Once established in a yard, crabgrass can be very difficult to eradicate. Your best bet once it has reached maturity is to use post-emergent herbicides against the crabgrass. After midsummer, crabgrass is typically too established to kill. At that point, even multiple applications of herbicide may not be effective and you will have to wait for the crabgrass to die off naturally after the first frosts of the year. Plan ahead for the next year and use pre-emergent herbicides to prevent crabgrass from taking root.
Water your lawn thoroughly using a garden hose or sprinkler the day before you plan to apply the herbicide. This relieves your lawn of any drought stress and loosens the soil around the crabgrass roots. Air around the roots gives the herbicide room to flow to the deeper roots as well.
Spray your lawn with an herbicide containing methanearsonate. These herbicides are available at home and garden supply or home improvement stores. Follow the package directions regarding how much to apply and do not water your lawn within the first 24 hours after you apply the herbicide. Do not mow your lawn directly after applying herbicide. Wait at least 24 hours, preferably longer, to allow the lawn to recover.
Reapply the methanearsonate herbicide after one week. Water your lawn the day before as with the first application of herbicide. If the crabgrass is still not completely dead, follow the herbicide's package directions regarding additional applications.