Herbicides applied postemergently were screened for efficacy in controlling crabgrass in established turfgrass. The study was located at the Iowa State University Horticulture Research Station north of Ames, IA. The experimental plot was in established 'Common' Kentucky bluegrass with a heavy crabgrass infestation. The soil in this area was a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with 2.4% organic matter, 14 ppm P, 66 ppm K, and a pH of 7.8. The study was arranged as a randomized complete block. Individual plots were 5 x 5 ft with three replications. Preclaim 3.09EC, Drive 75DF, and Acclaim Extra 0.57EW were included with an untreated control. Preclaim 3.09EC and Drive 75DF were applied early and mid-postemergent. Drive 75DF and Acclaim Extra 0.57EW were applied late postemergent.

Early post treatments were applied on June 24 when the crabgrass was in the 1-4 leaf stage. Mid-post treatments were made on July 8 when the crabgrass had 1-2 tillers and the late-post treatments were applied on August 5 when the crabgrass had 3 or more tillers. Estimations of crabgrass density were made weekly from June 29 through September 7 (Table 1). These data represent the percentage of area per plot covered by green, healthy crabgrass. Percentage crabgrass cover data were transposed to express percentage cover reductions as compared with the untreated controls (Table 2). Visual quality was monitored over this same period. Quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst quality (Table 3). The bluegrass also was checked for phytotoxicity following each set of treatments. Data were analyzed with the Statistical Analysis System (SAS, Version 6.12) and the Analysis of Variance procedure (ANOVA). Means were compared with Fisher's Least Significant Difference test (LSD).

The early-post treatments provided nearly total crabgrass control from July 7 through July 14 (Table 1). Reductions in crabgrass cover remained „ 90% in Preclaim 3.09EC treated grass until August 26 and in Drive 75DF treated through September 7 (Table 2). Preclaim 3.09EC applied mid-postemergently provided > 88% crabgrass control from July 21 through September 7 (Tables 1 and 2). The mid-post application of Drive 75DF reduced crabgrass by 60.7% on July 26 but the crabgrass seemed to recover. By August 19, percentage crabgrass cover in the untreated control was similar to cover in grass treated with Drive 75DF mid-post. Both Acclaim Extra 0.57EW and Drive 75DF were effective when applied as late-post materials (Tables 1 and 2). By August 11, Drive 75DF reduced crabgrass by „ 88% as compared to the untreated controls. After August 19, Acclaim Extra 0.57EW provided > 80% crabgrass control.

No phytotoxic symptoms were observed. There was some slight yellow coloration on the turf following the Preclaim 3.09EC applications. This color was gone after 48 hours with no turfgrass damage visible. There were no statistical reductions in the quality of treated grass as compared with the untreated controls (Table 3).

²Mean cover for early post treatments includes data for the entire season, for mid post only data after July 8, and for late post only data after Aug. 5.

Early post materials were applied June 24 when the crabgrass was in the 1-4 leaf stage. Mid-post applications were made on July 8 when the crabgrass had 1 to 2 tillers and late post applications on Aug. 5 when the crabgrass had > 3 tillers.

²Mean cover for early post treatments includes data for the entire season, for mid post only data after July 8, and for late post only data after Aug. 5.

Early post materials were applied June 24 when the crabgrass was in the 1-4 leaf stage. Mid Post applications were made on July 8 when the crabgrass had 1 to 2 tillers and late post applications on Aug. 5 when the crabgrass had > 3 tillers.

Table 3. Visual quality¹ of turf treated for the 1999 Postemergence Annual Weed Control Study.

Early post materials were applied June 24 when the crabgrass was in the 1-4 leaf stage. Mid Post applications were made on July 8 when the crabgrass had 1 to 2 tillers and late post applications on August 5 when the crabgrass had > 3 tillers.

NS = means are not significantly different at the 0.05 level.

-- = means comparisons tests can not be applied to these data.