The influence of temperature and daylength on flower formation of three leek cultivars ( Allium porrum L.) was studied in growth chamber experiments. After different treatments, plants were planted in the field where the percentage and date of bolting and the leaf number of plants were recorded later. Leek probably has an obligatory vernalisation requirement whereas the effects of daylength are quantitative only. There is a juvenile phase that ends at a minimum size of about 2 g or five visible leaves. Optimum vernalisation temperature is at 5°C, the inductive temperature ranges between 0 and 18°C. Temperatures above 18°C cause devernalisation. Bolting is promoted by short days during and long days after vernalization.