A preemergent herbicide is used in lawn and garden care to stop weeds before they appear above the soil surface. This type of herbicide creates a chemical barrier in the top layer of the soil to prevent weed seeds from successfully sprouting. Unlike post-emergent products, which kill existing, visible weeds, preemergents are a preventive measure against the annual invasion of unwanted plants. By disrupting the early growth stages of a germinating seed, preemergents help maintain a dense, healthy lawn.
How Preemergents Work
Preemergent herbicides function by establishing a thin, protective zone of chemical concentration just beneath the soil surface. When a weed seed begins to germinate and sends out its first root or shoot, it comes into contact with this chemical barrier. The product does not kill the dormant seed itself, but rather targets the newly developing seedling once the germination process has begun.
The primary mechanism of action involves inhibiting cell division in the young plant’s root system. Chemicals like prodiamine or dithiopyr interfere with the cellular processes required for the seedling to develop a robust root structure. This disruption prevents the seedling from anchoring itself and absorbing the water and nutrients it needs to establish a viable plant.
Preemergent products have a residual period, meaning they remain active in the soil for a set amount of time, typically ranging from 8 to 12 weeks, though some formulations last longer. Over time, soil microbes and environmental factors cause the herbicide to break down, which is why reapplication is necessary for season-long control.
Timing for Application
The success of a preemergent application depends entirely on correct timing, which must occur before the target weed seeds begin to germinate. Relying on a specific calendar date is unreliable because germination is triggered by soil temperature, not the day of the year.
For common summer annual weeds like crabgrass, the optimal time for application is when the soil temperature consistently reaches 55 degrees Fahrenheit at a depth of two to four inches. Applying the product too early risks having the chemical barrier degrade before the weed seeds begin to sprout, reducing its effectiveness later in the season.
Conversely, applying it too late—after the soil temperature has exceeded 70 degrees Fahrenheit, for instance—means the weeds are already germinating and the product will not be able to stop them. Homeowners can use a soil thermometer or consult local agricultural extension services to monitor soil temperatures for the most accurate timing.
Preparation before application is also an important step to ensure the herbicide reaches the soil surface. Any heavy layer of debris, such as leaves or thick thatch, should be removed to allow for direct contact with the ground. Once the granular or liquid product is applied, it must be activated by moisture, which is usually achieved by watering it in thoroughly with about a half-inch of irrigation or by applying it just before a predicted rainfall.
What Preemergents Control and What They Harm
Preemergent herbicides are most effective against annual weeds, which complete their life cycle within a single year and must germinate from seed each season. This includes common invaders like crabgrass, annual bluegrass (Poa annua), foxtail, and spurge. The effectiveness of the product is specific to the life cycle of the weed, meaning a spring application targets warm-season annuals, while a late-summer or fall application is often necessary to prevent cool-season annuals.
These products are generally ineffective against established perennial weeds, which return each year from existing root systems, rhizomes, or tubers, such as dandelions or thistle. Existing weeds must be removed or treated with a post-emergent herbicide before the preemergent application.
A significant limitation of preemergent herbicides is that they are not selective between desirable seeds and weed seeds; the chemical barrier will prevent nearly all types of seeds from establishing. If new turfgrass seed, vegetable seeds, or flower seeds are applied too soon after a preemergent application, the product will stop their germination just as it stops the weeds. Therefore, it is important to avoid using preemergents in any area where new seeding or overseeding is planned for at least 12 to 16 weeks to prevent killing the desirable seedlings.