Poa annua: An Annual Species?

Poa annua L. is ranked the most troublesome turfgrass weed but can also be a highly desirable turfgrass species. As the Latin name annua implies, the species is thought to persist via an annual life cycle; yet there are many reports in literature of P. annua persisting perennially. Considering that P. annua senescence patterns do not align with other true annual species such as Triticum spp. and Zea mays L., we hypothesized that P. annua presents itself similarly to other perennial, cool-season, turfgrass species that are subject to a confluence of environmental factors in summer that can cause mortality. Four experiments were conducted in Knoxville, TN with the objective of determining environmental factors lethal to P. annua. A field monitoring study assessed 100 P. annua plants across ten micro-environments in golf course fairways and roughs. Forty plants survived the duration of 2020, establishing the existence of perennial P. annua ecotypes. Bioinformatic analysis of environmental factors at the time of plant death for ecotypes that did not survive perennially indicated soil moisture, soil temperature, and pathogenic infection were associated with mortality. A series of individual greenhouse or field experiments were conducted to investigate the effects of each factor on P. annua mortality. Neither soil moisture nor soil temperature were lethal to P. annua, except under extreme conditions not typical in the field (e.g., 37.8°C soil temperature for 12d under optimum soil moisture, or 35°C soil temperature with no irrigation for 19d). A field study assessed mortality from disease and observed that P. annua plants treated with fungicide throughout the summer on a 14-d interval survived year-round, whereas plants not receiving fungicide applications senesced. These findings support our hypothesis that the name P. annua is a misnomer and that plants persist via a perennial life cycle unless environmental conditions are unfavorable.