Pre-emergent herbicides are a form of chemical defense intended to prevent weed problems before they start. The straightforward answer to whether they kill existing, visible weeds is no. These products are specifically designed as a preventative measure, stopping weed seeds from successfully developing into established plants. Their function is to create a protective barrier on the soil surface, intercepting the germination process.
The Mechanism of Pre-Emergent Herbicides
Pre-emergent herbicides work by forming a thin chemical zone in the top layer of the soil after application and activation by water. This zone acts as a barrier that targets germinating weed seeds. The active ingredients remain in the soil until a weed seed begins to sprout.
Once a weed seed germinates and its root or shoot emerges, it immediately absorbs the chemical from the surrounding soil layer. The herbicide’s mode of action involves inhibiting cell division, a process known as mitosis. Chemicals like Prodiamine or Dithiopyr prevent the rapid cell replication needed for the seedling to develop a viable root system and shoot.
The seedling is killed soon after germination, before it ever emerges above the soil surface. This mechanism explains why pre-emergents have little effect on established plants, such as perennial weeds or turfgrass, which rely on existing, mature root systems. The chemical barrier only impacts the rapidly dividing cells of new seedlings.
The longevity of this barrier varies depending on the product and environmental conditions, often lasting from a few weeks to several months. Effectiveness declines as the chemical naturally breaks down over time, since the herbicide must be present in the soil to intercept the growing seedling. This breakdown is faster in warmer temperatures and areas with higher rainfall.
Dealing with Weeds Already Growing
Since pre-emergent products only act on germinating seeds, a different solution is necessary for weeds that have already emerged and are actively growing. This is the domain of post-emergent herbicides, which are designed to kill established plant matter. Post-emergent chemicals are applied directly to the foliage of the visible weed.
These products are categorized as either contact or systemic. Contact post-emergents kill only the plant tissue they touch, resulting in a rapid burn-down of the leaves and stems. While fast-acting, they are less effective on perennial weeds because the root system often remains viable, allowing for regrowth.
Systemic post-emergents, such as those containing glyphosate, are absorbed by the leaves and translocated throughout the entire vascular system of the plant. This process is slower, taking several days or weeks, but it is more effective at killing the entire plant, including the roots and rhizomes. This makes them the preferred option for controlling perennial weeds.
When treating a lawn, it is important to choose between selective and non-selective post-emergents. Selective herbicides target specific types of weeds, such as broadleaf weeds, while leaving the lawn grass unharmed. Non-selective herbicides kill nearly all vegetation they contact and should only be used in areas like cracks in driveways or garden beds where no plant growth is desired.
Optimal Timing for Pre-Emergent Application
Achieving maximum effectiveness with a pre-emergent herbicide depends on applying it at the correct time, before weed seed germination begins. Monitoring soil temperature is more reliable than using the calendar date, as temperature is the trigger for germination. For many common annual weeds like crabgrass, germination starts when the soil temperature consistently reaches 55 degrees Fahrenheit at a depth of two to four inches for several consecutive days.
The pre-emergent must be applied and activated by irrigation or rainfall before this temperature threshold is met. Applying too early risks the chemical barrier breaking down before the weeds begin to sprout later in the season. Applying too late means that germination has already started, and the product will not stop the weeds that have already begun to grow.
To extend the period of protection, a “split application” strategy is recommended. This involves applying a portion of the product in the early spring, just before the first germination, and following up with a second, smaller application in the late spring or early summer. This method ensures a continuous chemical presence throughout the germination season, preventing late-season weed outbreaks that occur after the initial application’s active ingredients have degraded.