Vernalization

The word vernalization is Latin for ‘of the spring’ and it refers to a period of cold that stimulates growth and flowering. Many fruiting trees, such as almonds, need to accumulate a specific range of hours of cold temperatures in order to flower and produce a crop in spring. That range of hours is called the plant’s chilling requirement. Without enough hours of cold, vernalization cannot occur.

French lavender

Bulbs

Many commercial growers use artificially generated vernalization to boost crop production. Most flowering bulbs require vernalization, to one degree or another, to reach their full potential. In the case of onions, the sets to be planted are stored at low temperatures to induce vernalization. Before planting the sets, they are devernalized at high temperatures to cause the plant’s energy to go into bulb development, rather than flowering.

Biennial plants

Biennial plants are those that take two years to grow from seed to fruition, and then die. Many varieties of carrots, cabbage, turnips, beets and kale only grow foliage during the first year. These plants are normally harvested during their first year, so they never have a chance to go to seed. If left in the ground and exposed to enough cold during the following winter, these plants are stimulated to produce flowers and seeds. By harvesting only part of these crops, gardeners can create perennial foodscapes!

Herbaceous perennials

Not all herbaceous (non-woody) perennials require vernalization, but several varieties will not flower without adequate chilling hours. The need for vernalization is only loosely associated with hardiness zones, but the amount of light a plant receives each day plays a major role in stimulating flower production. This is similar to my chickens, when egg production drops off drastically in winter and kicks back in as daylight hours start getting longer in spring. Here are just a few plants that require vernalization to flower optimally:

• Creeping phlox
• Cushion spurge
• Candytuft
• Pasqueflower
• Coreopsis
• Campanula/Bell Flowers
• Columbine
• Evening Primrose
• Lavender (pictured)

Gardeners can get more and better flower and produce by selecting plant varieties suited to their microclimate. Rather than cooling plants in the greenhouse, let nature do the work for you!