Annual grass weeds are weeds that germinate from seed, grow throughout the season, produce seed and die within 12 months. A number of annual grass weeds include crabgrass, goosegrass, foxtail, barnyardgrass, fall panicum and annual bluegrass.
Crabgrasses are the most common annual grass weeds in most lawns. Both the smooth and hairy types of crabgrass are classified as summer annual weeds. These grass weeds are considered undesirable in quality lawns because of incompatibility with desirable turfgrasses.
These annual grass weeds are usually lighter green in color, have wider leaf blades and possess more spreading growth habits than the cultured turfgrasses.
To ensure crabgrass and other annual grass weeds do not get established in your lawn, preventive and control programs must be implemented. The invasion of crabgrass and other annual grass weeds can be prevented to a large degree by maintaining a dense, healthy lawn.
A quality lawn will develop a thick canopy which shades the soil and discourage germination and establishment of seedling annual grass weeds. Most annual grass weed seeds germinate in the top 1/2" of the soil.
For homeowners who cannot control of annual grass weeds in a preventive manner with just cultural controls, the best way to stop these weeds from developing is through the use of pre emergent herbicides.
Pre emergent herbicides are chemicals that prevent the germinating weeds from establishing in the lawn. These herbicides control annual grass weeds by inhibiting cell division in the young root system. The failure of the root system to develop results in the death of the young seedling weed shortly after germination.
Lawns with thin stands of grass that do not provide 100 percent cover may require yearly applications of a pre emergent herbicide to prevent the invasion of crabgrass and other annual grass weeds. Dense, high-quality lawns may not need yearly applications since crabgrass only occasionally establishes in lawns with good density.
Pre emergent herbicides are generally only effective if applied before the annual grass weeds emerge. Therefore, early spring applications are essential if satisfactory weed control is to be achieved. Herbicide applications should be completed and the herbicide watered-in at least 7 days prior to the initial germination date to allow time for the herbicide barrier to be established in the soil.
Pre emergent herbicide applications for annual bluegrass control should be made in late summer or early fall.
If you have a pre emergent herbicide applied to your lawn in the spring, you cannot reseed easily until the fall. Should the occasion arise where it is absolutely necessary to apply seed after a pre emergent herbicide has been applied, rake the area thoroughly to help break the chemical shield in place. Apply some peat moss and rake in, then reseed.
Heavy rains during the spring may degrade the chemical shield and thus weakening its effectiveness.