How do Weed Killers Kill Weeds and Not Lawns?

Herbicides are complicated chemicals and the way they work depends largely on the type being used. There are two primary types of herbicides or weed killers – pre-emergence and post-emergence. Pre-emergence herbicides kill plant life while the seed is germinating, while post-emergence herbicides kill weeds during active growth. Both types of weed killer can be selective or nonselective, which is the reason certain weed killers target weeds but not lawns.

An easier explanation of how weed killers work without killing lawns is to understand what weeds are. Weeds are simply plant life growing in an undesirable place. In other words, what may be considered a weed in one location is considered a plant in another. Herbicides are applied to kill weeds when they are growing in an undesirable location. By using a selective herbicide that targets broadleaf plants, for example, weeds can be killed without harming most grasses.

The science behind how herbicides kill weeds is linked to cell growth and development. Because there are different types of weed killer, there are different ways to get rid of weeds, but essentially all herbicides destroy certain plant enzymes that are necessary for growth. There are also different types of plant life and since grassy plants develop differently than broadleaf plants, they respond differently to different weed killers. So while some herbicides kill weeds without harming grass, others will kill all plant life.

Nonselective herbicides are a category of weed killers that kill weeds, grass, and basically all plants. Roundup® is an example of a popular brand nonselective herbicide. When applied, nonselective herbicides kill all plant life, but they still remain useful for areas such as patios, walkways, and driveways where all plant growth is undesired. Caution should be used when applying nonselective herbicides because they can harm shrubs, trees and other nearby plants it may come in contact with.

To successfully kill undesirable broadleaf plants, such as crabgrass, clover, dandelions, and thistle, without killing the grassy plants that comprise a desirable lawn, choose a selective herbicide. These will attack the broadleaf plants and thus kill weeds without harming grass. Since methods of application vary depending on the type of weed killer used, be sure to thoroughly read the manufacturer’s directions for product use before applying any weed killer.