Nothing ruins a green expanse of lawn like an invasion of clover or dandelions. These pests are difficult and time consuming to hand weed, but selective herbicides remove these weeds while leaving the lawn unharmed. Grasses metabolise the herbicides, while the same herbicides interfere with cell division in broadleaved weeds. Although manufacturers design these weed killers for lawn use, always follow the directions carefully, and take precautions to prevent unwanted herbicide exposure to people, animals and vulnerable plants.
MCPA is an abbreviation for the chemical formula ((4-chloro-2-methylphenoxy) acetic acid). Retailers sell MCPA under various trade names, including Agritox and Weed-Rhap, and often combine MCPA with other herbicides. MCPA is effective against both perennial and annual weeds. Growers use MCPA to control thistles, dock and other weeds in sod, grasslands and cereal crops. Although it usually breaks down within weeks in average soils, it is very water soluble and may contaminate groundwater, according to the Extension Toxicology Network.
2,4-Dichlorophenoxyacetic acid, or 2,4-D, is effective against broadleaved weeds and is a common ingredient in many herbicides, including granular "weed and feed" lawn fertilisers like Scotts Green Sweep Weed & Feed. Manufacturers for large-scale herbicide applications market 2,4-D as Barrage, Aqua-Kleen, Lawn-Keep and other names. Soil microbes break down this herbicide relatively quickly: only small amounts remain in soil after 56 days, according to the Cornell University Cooperative Extension.
Unlike 2,4-D and MCPA, the broadleaved weed herbicide dicamba remains active in soils for up to 12 weeks, and traces of dicamba may remain for over a year. This quality makes dicamba a useful ingredient in both pre- and post-emergent herbicides, and it is an ingredient in Scott's Pro-Turf, Trimec and Ortho Weed B Gone Lawn Weed Killer. However, dicamba also leaches readily into soils, and, because it does not break down quickly, it may contaminate groundwater. Trees and shrubs near treated lawns take up dicamba through their extended root systems.