Effect of Cultural Practices On Southern Crabgrass (Digitaria ciliaris) Management in St. Augustinegrass.

Southern crabgrass (Digitaria ciliaris) is a common weed found in St. Augustinegrass (Stenotaphrum secundatum) in Florida. New regulations and loss of registered herbicides have greatly limited postemergence control options for crabgrass.  Cultural practices such as mowing height and irrigation  have impacted weed populations in other turfgrasses.  Field trials were conducted in two separate locations during the summer of 2009 to determine the effect of mowing height, nitrogen fertility, and irrigation on crabgrass management.    Three mowing heights were used (5.1, 7.6, and 10.2 cm), and plots were mowed weekly.  Nitrogen was applied at four rates (0, 8, 12, and 16 kg N/ha^-1), split over three application during growing season. Irrigation was applied at three rates; no supplemental irrigation, water replacement based on evapotranspiration rates for St. Augustinegrass, and irrigation applied daily. Weed density was measured using a Scotts grid (a 1 m by 1 m aluminum frame with strings stretched across both vertical and horizontal directions resulting in a total of 36 compartments). Weed "counts" ranged from 0 to 36 with 0 indicating no weed in any of the compartments and 36 indicating at least one plant in each of the compartments.  Differences in weed counts were not significant for the three mowing height treatments.  Weed counts were highest at the 16 kg N/ha^-1 rate, at >10 per plot.  The lowest weed counts were at the 0 kg N/ha^-1, at <3 per plot.    Weed counts were highest when no supplemental irrigation was applied, at < 10 per plot.  Plots irrigated based on ET had the lowest weed counts of >5 per plot.   The results indicate that the levels nitrogen fertility and irrigation are important in crabgrass incidence and could be used as an effective management in residential lawns.