Types of Weed Killers
Weed killers, or herbicides, are a combination of chemicals that when applied kill plants. Some weed killers kill every plant they come in contact with while others are formulated to kill only certain types of plants. Weed killers are divided into two primary categories, pre-emergent and emergent, that are broken into either selective or non-selective. Selective weed killers kill only certain types of plants, such as broadleaf weeds. Non-selective kills all vegetative growth in the area where applied.
Pre-Emergent Selective Herbicides
Selective pre-emergents prevent weed seeds from germinating. Most pre-emergents are selective and don't harm lawn grass. Most commonly used in weed-and-feed-style lawn fertilizers in the spring and fall, the pre-emergent creates a chemical barrier that stops weed sprouting. Corn gluten is a common ingredient in this style of weed killer. Naturally occurring in corn plants to prevent weed growth where they are growing, it does the same thing on the lawn when used as a herbicide.
Emergent Selective Herbicides
The most common type of emergent selective is broadleaf killer. Most weeds that grow in lawns are of the broadleaf variety. Broadleaf weeds fall in the category of dicot plants---their primary leaves are not the same as the first leaves they sprout when growing. Grass is a monocot in that the first leaf to emerge is the only type the plant produces. A plant growth hormone called Trimec is applied to lawn. It confuses the dicot broadleaf weeds so they grow deformed and die.
Emergent Non-selective Herbicides
Non-selective weed killers are primarily sold as a spray and are available only in emergent varieties. The chemicals used in non-selective herbicides kill all growing things, and some are available that render the soil sterile for up to a year. Applied while plants are actively growing, the herbicides usually use Glyphosphate as the primary active ingredient. Glyphosphate destroys plant enzymes so that the plant can no longer produce the protein it needs to survive.
Apply emergent herbicides only when the weeds are actively growing; they do not work on dormant plants. Avoid spraying or applying herbicides in areas where pets or children play. Pre-emergent herbicides are applied year-round but most often in spring before the first weed seeds sprout. The weed killer is best applied during a dry period of two to three days so it is not washed away by rain or irrigation before it has a chance to act.