Annual Bluegrass Population Changes Due to Fertilizer Source, Flurprimidol and a Soil Surfactant.

Annual bluegrass (Poa annua L.: ABG) is an undesirable and highly invasive plant in cool-season putting greens. Although ABG suppression programs have been widely studied, variability in chemical responses due to ABG biotype and geographic location are frequently reported. Further, the effect of various cultural inputs like fertilizer source remains poorly understood. A field study evaluated the effects of three factors: fertilizer source (urea vs. complete:15 kg N ha-1), the root-absorbed plant growth regulator (flurprimidol:FL) applied every 14-days and monthly soil surfactant (SS) applications on seasonal turf appearance and ABG populations for two years. Treatments were applied to two adjacent research putting greens with contrasting rootzones (fine-textured native soil versus sand-based) located in full-sun. It was hypothesized that the SS might enhance fertilizer efficiency and FL efficacy. Initial ABG populations ranged from 14-17% and 7-11% for the native soil and sand-based rootzones, respectively. After two years, ABG populations ranged from 1-23% and 2-45% for the native soil and sand-based rootzones, respectively. The most ABG suppression occurred in any treatment receiving FL, 2-8% ABG present. When FL was not applied, significantly more ABG was present, 16-45% and 20-35%, for the native soil and sand-based rootzones, respectively. Applying the complete fertilizer and/or SS increased ABG. Although the SS resulted in more ABG, a more uniform turf with a higher turf appearance was observed during late-summer stressful environmental conditions, particularly on the sand-based rootzone. If minimizing ABG encroachment is a goal, these data demonstrate the importance of fertilizer source selection, and including a PGR like FL in the management program.