Managing unwanted plants in lawns and gardens requires a targeted strategy, which depends entirely on the weed’s stage of development. Herbicides are specialized tools categorized by how and when they act upon the plant life cycle. Understanding the mechanism of these products is the foundation for successfully maintaining a healthy landscape. The two main categories of weed control are designed to be used either before or after a weed has begun to grow.
How Pre-Emergent Herbicides Work
Pre-emergent herbicides are a proactive measure, targeting the problem before the weed seedling is visible above the soil surface. They do not kill existing, established weeds but prevent new ones from sprouting. These products are typically applied to the soil in the early spring or late fall, right before the target weed seeds are expected to germinate. For the product to work, it must be watered into the top layer of the soil, which activates the product and creates an invisible chemical barrier.
This barrier works by disrupting a specific biological process in the weed seed as it begins to sprout. When the seed begins to germinate and its root or shoot emerges, it absorbs the herbicide molecules in the soil layer. The active ingredients, such as prodiamine or dithiopyr, function as cell division inhibitors. This prevents the rapid cell division necessary for the seedling to develop a robust root system and grow past the initial germination stage.
The seedling is halted in its development, often dying before it can break through the soil surface and establish itself. The product’s effectiveness is tied directly to the integrity of this barrier. Disturbing the soil after application, such as by heavy raking, can therefore reduce its effectiveness. This category of herbicide offers residual control, meaning the chemical remains active in the soil for a period, providing protection over several weeks or months.
How Post-Emergent Herbicides Work
Post-emergent herbicides are curative tools intended to kill weeds that have already emerged from the soil and are actively growing. These products are applied directly to the foliage of the visible weed, relying on the plant’s leaves to absorb the active ingredients. They are classified into two types based on how they move within the plant: contact and systemic.
Contact Herbicides
Contact herbicides work quickly, causing visible damage to the plant tissue they directly touch, but they do not travel extensively within the plant. They are effective for controlling small, annual weeds. However, they often fail to kill the underground parts, like the roots or rhizomes, of perennial weeds.
Systemic Herbicides
Systemic herbicides, in contrast, are absorbed by the foliage and then translocated throughout the entire plant, including down to the root system. This movement is useful for achieving complete control of well-established perennial weeds, which can regrow if their roots remain viable.
The performance of post-emergent products is sensitive to environmental factors, as the weed must be healthy and actively growing to absorb and transport the chemical efficiently. Application during periods of drought or extreme heat can reduce absorption and translocation, resulting in poor control. Herbicides like glyphosate are systemic, while others such as diquat act primarily as contact killers.
Deciding Which Herbicide to Use
The choice between a pre-emergent and a post-emergent herbicide is determined by the target’s life stage and your overall goal. Pre-emergent products are for prevention, targeting seeds before they sprout. They are best used in the early spring before soil temperatures rise, or in the fall to control winter annuals. Post-emergent products are for eradication, targeting existing weeds that are physically visible and actively growing.
Selective and Non-Selective Types
Post-emergent herbicides are differentiated into selective and non-selective types. Selective herbicides are formulated to kill specific types of weeds, such as broadleaf weeds, without harming desirable plants like turfgrass.
Non-selective herbicides, like those containing glyphosate, will kill almost any plant they contact. This makes them suitable for areas like cracks in driveways or fence lines where no vegetation is desired.