Weed and feed combinations of fertilizer and broadleaf weed control herbicides such as 2,4-D often fail to effectively control target weeds like dandelions because of poor herbicide contact and uptake by the weed. To effectively kill the dandelion the herbicide must be taken up by the foliage.

Properly maintained turfgrasses provide competition against weeds, diseases and insects. Fertilizer applications alone, will give 50 to 60 percent control of summer annual grassy weeds. That is where Pre-emergent herbicides come in.

The primary control measure for these summer annual grassy weeds are pre-emergent herbicides.

Proper Timing

Pre-emergent herbicides are so-named because they must be in place before crabgrass seedlings and other weeds begin to emerge. As a general rule, crabgrass may begin to germinate when daily high temperatures begin to reach 70 degrees F or above. A more precise rule to follow is when soil temperatures reach 55 degrees F in the top inch for four to five consecutive days. In the transition zone this may occur during the first two weeks of April.

Keep in mind that these dates will vary greatly from region to region, so check with your local extension office for recommended dates of application.

Sedges are grass-like weeds that become major problems in low lying, wet thinned turf areas. Sedges are perennial weeds with a triangular shaped stem that develop and spread from small nutlets in the soil. Bentazon, halosulfuron-methyl and MSMA are excellent post-emergent herbicides for control of sedges.

Following the soil temperature rule allows for the most optimum application time to have your pre-emergent herbicide work the maximum length of time into the growing season. If you regularly apply pre-emergent herbicides, lets say March 15th, and the soil temperature indicator occurs April 15th, you essentially lost 30 days of residual activity for late season control. Highest crabgrass emergence begins to occur as daily high temperatures reach 80 degrees F.

Most pre-emergent herbicides will not kill crabgrass that has already emerged. Herbicides must be present in the soil surface to kill the summer annual grassy weed seedlings when its first root contacts the soil.

Effective Products

Most effective products are combinations of fertilizer with the pre-emergent herbicide (or crabgrass preventer) in the same bag. These combinations are an effective way to fertilize lawns and control annual grassy weeds with one application. Both need to be watered in to be activated (effective). All products are available as sprayable material and common names of available products include benefin, benefin+trifluralin, bensulide, dithiopyr, oryzalin, oxadiazon, pendimethalin, prodiamine and simazine.

For further information on lawn weed control, you can refer to MU Extension Guide G6752 muextension.missouri.edu/xplor/agguides/hort/g06752.htm.