Pre-emergent herbicides or “weed preventers” are often used to control annual lawn weeds such as crabgrass, foxtails, barnyardgrass, spurge, knotweed, purslane and others. Examples include various “crabgrass preventers” on the market.
A pre-emergent herbicide does not prevent weed seed germination or kill the seed. Instead, the root system development of a young weed seedling is severely limited by the action of the pre-emergent herbicide, killing it before it “emerges.” Pre-emergents will not control existing weeds, but will, if applied before germination, control seedlings of annual or perennial weeds.
After application, pre-emergents must be watered in or rained on to activate them. Some are more effective on grassy weeds; others on broadleaf (non-grass) weeds; others are equally effective on grassy or broadleaf weeds. Pre-emergents remain effective for a couple of weeks to 3 months or so, depending on the type or brand name used.
Follow label directions and do not exceed label rates of application. When applied in excess, some pre-emergents may damage the lawn grass or other desireable plants. As the name implies, a pre-emergent must be used prior to germination of the weed seeds. For most uses in the lower elevation areas of Colorado, this translates to a March application; April 1 to May 1 for higher elevations.
For more information, see the following Colorado State Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s). http://www.ext.colostate.edu/ptlk/2118.html