Influence of Nitrogen, Plant Growth Regulators and Iron Sulfate on Annual Bluegrass Populations.

Annual bluegrass (ABG, Poa annua L.) is widely adapted to most growing environments and often is considered a weed on golf greens. Although numerous sustainable and low input management practices have been gaining popularity in the United States and around the world, limited to no research-based information is available on their ability to reduce ABG while promoting more desirable species such as creeping bentgrass (CBG, Agrostis stolonifera L.). The objective of this field study conducted in University Park, PA between 2010 and 2015 was to elucidate the interaction of nitrogen (N) rates, plant growth regulators (PGRs) and iron sulfate (Fe) rates as they relate to reducing ABG  populations in a simulated golf green.   Turf consisted of mature stand of ‘L-93’ CBG  (~75%) and ABG  (~25%).  Main factors included N from ammonium sulfate (21-0-0) (24 or 147 kg N ha^-1 year^-1), the PGRs trinexapac-ethyl (TE; 0.043 kg a.i. ha^-1 application^-1), and flurprimidol (FL; 0.28 kg a.i. ha^-1 application^-1), and Fe from iron sulfate (0, 12, or 49 kg Fe ha^-1 application^-1). Annual bluegrass populations varied by season and year. Over the course of the study, turf treated with Fe generally had lower ABG populations with reductions more pronounced in plots receiving 24 kg N ha^-1 year^-1. Populations of ABG were lower in plots treated with low N rates, but interactions with Fe and PGRs were present in some years. The greatest influence on populations of ABG were PGRs. By the final year (2015), populations were lowest in plots treated with FL regardless of N rate (4 to 10%). Moderate levels of ABG (18 to 28%) were observed within plots treated with TE or no PGR in combination with low N rates. Turf treated with TE or no PGR in combination with 147 kg N ha^-1 year^-1 had the highest percentages of ABG (66 to 68%) in the final year. Results of this 6-year study indicate that while ABG exhibits natural seasonal and annual population shifts, PGRs and to a lesser degree N and Fe rates can impart significant influence over percentages of ABG in mixed stands with creeping bentgrass.