Products used to prevent and kill weeds are called herbicides. Although most are chemical formulations, many are now available in organic varieties. Herbicides work in different ways to perform their duty. Some prevent seed germination, while others interfere with the plant's photosynthesis process or membrane development. Weed killer types have two primary classifications: pre-emergent and post-emergent. Also, an often used subcategory of post-emergent herbicides is of the "selective" variety.
Pre-emergent Weed Killer
Seeds lay dormant in the soil during the winter months. When soil temperatures begin to rise in early spring, these seeds begin to germinate, sprout and grow. According to Colorado State University's Extension service, soil temperature of 55 degrees F is the trigger point for weed seed germination. It is at this time of the year when pre-emergent herbicides must be present in the soil for weed prevention. Pre-emergents form a shield along the soil's surface and thwart seedling germination. Types of pre-emergent herbicides include benefin, bensulide and prodiamine.
Post-emergent Weed Killer
Once weeds are present in the lawn, pre-emergent herbicides will no longer work. This is when post-emergent herbicide applications must be used. Post-emergents, as the name implies, kill weeds after they've begun to grow. The herbicides work on contact with the weed's leaves. Applied directly to the weeds, the chemical is absorbed and translocated throughout the plant. After several days, the weed dies, root and all. Today, post-emergent herbicides treat most weeds normally found in a lawn. Some of the weed killer types in this category include Trimec, Vantage and Image.
Selective Weed Killer
Selective herbicides consist of formulas that will kill weeds, without harming desirable lawn grasses and other vegetation. Closely following the directions available on the container of these products is critical to prevent harming, or killing, non-weed plants. Selective weed killers are a subcategory of post-emergent weed killers. They are typically applied during sunny days when temperatures are between 70 and 90 degrees F. Some of the more popular types of selective weed killers are MSMA (crabgrass control) and 2,4-D (broadleaf control).
Non-Selective Weed Killer
Herbicides for killing all vegetation are of the non-selective classification. Products such as Roundup and Rangemaster contain the chemical glyphosate.