The control of flowering in the monocarpic perennial Carlina vulgaris

In natural populations of the "biennial" Carlina vulgaris more than 95% of the individuals delayed flowering past the second year. The probability of flowering increased with (in order of importance) increasing size before bolting in June, size before the winter (November of the year before flowering) and growth in spring (April to June). No significant effect of age was found. In a nutrient rich experimental garden 58% of the individuals flowered in their first year. Laboratory experiments showed that all individuals flowered without cold if they were given "optimal" growing conditions. There was an interaction between the effects of cold and the effects of the long-day period. In large plants a cold treatment reduced the number of days before bolting and reduced the weight at which plants started bolting. Without a cold treatment plants flowered only at a very high weight or after a large number of long days. Small individuals were not responsive to cold. Because there is a size-related effect of cold on the probability of flowering, autumn or spring seedlings are not likely to flower the following season in natural habitats.