Crabgrass is a common summer annual. This weed is controlled with preemergence herbicides that form a chemical barrier in the soil prior to germination or emergence. The barrier prevents the seedlings from emerging and developing normally. Most preemergence herbicides remain active in the soil, so seeding should be postponed for the amount of time specified on the manufacturer’s label. The best time to apply preemergence herbicides is approximately 10 to 14 days prior to the expected germination period in spring. Crabgrass usually germinates between March 15 and April 15. Depending on the product, time of application, location and rainfall in the spring, reapplication may be necessary within 60 days for season-long control. If preemergence herbicides are applied too late, you may need to use a postemergence herbicide for summer control. Apply postemergence herbicides when the crabgrass is fully visible and apply only to patches of weeds. As crabgrass becomes larger, it is more difficult to control. You may need to use repeat applications at 10 to 14 day intervals and expect to see some yellowing on desired turfgrass.
Wiregrass, a perennial weed, spreads rapidly by tough wiry stems. This warm season grass goes dormant in the winter. The only mechanism for control of these perennial grasses is to use a non-selective herbicide, kill the entire patch of lawn infested with the weed, and reseed with desirable turfgrass once the weed is gone. Glyphosate is a non-selective herbicide that can be used for this type of control. The best time to spray wiregrass is August.
For more detailed information on lawn weeds see this fact sheet:: http://extension.udel.edu/factsheet/weed-control-in-turf/