Flowering Phenology: Trends Over 32 Years in a Common Garden

Flowering times of some early-season plants vary with temperature. First flowering dates of 35 native spring herbs and one shrub (Lindera benzoin [Northern Spicebush]), recorded yearly from 1976 through 2008 in a common garden in western North Carolina, were analyzed to determine if they correlate with January or spring (February–May) temperature or show a trend over time. We also asked if species that flower earlier in the season are more tightly correlated with temperature than later-season species. One early-season (average flowering date = March 6) species, Carex plantaginea (Plantainleaf Sedge) and one later-season (average flowering date = April 11) species, Geranium maculatum (Spotted Geranium), showed a significant trend of flowering earlier over time; seven April-flowering species showed weak, statistically non-significant, trends of earlier flowering over time; and one species (Iris cristata) exhibited a weak trend of later flowering over time. Four mid-season (mid-March—early April) species showed a significant trend, and seven species showed a weaker, not significant trend, of earlier flowering in years with warmer January or spring temperatures. Overall, weak seasonal and longer-term responsiveness despite variation in soil and air temperatures over the site, multiple plantings for some species, and only a modest 0.28 °C increase in average January temperature over the 32-year observation period suggest flowering phenology of at least some plants in this common garden will track future climate change.