Proxy 2SL (ethephon) was compared to Primo 1EC (trinexapac-ethyl) to evaluate their effects on growth suppression and visual quality, e.g., color, density and uniformity, of ‘1020’ creeping bentgrass (Agrostis palustris Huds.). The trial was organized as a completely randomized block design, which was replicated three times. There were five treatments within each block- two Proxy 2SL and Primo 1EC treatments and an untreated control. Application timing and rate differed between the plant growth regulator treatments. Proxy 2SL was applied either at two- or six-week intervals. Two-week interval treatments were made at a rate of 2.5 fl oz/1000 ft² and six-week interval treatments were applied at a rate of 5.0 fl oz/1000 ft². Similarly, Primo 1EC treatments were also made at two- or six-week intervals. Two-week treatments were applied at rates of 0.125 fl oz/1000 ft², while the six-week treatments were made at 0.25 fl oz/1000 ft².

Treatment applications began on May 10, with subsequent applications occurring at either two- or six-week intervals, depending on the treatment protocol. Plant growth regulator applications were made to 5x5 ft plots using a C0₂-powered backpack sprayer. The sprayer was calibrated to apply 3.0 gallons of material/1000 ft². Established stands of ‘1020’ creeping bentgrass at the Iowa State University Horticulture Research Station were used for the study. The turf was maintained at a height of ¹/₂ inch and watered as necessary to facilitate normal growth and prevent desiccation. The turfgrass stand was grown in a medium consisting of ¹/₃ Nicollet soil, ¹/₃ peat and ¹/₃ sand. The medium had a pH of 8.05, an organic matter content of 3.04%, 3 ppm of nitrogen, 2 ppm of phosphorus and 41 ppm of potassium.

Turf evaluations of color, density, uniformity, and clipping yield were performed on a weekly basis following initial chemical treatment. Visual quality ratings were determined using a 1 to 9 scale, with 9=best, 6=lowest acceptable and 1=worst. Clipping yield was a measurement of the fresh weight, in grams, of clippings obtained from mowing the treated turf at 0.50 inch. The weekly evaluations for color are located in Table 1. In general, both application intervals and rates of Primo improved the color ratings of creeping bentgrass. Proxy, on the other hand, resulted in a slight, but acceptable, reduction in color. Proxy treated plots appeared to have a florescent yellow hue. Primo applied at two-week intervals consistently produced the highest color ratings. Additionally, phytotoxicity was not noted after any of the plant growth regulator treatments.

Proxy at both application intervals enhanced the density of creeping bentgrass (Table 2). Both Primo treatments also improved density, but not as much as the Proxy treatments. Changes in density associated with Proxy treatments did not appear until the third week of the study, but continued throughout the remainder of the study. Proxy applied at a two-week interval resulted in the most consistent enhancement of turfgrass density, although Proxy applied at six-week intervals produced similar results. The uniformity of the grass plots was also influenced by plant growth regulator treatments (Table 3). Proxy and Primo treated plots produced a grass stand that was far less grainy, with a more consistent color and density than the untreated controls. Primo treatments did not impact uniformity as much as Proxy treatments, and occasionally resulted in lowered uniformity ratings in comparison to controls. Proxy treated plots, however, appeared to be much tighter and denser, with a more consistent color— more uniform. Once again, Proxy applied at a two-week interval produced the highest uniformity ratings.

Growth regulator treatments also affected clipping yield, although the extent and persistence was variable and treatment-dependant (Table 4). Primo applied at six-week intervals initially resulted in a dramatic decrease in a relative clipping yield, but after four weeks there was an equally dramatic post-inhibition stimulation of growth that lasted until the next chemical application. Primo treatments applied at two-week intervals produced much more consistent reductions in clipping yields than those achieved with the six-week application interval. Both of the Proxy treatments resulted in an initial flush of growth the week after the first chemical application. Relative clipping yields in the Proxy six-week treatments were reduced in the ensuing 17 weeks of the study. Proxy two-week treatments, however, resulted in a flush of growth for two weeks after initial chemical applications and did not result in a satisfactory reduction in relative clipping yield until the sixth week of the study. Primo two-week treatments produced the most consistent reduction in relative clipping yield and were not susceptible to the post-inhibition growth response associated with the Primo six-week treatments.

Primo and Proxy are both plant growth regulators that can be used as part of a balanced maintenance program to reduce mowing costs and improve efficiency. An additional benefit is improvement in turf quality. While Primo readily improves the color of treated turf, Proxy greatly improves density and uniformity. Two main drawbacks, however, associated with Proxy are its reduction of turf color and the flush of growth after initial application. Future research considerations should look at ways to mask or reduce Proxy’s effect on turf color, such as addition of supplemental iron or magnesium. Moreover, it may be possible to limit the initial growth flush, while maintaining improvements in density and uniformity, with the addition of a type II plant growth regulator. The growth flush is most likely the result of either increased production of, or sensitivity to, gibberellic acid or to the accumulation of carbohydrates during growth suppression. Addition of a type II plant growth regulator could potentially counteract the Proxy-induced stimulation of internode cell elongation and limit the initial growth flush.

Table 1. Visual assessment ¹ regarding the color of ‘1020’ creeping bentgrass

Table 2. Visual assessment ¹ regarding the density of ‘1020’ creeping bentgrass

Table 3. Visual assessment ¹ regarding the uniformity of ‘1020’ creeping bentgrass

Table 4. Relative yield ¹ of ‘1020’ creeping bentgrass treated with Primo 1EC and Proxy 2SL