Crabgrass is showing up frequently in lawns at this time. Crabgrass is a warm season annual weed that germinates when soil temperatures reach 55 to 60 F. and thrives throughout the summer if not controlled. Two species of crabgrass are commonly found in Iowa lawns, smooth crabgrass (Digitaria ischaemum) and large crabgrass (Digitaria sanguinalis). Both plants have light green foliage color and prostrate growth habit. Seedheads, which look like finger-like projections, begin appearing in late summer and continue until frost. Individual plants have the potential to produce thousands of seeds each. Ideally, crabgrass is controlled with preemergence herbicides in the spring, which prevents the seeds from germinating. This year, however, abundant rainfall may have leached the preemergent herbicides (if they were applied) below the control region allowing the seeds to germinate. Postemergence control is possible with MSMA or DSMA herbicides. Two applications should be applied 4 to 7 days apart. These herbicides work most effectively when applied to small, actively growing plants. If the lawn is severely infested with crabgrass, control would be easier if the lawn is totally renovated next month.
This article originally appeared in the July 28, 1993 issue, p. 127.