Chemicals that prevent or kill existing weeds are known as herbicides. Their mode of action can vary dramatically - from inhibiting weed seed germination, to preventing cell division and growth, to destroying cell membranes and disrupting photosynthesis.
While all herbicides control weeds, the key is knowing which one is most effective for your weed problem. It's important to first properly identify the weed you wish to eliminate and understand its growth cycle. This will enable you to choose the appropriate herbicide product and correctly time its application to achieve optimum weed control.
Herbicide products can be classified several ways. One is by their activity - selective or non-selective. Generally most herbicides are selective, meaning they will only control specific weed species without damage to nearby grasses and ornamentals. When controlling weeds in lawns, it is also wise to know your turfgrass variety, since their tolerance to herbicides do vary.
Non-selective herbicides will kill or damage all vegetation, so it is critical to apply them precisely to avoid injuring desirable plants and grasses. They are commonly used in landscaping for lawn renovation and for controlling weeds and brush along highways and railroad beds.
Another classification for herbicides is based on the time they are applied in the weed's life cycle. Preemergence herbicides are most effective when applied a few weeks prior to the germination and emergence of weed seeds. They will not control established weeds. These herbicides create a weed control zone on the soil surface, preventing seeds from germinating. Because most preemergents have a long residual, be aware that they can affect newly seeded turfgrass. Annual weeds such as crabgrass, poa annua (annual bluegrass) and goosegrass are typically controlled by preemergence herbicides.
Postemergence herbicides are applied after the weed is established and actively growing. While a few products work through soil application, the majority of postemergents work on contact. They are absorbed through the surface of leaves and translocated throughout the plant. It is recommended that applications be made during a dry, eight-hour period, since rain can wash off the material and reduce control. Most broadleaf weeds are controlled by postemergent herbicides.
In the past, herbicide products simply controlled one type of weed - annual weeds or broadleaves, and offered either preemergent or postemergent control. Today, new chemistries have been developed that cover a wider range of weed species, and can supply both pre and postemergence coverage.
Before you buy and apply any herbicide, carefully read the entire label for exact weed species controlled and sites where it can be safely used. Always review precautionary information and restrictions, follow directions for use, and store and dispose of empty containers as indicated on the label.