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Abstract Turfgrasses are perennial components of urban greenspaces found in parks, recreational areas, golf courses, sports fields, and lawns that confer many ecosystem services. A copious seed producer, Poa annua is the most troublesome weed of turfgrass and continually threatens the ecosystem services provided by urban greenspaces. Field research was conducted in Knoxville, TN to better understand environmental conditions triggering P. annua seedling emergence patterns to assist managers with optimally timing interventions—both chemical and non-chemical—for control. Fluctuations in cooling degree day (CDD21C) accumulation accounted for 82% of the variance in yearly cumulative P. annua emergence data collected in a single irrigated sward of hybrid bermudagrass [C. dactylon (L.) Pers. x. C. transvaalensis Burtt-Davy]. However, non-linear models using CDD21C data developed ex post were not able to accurately predict P. annua emergence patterns ex ante. In both years, P. annua emergence changed most rapidly between the 40th and 43rd week of the year when seven-day mean soil temperature and rainfall were 18.9 °C and 12.7 mm, respectively. Future research should explore the efficacy of herbicide mixtures applied when P. annua emergence is most rapidly changing in lieu of developing models to predict when specific emergence thresholds occur.

As the Latin name annua implies, the species Poa annua L. is thought to have an annual life cycle. Yet, there are many reports in literature of P. annua persisting as a perennial. Considering that P. annua senescence patterns do not align with other true annual species, we hypothesized that P. annua is similar to other perennial, C3 turfgrass species that are subject to a confluence of environmental factors 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 grassland micro-environments from May to October 2020. Forty plants survived the summer and confirmed the existence of perennial P. annua ecotypes. Analysis of environmental factors at the time of plant death indicated soil moisture, soil temperature, and pathogenic infection were associated with mortality. A series of glasshouse or field experiments were conducted to investigate the effects of each factor on P. annua mortality. Soil moisture and soil temperature were not lethal to P. annua in the glasshouse, except under extreme conditions not typical in the field. A field study assessed mortality of plants from pathogenic infection and indicated that P. annua plants treated with fungicide throughout the summer survived year-round, whereas plants not receiving fungicide applications senesced. These findings support our hypothesis that P. annua is of a perennial life cycle, which can be influenced by environmental conditions. We suggest that the name P. annua is likely a misnomer based on its modern interpretation.