Smooth crabgrass (Digitaria ischaemum [Schreb. Schweig.] Schreb. Ex Muhl.) is perhaps the most invasive weed in tall fescue (Festuca arundinacea Schreb.) lawns and sports turfs in Maryland. The performance of preemergence herbicides that target crabgrass can be greatly influenced by weed pressure, environmental conditions and turf vigor. Nitrogen (N) is important in stimulating turf density and thus promoting its competitiveness with weeds. Two studies will be reviewed. Study I was conducted in 2009 and 2010 and was designed to take into consideration the use of corn gluten natural herbicide (Espoma Corn Gluten 9-0-0, Millville, NJ) and chemical herbicides (prodiamine and dithiopyr) applied to plots receiving organic or synthetic organic N. The natural organic N source was OceanGro 5-5-0 sewage sludge (Bayville,NJ) and the synthetic N sources were sulfur coated urea (SCU) and urea. The treatments were as follows: corn gluten (CG) + organic N; dithiopyr + organic N; prodiamine + SCU; dithiopyr + urea; dithiopyr + no N; no herbicide + urea; and an untreated control. The N was applied early April (0.75 lb N/ 1000 ft2), early July (0.5 lb N/1000 ft2) and early Sept. (0.75 lb N/ 1000 ft2) in 2009 and 2010. All herbicide treatments and CG were applied 2 April 2009 and 31 March 2010 and were watered-in by rain or irrigation within 24 hrs. Rates were as follows: CG at 20 lb product/1000ft2 and prodiamine at 0.75 lb ai/A in both years; dithiopyr at 0.50 and 0.38 lb ai/A in 2009 and 2010, respectively. In 2009, rainfall was plentiful. In 2010, the turf entered a state of dormancy in response to drought in June. Rain in late June was followed by routine irrigation throughout July to break dormancy. In Study II, the herbicides, N from OceanGro and CG were applied at the rates and dates shown in Table 1. Both studies were conducted in mature stands of tall fescue mowed weekly to a height of 2.5 to 3.0 inches. Sprayable herbicides were applied in 50 GPA using a CO2 pressurized (35 psi) sprayer equipped with an 8004E flat fan nozzle. Granular products were applied using a shaker jar. Plots were 5ft by 10ft and 5ft by 5ft in Studies I and II, respectively. In both studies, plots were arranged in a randomized complete block with four replications. Percent of plot area covered by smooth crabgrass was visually assessed using a 0 to 100% scale where 0 = no crabgrass present and 100 = entire plot area covered with crabgrass. Treatments with crabgrass ratings > 5% cover were subjectively judged to be commercially unacceptable. Data were subjected to ANOVA and significantly different means were separated using Fisher’s LSD at P ≤ 0.05. Crabgrass cover was assessed on 25 Aug 2009 in Study I and the untreated control had 28% crabgrass cover. Plots treated with prodiamine + SCU and dithiopyr + urea or organic N had 1% crabgrass cover (94 % control); whereas, plots treated with CG + organic N and no herbicide + urea had 18% cover (36% control). In 2010, all treatments had lower crabgrass cover compared to the control (89% cover) when plots were rated on 9 Sept. Plots treated with prodiamine + SCU (3% cover = 97% control); dithiopyr + organic N (13% cover = 85% control), and dithiopyr + urea (17% cover = 81% control) had statistically equivalent levels of crabgrass cover. Poor control was observed in plots treated with CG + organic N (48% cover = 46% control); dithiopyr + no N (51% cover = 43% control) and no herbicide + urea (52% cover = 42% control). These data show that the performance of dithiopyr was improved by using it in conjunction with N (organic or urea) and that N alone from urea was as effective in suppressing crabgrass as CG + organic N. In Study II, crabgrass cover in plots treated with CG, Oceangro and mesotrione applied preemergence was excessive and ranged from 24 to 39% (Table 1). All other treatments provided a statistically equivalent level of crabgrass control and most had provided commercially acceptable crabgrass control (< 5.0% crabgrass cover). This included benefin + trifluralin, which was product received in 1992 and stored in an open bag for 18 years. Data again showed that N (i.e., OceanGro) was statistically as effective as CG in reducing crabgrass cover.

Table 1. Preemergence smooth crabgrass control in tall fescue with corn gluten and commercial herbicides in College Park, MD 2010. Rate % crabgrass cover Herbicide* (lb ai/A)12 Jul13 Aug Dithiopyr 2 EW0.38 0.3 c 4.8 c Dithiopyr 0.21G0.38 0.5 c 5.3 c Prodiamine+Sulfen. 4SC0.75 0.0 c 5.5 c Prodiamine+Sulfen. 0.3G0.75 0.0 c 1.3 c Pendimethalin 3.8ME0.75 0.0 c 0.3 c Pendimethalin 3.8ME1.5 + 1.5 0.1 c 5.0 c Benefin+Trifluralin 2G3.0 0.0 c 0.9 c Mesotrione 4SC0.187 + 0.187 4.9 bc37.8 b Corn Gluten20 lb product/1000ft2 2.6 bc24.5 b Corn Gluten15 + 15 product /1000ft2 7.0 b26.5 b OceanGro 5-5-02.0 lb N/1000ft2 7.5 b39.3 b Untreatedþu12.8 a63.5 a *Treatments were applied 2 April and sequential treatments were applied 18 May 2010. **Means in a column followed by the same letter are not significantly different according to Fisher’s LSD, P ≤ 0.05.