PROC. BIOL. SOC. WASH. 100(4), 1987, pp. 993-1017 FIRST FLOWERING DATES FOR SPRING-BLOOMING PLANTS OF THE WASHINGTON, D.C., AREA FOR THE YEARS 1970 TO 1983 Stanwyn G. Shetler and Susan K. Wiser Abstract. —The first-flowering dates for spring-blooming plants (both native/ naturalized and cultivated species) have been recorded by volunteers in the Department of Botany, National Museum of Natural History, since 1970. The first fourteen years (1970-1983) of these observations are tabulated, and the native/naturalized records are analyzed briefly. Over 1 00 persons contributed observations, recording 397 native/naturalized species and 704 cultivated species. Bar graphs of first-flowering dates are presented for all native/natu-ralized species for which there are four or more years of records, and compar-isons of cumulative numbers of woody, perennial, biennial, and annual species coming into flower by a certain date are graphed. Variation in first-flowering dates from year to year within and among species is described and possible explanations are discussed. Phenology is the study of periodic cycles in biological events and their relation to cli-mate. One important phenological cycle is flowering in plants. In temperate areas, such as the mid-Atlantic region, this cycle is an annual one. Flowering plants are generally dormant throughout the winter but renew growth in spring. Many species flower in the spring within a few weeks after growth re-sumes. Several factors, which are seasonal themselves, are well known to correlate with flowering time. Among these are (1) tem-perature, (2) day-length, (3) degree of can-opy closure, (4) potential for pollination, i.e., availability of pollinators or suitability for wind-pollination, and (5) microclimate. Re-cently, Fagerstrom and Agren (1980) have proposed that competition for seedling es-tablishment may also influence the pheno-logical spread of flowering. While flowering time may vary greatly from year to year, each species has its own approximate time-table, and there is an overall flowering pat-tern as the season progresses. This study was begun by Shetler in 1970 as an informal eflbrt to record the first dates of blooming for as many spring-flowering woody and herbaceous species as possible in the immediate Washington, D.C., area. To ensure as wide a coverage as possible, he enlisted the help of other interested per-sons by posting a recording chart outside his office in the Department of Botany at the Smithsonian Institution, and soon oth-ers were contributing regularly. The project quickly proved popular as a means of fol-lowing the progress of spring and became an annual tradition, which has been contin-ued down to the present. Each year the chart is put up at the beginning of the calendar year, and recording is continued through May 31st. At the outset, June 1st was se-lected as the arbitrary cutoff" for the spring season. After the annual observations had accu-mulated for several years, it became appar-ent that they were adding up to a unique phenological record for the area, and efforts were begun to computerize the data. De-spite the large number of observers and re-cording variability introduced thereby, the flowering dates were becoming predictable
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