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We recorded flowering stages for 97 species of native and introduced angiosperms, most of which were perennials. These stages included dates of first flowering (or emergence of inflorescences for grasses), first ten flowering individuals, full flowering aspect, flowering 95% complete, and last flowering. For all species, we recorded at least four of these five stages for at least two years; for most species we have records for four or five years. We obtained records by walking through fields of native prairie and seeded nesting cover for waterfowl throughout the study area. We visited fields at least weekly and often twice or thrice weekly during the growing season, but surveyed fields in no particular order. We gathered no data on fields with stands of seeded grasses less than two years old or on fields burned or flooded during the observation year. We considered the flowering period for each species as the number of days between the median date of the first ten flowering individuals and the median date when flowering was judged by visual estimate to be 95% complete. We regressed length of flowering period on date of first flowering. For the regression analysis we used the median date of first ten flowering individuals for the 84 species for which we had enough records to calculate flowering period. We also calculated the number of species initiating flowering earliest and latest during the six years of the study. We used a Chi-square test to determine whether these earliest blooming-date frequencies were equally distributed among years.

We assembled departures from 30-year mean temperature and precipitation for March through August, 1979-1984, from records for the weather station at Pettibone, ND (NOAA 1979, 1980, 1981, 1982, 1983, 1984). We supplemented these data with precipitation measurements from the NOAA rain gauge on the study area.