The time to harvest overwintered carrots is now. Carrot, Daucus carota, is a biennial crop grown for its taproot and requires two years (or seasons) to flower; you'll want to dig your carrots before they transition to the flowering developmental stage since once the floral stalk has emerged, carrot roots become fibrous and inedible.

Flowering in biennials is stimulated upon vernalization, a period of extended exposure to low temperatures, usually experienced under winter conditions. Vernalization is a physiological response of phytohormones present in shoot apical meristems; in a carrot the shoot apical meristem is located in the layer of cells of the compressed stem found at the interface between the carrot root and the interior of the rosette of leaves (the tissues usually cut off before eating). When the cold period is satisfied in a biennial, a cascade of events occur, leading to flowering.

Carrots remain in a juvenile nonflowering form until subjected to cold exposure. Carrots planted in last year's fall garden and overwintered are still in their juvenile stage of growth and the root is still edible; but soon, a spring flush of shoot growth will occur in tissues that have been induced to flower commencing the irreversible transition from the juvenile (vegetative) edible stage to reproductive maturation (flowering).

Classical studies conducted in 1937 by M. K. Chailakhyan proposed that the floral stimulus is a hormone called "florigen." A single compound has never been isolated; however, subsequent numerous physiological studies of floral transitioning have identified several putative signals that stimulate flowering: the carbohydrate sucrose and the plant hormones cytokinin and gibberellin. In response to cold exposure, these compounds are translocated from leaves to the rosette of the shoot apical meristem.

Seems science is never simple. In science, when one question is answered, other facets become exposed, leading to further research. This is the fun and excitement of science. The story of flowering in carrot is a little more complicated than just vernalization of carrots, the cold-induced flowering in response to hormonal stimuli. Genetics of an organism must be considered.

The carrot, with which most Americans are familiar, is known as a "Western" carrot. These have orange, purple or white roots (see next week's column) likely derived by hybridization with Mediterranean "Eastern" carrots. The long orange Western carrot was bred in the 16th or 17th centuries and is the progenitor of present-day cultivars in commerce.

Such is the much-abbreviated story of flowering in the simple carrot. To summarize: carrots left in the ground will flower, not a good thing if you want a carrot root; but flowering could be a good thing if it is the bloom you want, as the carrot inflorescence is a stunning umbel resembling Queen Anne's Lace.

(Some information from C. F. Quiros, UC Davis; J Exp Bot 10.1093)

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