Large crabgrass and smooth crabgrass are problematic weeds in all areas where turf is grown. Often considered to be the most problematic weed in lawns, crabgrass is a summer annual that germinates in spring and dies with the first killing frost (in areas where there is frost).
Once conditions are favorable for germination, crabgrass can continue to germinate throughout the summer until August. One square foot of crabgrass can produce at least 10,000 seeds, so crabgrass becomes an annual problem in most turf areas. Proper mowing (higher mowing heights), proper fertilization (some rather than none to improve turf density), irrigation to prevent summer dormancy during drought, and aerification of compacted areas to improve turf health are all cultural practices that can be used to reduce crabgrass. However, despite good cultural practices, crabgrass can still occur requiring the use of a postemergence herbicide for control.
Several things are important for achieving adequate postemergence crabgrass control including using the right adjuvant in your spray tank (if one is needed) to improve control, and making applications when temperatures are <85°F to reduce the risk of turf injury. However, nothing is probably more critical than both knowing the stage (size) of crabgrass present and the safety of available herbicides on your desirable turf before you select the right product and apply the herbicide. The size of the plants will affect how well the herbicide works and which herbicide you should select.
The developmental stages for large crabgrass are shown below (Fig. 1), but other crabgrass species, goosegrass, foxtails, barnyardgrass, and sandbur developmental stages can also be identified using the same leaf and tiller terminology.
Regardless of herbicide selection, remember that smaller crabgrass (pre-tiller) is easier to control than larger, tillered crabgrass. In most cases, tillered crabgrass requires two postemergence applications made at a two- to three-week interval for complete control. The table shown below provides a summary of details found regarding crabgrass control from key herbicide ingredients with postemergence activity on crabgrass. Many other herbicide mixtures contain these key ingredients (such as Last Call, Q4 Plus, Quincept, Solitare, etc.) but are not listed for brevity.