Vernalization: Why We Still Need Winter


Vernalization (from the Latin Vernus, “of the spring”) is the process that plants undergo in the winter when cold temperatures signal them to go into dormancy and to then transition from a vegetative to a reproductive state. Vernalization is a vital part of the year for all cold-accustomed plants—seeds need it to germinate, flowers need it to bloom, and fruits, vegetables, and nuts need it to bear crops.

When cold weather arrives, the plant hardens to protect itself from freezing, drawing all soluble water deep into its tissue and retains on the surface the proteins and plant alcohols that form a natural antifreeze. Plants are so in tune with the cycle of the seasons that they have a cellular level memory of winter.

Throughout winter, several metabolic pathways change their function to prepare the dormant plant for the blooming/fruiting phase. This process is governed by three different genes known as VRN1, VRN2, and FT (VRN3). During vernalization, changes in the chromatin structure of these key genes encodes how long winter lasts, and the plant adapts to this amount of winter throughout its lifecycle.

The internal workings of the plant are complex, but the conditions to produce vernalization are simple: cold weather and less sunlight, just like nature typically provides in the winter. In 1927 it was discovered that this process can be artificially induced with refrigerators. Artificial vernalization was an agricultural breakthrough that has since been applied to all types of growing.


Thanks to artificial vernalization we can now plant many flowers in the spring and enjoy them in the summer, rather than having to plant in fall and wait the better part of a year for flowers. This can effectively turn biennials into annuals and encourage many slow-to-flower perennials to flower the first year. Artificial vernalization is the inverse of forcing. In artificial vernalization, we provide cold and dark conditions to simulate winter and tell a plant to recharge for blooming. In forcing, we provide warm and sunny conditions to simulate summer and tell a plant to bloom.


In the south, artificial vernalization allows us to grow plants with higher chilling requirements, like tulips and many other bulbs. A plant’s chilling requirement is expressed in terms of “chill hours”, the number of hours the plant needs to spend between 32- and 45-degrees F to properly vernalize that year. Plants that originate from warmer climates may require very few chill hours, and chilling alone might be optional for them (this is called facultative or quantitative vernalization). But plants with colder origins tend to have obligate vernalization, meaning that they must undergo chilly weather, and they can require as many as 1000 chill hours to properly vernalize.

phlox twister

As hardiness has been bred into modern perennial varieties, fewer perennials need extended vernalization. The list of those that do include plants like phlox and coreopsis. Fruit trees and shrubs often need vernalization to develop sweet and abundant fruits—this list includes blueberry, strawberry, cherry, and especially apple. Even citrus plants produce better with a hint of cold—though, of course, not too much. The plants that are best known for requiring vernalization are bulbs.



If you have garden bulbs that require more chilling hours than your area is likely to provide, the process of artificially vernalizing bulbs is quite simple. Once your blooms are done for the year, and the plant is ready to go dormant, cut the foliage down to a few inches from the ground (exactly how close varies by genus), then carefully dig up the bulbs and separate them.

Place the bulbs in a cardboard box with some newspaper or vermiculite, and place the box in the refrigerator. The right moisture level is important since bulbs do absorb water while they are dormant, and too much water can promote rot, so some air flow is important. At the same time, you do not want your bulbs to dry out, thus the importance of wrapping them in a medium that holds moisture well. Being dark, still, and around 40-degrees F, the average refrigerator is perfect for producing the winter-like conditions needed for good vernalization.


Check on your vernalizing bulbs monthly to make sure they aren’t too wet or drying out. Do not keep bulbs in a refrigerator with fruit. The ethylene gas released by ripening fruit—particularly apples—will kill the flower bud. After enough chill hours (the time frame varies by variety), plant as you normally would, and enjoy the full beauty of your over-wintered flowers.

But before you buy an extra refrigerator, research your favorite varieties. Success with gardening is most easily achieved with plants intended to grow naturally in your USDA Growing Zone. Additionally, many new cultivars have been carefully bred to tolerate a fewer vernalization hours, so check what might be new for your zone. You might be surprised at what can grow in your garden!

Photos: Shutterstock