Life at the edge: a 14-year study of a Verbena officinalis population's interactions with climate

1 Controlled environment studies have demonstrated that absolute minimum winter temperature is critical for the geographical distribution of Verbena officinalis in the UK, with full survival if temperatures remain above -4 degrees C and full mortality if they fall below -17 degrees C. Flowering only occurs above a threshold temperature of 16 degrees C, and seed germination when the mean temperature is greater than 14 degrees C and the day temperature is greater than 19 degrees C. 2 Field observations on a population of Verbena indicated marked fluctuations in young and old plant numbers between 1980 and 1993. Population size was at a minimum in 1982, following extremely low temperatures in January 1982, and reached a maximum in 1984 following the very warm summer of 1983. 3 The northern limit to the distribution of Verbena in the UK cannot, however, be predicted correctly from these survival criteria and mean values. An improved prediction may be achieved on the basis of the frequency of warmer than average years suitable for flowering, allowing for a typical life span for Verbena of 5-10 years.