|Dandelion, photo courtesy extension.usu.edu|
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” preventing the weed from establishing. 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. Read the label to determine target weeds for particular brands. 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 desirable plants. As the name implies, a pre-emergent must be used prior to germination of the weed seeds. Make sure not to plant seed in an area with active pre-emergent pesticides as they, too, will likely not succeed. 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 University Extension fact sheet(s).