Can You Kill Grass by Putting Weed Killer on It?

Pre-Emergence Herbicides

• Pre-emergence herbicides are ones that prevent a seed from sprouting. The most frequent reason for using pre-emergence herbicides is to prevent the growth of crabgrass in the soil. This type of herbicide is typically scattered over lawns on a monthly basis in early spring. These herbicides prevent the germination of weed seeds. They typically have no effect on grasses that spread through stolons or rhizomes. If grass seeds are present in soil when pre-emergence herbicides are spread, they will not germinate.

Selective Herbicides

• Grass and weeds both are considered broadleaf plants. Broad-spectrum, broadleaf herbicides frequently harm all broadleaf plants, including grasses. But selective herbicides that are formulated for specific plants such as crabgrass, dandelions or clover without harming grass plants through overspray. Selective herbicides may include ammonium salt of imazaquin or halosulfuron-methyl. Selective herbicides can still negatively affect some grasses, however, such as bahia grass or St. Augustine grass. Always read the labels of selective herbicides to determine which plants are safe for their use before using the herbicide.

Non-Selective Herbicides

• Non-selective herbicides contain active ingredients such as glyphosate and trichlor. These types of herbicides will kill both weeds and grass. They are most widely used for spot-treatment of weeds such as dandelions or even for grass that creeps into walkways. Overspray of these herbicides when treating weeds can easily kill grass. Multiple applications of non-selective herbicides may be required to completely kill grass weeds. Because of this, occasional overspray will not irreparably damage grass.

Herbicide Tolerance

• As herbicides are widely used to treat weeds, eventually the weeds develop a tolerance to the herbicide. Weeds such as Palmer amaranth and waterhemp have shown some resistance to glyphosate in Ohio and Kentucky. This means that while glyphosate will kill grass that it is sprayed on, it leaves tolerant weeds alone. Rotating a variety of weed killers can reduce the resistance of certain weeds to herbicides. Resistant strains of weeds may travel from one region to another on seeds stuck to the blades of field cultivators and rototillers.