Vernalization

Vernalization is a temperature-driven developmental response: a plant must accumulate enough cold, at appropriately low but non-freezing temperatures over a sufficient period, before it becomes competent to flower. The cold does not itself cause flowering immediately; rather, it removes a block, so that once warmth and, often, appropriate day length return, the plant can proceed to reproductive development.

This is why vernalization is described as a requirement for, or acceleration of, flowering rather than a direct trigger. It is a distinct phenomenon from the chilling requirement of temperate fruit trees, which governs the breaking of bud dormancy, although both involve a need for winter cold.

In cereals, the vernalization requirement underpins the winter and spring cropping types. Winter wheat, barley, and rye are sown in autumn, grow vegetatively, receive their cold requirement over winter, and flower and mature the following season; spring types lack a strong requirement and can be sown in spring for the same year. Choosing the correct type for the sowing season and climate is essential, because a winter type sown in spring may not flower properly, and a spring type sown too late in a cold winter area is exposed to frost risk.

• Determines whether a variety should be sown in autumn (winter type) or spring (spring type)
• Governs bolting and flowering in biennial vegetables, affecting seed production and quality
• Premature bolting from unintended vernalization can spoil the marketable part of some vegetable crops
• Influences how crop varieties are matched to regions with different winter conditions
• A consideration in breeding varieties adapted to warmer areas with less reliable winter cold

Because reliable winter cold is part of what makes a region suited to winter cropping, the vernalization requirement links crop choice directly to climate.

The amount and temperature of cold needed for vernalization vary by species and variety and interact with day length and developmental stage. This entry explains the concept and its agronomic importance; it does not give the specific cold durations or temperature thresholds required by any particular crop or variety, which are determined through research.