When selecting an herbicide, there are a wide range of choices that are commercially available. The most important piece of information needed when choosing the right herbicide is what type of weeds you're trying to eradicate. Most weeds and undesirable plants fall into three categories: broadleaf weeds, grass and grass-like weeds and sedges. Herbicides are divided into two main categories: pre-emergent and post-emergent, with several subcategories based on what types of weeds they kill.
Select a pre-emergent herbicide to apply to your soil in areas where you want to prevent weed seeds from germinating. Most pre-emergent herbicides are applied in late winter or early spring, before the weeds begin to emerge. Many pre-emergent formulas will kill desirable grass and plant seeds along with the weed seeds.
Choose a post-emergent herbicide to get rid of weeds after they've begun to grow. You'll need to select the correct post-emergent herbicide for the specific type of weed you're trying to kill and at the correct time of year recommended for that weed type.
Select a broad-spectrum herbicide to kill a wide range of weed species or undesirable plants after they begin to grow. Broad-spectrum herbicides will kill any plants or grasses that it comes into contact with, so you'll need to use this herbicide with extreme caution. Broad-spectrum herbicides are often used to kill weeds and grasses that grow in sidewalks, walkway pavers and driveways.
Apply a post-emergent herbicide designed specifically for broadleaf weeds after the weeds begin to grow, if broadleaves are the type of weeds you're trying to get rid of. Other types of post-emergent herbicides won't be effective at killing broadleaf weeds.
Select a post-emergent herbicide designed for sedges to kill sedge weeds, such as nutsedge, after they begin to grow. For nutsedge, you'll need to spray the weed's foliage when there are at least three leaves present.
Choose a post-emergent herbicide made for grasses or grass-like weeds to kill grassy weeds like crabgrasses, after the weeds are growing but preferably before they mature and form seed heads. To get rid of crabgrass, for example, you'll need to spray the weeds before the second leaves emerge.