An Integrated Nutritional and Chemical Approach to Poa Annua Control in Creeping Bentgrass Putting Greens.

Poa annua suppression and control in creeping bentgrass (Agrostis stolonifera, CB) putting greens is a perennial challenge in many temperate climates. A nutritional approach to Poa control, common in the UK and becoming more popular in the US, is frequent use of sulfates of iron (Fe) and ammonium to provide plant nutrition plus gradual rootzone acidification. Also common is the repeated application of paclobutrazol (PAC) to selectively injure Poa throughout the growing season. The ultimate goal is a smooth transition to mostly CB, without sudden stand collapse. Our objective was to apply very high foliar rates of FeSO₄, in combination or not with PAC or seaweed extract (SWE), and determine effects on Poa annua populations in a creeping bentgrass putting green over time. The trial is on a mature sand-based putting green, originally seeded with ‘Penneagle’ CB. At trial initiation line intersect counts indicated that the green consisted of 50 to 65% Poa. Main plots were foliar FeSO₄ rates of 0, 1.22, 2.44, and 4.88 g m^-2 (0, 0.5x, 1.0x and 2.0x rates, respectively) applied every two weeks from March through October. Main plots are split by SWE (1.27 mL m^-2) or PAC (22.3% a.i. at 0.16 mL product m^-2) on the same application schedule. Nitrogen was supplied to all plots uniformly via ammonium sulfate at 8.8 g N m^-2 yr^-1. Initial soil pH (2.5 cm) was 6.22. FeSO₄ treatments had no effect on soil pH at the end of year one. The 1x and 2x FeSO₄ rates reduced Poa to 11 and 9%, respectively, by the beginning of year two, but SWE had no effect. Adding PAC to the 1x and 2x FeSO₄ rates dropped Poa from 20% (PAC-alone) to between 3 and 5%. Preliminary results indicate that season-long application of high rates of foliar FeSO₄ have the potential to significantly reduce Poa annua in CB putting greens.