Weeds such as crab grass or creeping Charlie can invade a lawn and make it look less than perfect. One method for dealing with weeds is to spray them with a commercial weed killer. With some weed killers you must protect your turf from accidental overspray. But other weed killers will target specific weeds and leave your lawn alone.
A broadleaf herbicide is called such because it specifically targets the leaves of plants with broad, spreading leaves. Broadleaf plants are dicotyledonous plants, which means that they have two seed leaves when they sprout. Grass varieties are monocotyledonous, which means that they have only one seed leaf. Broadleaf herbicides block chlorophyll production in leaves, which prevents a plant from creating food through photosynthesis. The leaves of plants treated with a broadleaf herbicide will slowly turn yellow before the plant dies.
A pre-emergent herbicide is formulated to attack annual weeds. Annual weeds sprout from seed in spring and are killed by frost in fall and winter. These herbicides are effective on turf weeds such as crab grass, annual bluegrass and goosegrass. Pre-emergent herbicides prevent weed seeds from germinating and growing into weeds. Turf is not affected by pre-emergent herbicides because the plants are already established. The herbicide will not prevent new grass plants from spreading on runners or stolons. Knowledge of the life cycle of weeds that you want to control is important to stop pre-emergent weeds from sprouting. If you apply pre-emergent herbicides after weed seeds germinate, they will grow despite the presence of the herbicide.
Organic arsenicals are registered in many states for the specific control of weeds in grass and cotton varieties. Examples of organic arsenicals include monosodium methanearsonate (MSMA), calcium acid methanearsonate (CAMA) and disodium methanearsonate (DSMA). Due to concerns over arsenic presence in water, the EPA has ruled that MSMA could no longer be used on home lawns after 2009, but could still be used for commercial application such as control of weeds in cotton and on golf courses. Commercial application of MSMA will stop after 2012.