A total of 18 unique nursery catalogs were sampled for this
research. The number was restricted by availability in the archival
collections. The number of catalogs sampled in each time period differed such that period one (1900–1929) had three catalogs, period
two (1930–1959) had four catalogs, period three (1960–1989) had
six catalogs, and period four (1990–2011) had five catalogs. We
streamlined the study design by sampling at least one nursery catalog to represent each decade (1900–1909, 1910–1919, and so on).
While it is likely that having more catalogs, and in turn, having a
larger sample size, especially in the earlier decades, would have
yielded more robust trends and results for the earlier decades, it is
unlikely this would have changed the overall results.