Delay of Flowering in the 'Biennial' Cirsium vulgare: Size Effects and Devernalization

Delay of flowering is a common phenomenon in natural populations of Cirsium vulgare. In all years more than 45% of the rosettes that were at least one year old delayed reproduction. The probability of flowering increased with (in order of importance) increasing size, growth over the period immediately prior to bolting, and age. In contrast with the general idea about biennials that only large plants are vernalized during winter, we show that in C. vulgare all individuals can be vernalized in winter. Rosettes, from the smallest size-classes, collected from the field in Februari showed 80% flowering when placed under optimal conditions. We argue that in C. vulgare no size requirement exists at the time of vernalization but that flowering is delayed if plants fail to attain a required minimum size for bolting (probably induced by long days) in May.