Swiss Federal Research Station for Fruit-Growing, Viticulture and Horticulture, Wädenswil

Development of fennel and bolting

“Fennel is recognised as a long-day plant which develops inflorescence (bolting) at day lengths greater than 14 hours. From the day of planting until bolting the development of the swollen leaf base, “”bulb””, lasts approximately 50 to 65 days, dependent on the variety. The development of the bulb until bolting of different varieties and planting dates was studied. At weekly intervals bulb height and width were measured on single plants. The growth curve is expressed as the quotient of bulb height and width as function of time. First, when the value of the quotient is lower than one, indicating the full development of the bulb, or optimal harvest time. Second, when the value of the quotient is greater than one again, indicating bolting of the plants. The time gap between these two intersection points varies for different varieties. For Amigo this time gap was only eight days compared to that of Zefino of twelve days. Further analysis of the data from Zefino showed an even better fit of the growth curve when using the sum of daily maxima temperature, starting at the day of plantation. This would allow to make some predictions of optimal harvest time at different planting sites.”

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