Field-grown fennel can be harvested from about 15 June to the autumn frost. To force cultivation or to improve emergence, fennel is covered with fleece. It is usually removed during the second half of May in overcast weather. Depending on the cultivation period, plant propagation takes three to eight weeks.
Fennel belongs to the category of long-day plants. This means that long days (longer than 13 hours) stimulate flowering induction (also called ‘blooming’) while short days delay this process. The optimal germinating temperature is 20-22 degrees Celsius. For fennel to grow properly temperatures of 16-18 degrees Celsius are optimal. Temperatures above 24 degrees Celsius and below 7 degrees Celsius inhibit growth. Since fennel is susceptible to night frost, temperatures below -4 degrees Celsius cause damage to the plants. Plant growth and bulb development must proceed as uniformly as possible. Extremely hot summers, therefore, are unfavourable while cool, not too wet summers are beneficial to the plant. During the growing period, the plant requires little crop protection on average. A good water supply during bulb formation is important.