One weed that gives turfgrass producers a bad case of the production blues is annual bluegrass (Poa annua L.), a common winter weed in both warm- and cool-season turfs. It disrupts uniformity of the desired turfgrass, produces unsightly flowers even at low mowing height, and is competitive with the desired turfgrass. In cool-season turfgrasses, P. annua can be so competitive as to eventually replace the desired turfgrass species.

Augusta ecotypes.

Both annual and perennial varieties of Poa annua exist. The perennial variety, Poa annua var. reptans Hauskn. (Timm), tends to favor more highly irrigated and fertilized areas, such as golf course putting greens, while the annual variety (Poa annua var. annua) is found in less irrigated and fertilized areas, such as home lawns and golf course roughs. Research has shown that Poa annua var. annua has fewer leaves, nodes, secondary tillers, adventious roots; reaches reproductive maturity quicker; and produces a greater percentage of flowering tillers than var. reptans. Also, var. reptans does not exhibit a post-harvest seed dormancy, while var. annua requires a ripening period of up to three months.

Traditionally, the dinitroaniline herbicides (Pre-M, Pendulum, Balan, Team Pro, Surflan, Barricade), Dimenson, and Ronstar have been used effectively for preemergence control of annual bluegrass. The triazines, (Simazine, Atrazine) and Kerb have been used for both preemergence and postemergence control. But, if the warm-season turf is to be overseeded with cool-season species, 30 to 60 days must be allowed between application of any of the above herbicides and overseeding to prevent injury to the overseed species. Resistance to the triazines, dinitroanilines, and Prograss (postemergence only) has been documented. Herbicide resistance coupled with perennial biotypes has increased the need for additional herbicide-based control options, particularly those with postemergence activity.

Research was initiated 1997 at the Auburn Turfgrass Research Unit to evaluate candidate herbicides for annual bluegrass control in turf. Rimsulfuron, a sulfonylurea herbicide, was selected for further evaluation. It has low mammalian toxicity and is used in other crops at rates that range from 0.008 to 0.13 pound active ingredient (a.i.) per acre. Rimsulfuron provides postmergence activity apparently through both shoot and root uptake. Some preemergence activity is evident, but the soil half life averages approximately seven days.

Research has identified rimsulfuron to be effective in the following situations: (1) control of annual bluegrass in non-overseeded bermudagrass; (2) annual bluegrass control prior to overseeding bermudagrass; and (3) removal of perennial ryegrass and rough bluegrass from overseeded bermudagrass. These uses received Federal registration in September 2001 with TranXit 25 DF herbicide. Use sites include professionally-managed sports facilities, golf courses, sod and seed farms, and commercial and industrial lawns. TranXit 25 DF is a 25% dry flowable formulation and will be marketed by Griffin LLC of Valdosta, Georgia.

Research was initiated in the spring of 1998 with the objective of evaluating four TranXit 25 DF rates with and without non-ionic surfactant for postemergence control of annual bluegrass. A Tifdwarf bermudagrass putting green with an annual bluegrass infestation that averaged at least one plant per square inch was selected as the experimental site. The putting green received daily mowing at a height of 0.25 inches and water and fertility levels were high. The soil is a native loamy sand, with a pH of 6.2.

Herbicide application was made on April 20, 1998, and the annual bluegrass was in full bloom. Herbicides were applied in a diluent volume of 30 gallons per acre using a small push boom connected to a CO₂ backpack. Individual plots were four feet wide and 10 feet long and were replicated three times in a randomized complete block design. Annual bluegrass control and Tifdwarf injury were evaluated over time.

Initial observations showed that TranXit 25 DF acted slowly requiring approximately three weeks to kill annual bluegrass. However, further growth of target plants ceased within a few days after herbicide application. The addition of non-ionic surfactant increased control 12% and 7% with the one and two ounces per acre rates, respectively at 22 days after application (DAA). When surfactant was added, all rates controlled annual bluegrass 93 to 97%. Tifdwarf bermudagrass exhibited slight yellowing but within acceptable limits. The reference herbicide Kerb 50 WP produced similar bermudagrass injury, but annual bluegrass control averaged only 68% (22 DAA), Table 1.

Experiments were conducted in 1999 and 2000 to evaluate response of three ecotypes (collections) of annual bluegrass to postemergence-applied herbicides. Three ecotypes evaluated were: ‘Auburn’, ‘Augusta’, and ‘Fresno’. Seed of ‘Fresno’ was obtained from a weed-seed supplier in Fresno, California; ‘Auburn’ from plants at the Auburn Turfgrass Research Unit; ‘Augusta’ from plants collected from fairways 4, 8, 14, and 17 at Augusta National Golf Club.

Collected seeds were sown into a Tifdwarf bermudagrass sod during the fall of both years and allowed to grow for approximately eight weeks prior to herbicide application. Herbicides evaluated were: TranXit 25 DF at two ounces per acre; Kerb 50 WP at 48 ounces per acre; and Prograss 1.5 EC at 107 fluid ounces per acre. Each treatment was applied twice at these rates and a non-ionic surfactant was included in all treatments. The first experiment received treatments on February 8, 1999, and repeated April 22, 1999, and the second experiment dates were December 18, 1999, and repeated February 16, 2000. Herbicides were applied as previously described.

Results indicate that Prograss 1.5 EC was not effective in controlling any of the ecotypes. TranXit 25 DF and Kerb 50 WP provided excellent control of ‘Auburn’ and ‘Augusta’, but less control of the ‘Fresno’ ecotype was evident, with greater reduction in control for Kerb 50 WP than for TranXit 25 DF (see Table 2). No information is available on the ‘Fresno’ ecotype that would help in explaining the difference in herbicide response. However, these data do indicate that there is differential response among Poa annua ecotypes/biotypes and all have not been tested.

Small-plot research was conducted fall 2000/spring 2001 with the objective of evaluating TranXit 25 DF for annual bluegrass control and perennial ryegrass establishment when overseeded into Tifway bermudagrass managed as an athletic field. TranXit 25 DF was applied at rates ranging from one to eight ounces per acre either one, two, three and/or four weeks before overseeding. Herbicide application was the same as previously described. Overseeding was done October 25, 2000, with a Mat-O-Way seeder that delivered 15 pounds of seed per 1,000 square feet. Clipping weights were obtained and ryegrass stunting, stand reduction, and annual bluegrass control were visually evaluated over time.

Data (Table 3) illustrate that TranXit 25 DF provided greater than 90% annual bluegrass control 107 days after overseeding with all rate and timing scenarios. All treatments produced acceptable perennial ryegrass turf except where the eight ounces per acre rate was used. The treatment having the least adverse effect on ryegrass establishment and growth was a single application at two ounces per acre applied two weeks before overseeding.

Research to determine potential to use TranXit 25 DF for annual bluegrass control prior to overseeding bermudagrass putting greens is well under way. Annual bluegrass control and Tifdwarf and ultradwarf bermudagrass tolerance has been good. It is anticipated that use on bermudagrass putting greens will receive Federal labeling in the near future.

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