Pre-emergent herbicides are a type of herbicide applied to soil before weeds have a chance to germinate. These herbicides work by forming a barrier in the soil that prevents weed seeds from sprouting and growing. The active ingredient in pre-emergent herbicides typically targets enzymes essential for plant growth. When a weed seed attempts to germinate and grow, it takes up the herbicide from the soil, which then interferes with the plant’s ability to produce necessary proteins and enzymes for growth. This causes the weed seed to die before it can emerge from the soil and establish itself. Pre-emergent herbicides are particularly effective against annual weeds, which complete their life cycle within a single growing season and rely on seeds to reproduce. By preventing these weeds from germinating, pre-emergent herbicides can significantly reduce the number of weeds that appear in each area. It is essential to apply pre-emergent herbicides before weed seeds have a chance to germinate, which typically means they are applied in early spring before soil temperatures rise and trigger seed germination. Pre-emergent herbicides have residual effects that last for several weeks to several months in the soil. The goal of the residual is to create a barrier that extends through the germination season of a particular target weed. The most common use of pre-emergent herbicide is to prevent summer crab grass. This weed germinates in April and begin to appear above ground from May to September. The drawback of pre-emergent is they do not prevent all weeds from germinating. Depending on the type of grass you have, professionals have several product options. Each product has a similar list of weeds they prevent, with a few caveats, but some weeds can overcome this protection and germinate despite a properly applied pre-emergent barrier. Fescue lawns are challenging as the pre-emergent barrier needs to be worn off prior to aerating and overseeding the lawn. This leaves the lawn susceptible to weed invasion in the fall, when poa annua germinates in all grass types and begins to show up in late winter and early spring. Given this cycle, it has become one of the more difficult weeds to prevent. Even though pre-emergent herbicide can target this weed, the timing of the application is required in the fall, which eliminates the homeowners’ opportunity to perform an important aeration and overseeding.
