preemergent

Preemergent refers to a class of substances or strategies that act before the emergence of seedlings. In horticulture and lawn care, the term most commonly describes preemergent herbicides, chemicals applied to the soil to prevent germination or early growth of weed seeds, thereby reducing future weed pressure.

These products are applied to bare soil or to prepared turf, then activated by water. They form a chemical barrier in the topsoil that interferes with the cell division or growth of germinating seeds, limiting their ability to establish. Some formulations are granular and others are liquid blends, and they may be designed for lawns, ornamental beds, or agricultural settings.

Timing and targets: applied before seeds germinate, typically in spring for summer weeds or in fall for winter annuals. They do not control existing weeds and are chosen based on the weed spectrum and crop tolerance. Activation often requires rainfall or irrigation after application.

Common examples include pendimethalin, prodiamine, dithiopyr, oryzalin, and oxadiazon. Some products are labeled specifically for lawns, others for ornamental landscapes or crop settings. These products may be used alone or as part of a broader weed management program, and they often come in granular formulations that require watering to activate.

Limitations and risks: potential injury to desirable plants if misapplied or if plant sensitivity is not accounted for, and efficacy can vary with soil type, temperature, and rainfall. Reapplication is typically needed each growing season. Overreliance on preemergents can contribute to herbicide resistance and environmental concerns, underscoring the value of integrated weed management.

In practice, preemergents are one tool among cultural practices such as mowing, preventive seeding, mulching, and hand weeding, used within a program tailored to local weed species and turf or crop conditions.