperative Extension: Weed Control in the Garden
• University of Vermont Extension: Ten Steps towards Organic Weed Control
• Garden Organic: Weed Management