Gardeners have a love/hate relationship with winter. The cold is one of the biggest killers of plants, but at the same time many plants have a chilling requirement—having adapted to a cold climate, they now require a certain length of wintry conditions to allow them to undergo the mysterious process of vernalization.
2014 was one of the harsher winters in US history. Weather.com reports that “Monday, Jan. 6, 2014 ranked as the 40th-coldest day on record since 1900 for the continental U.S., with an average temperature of 17.9 degrees for the Lower 48.”
This winter may not be any different, and all this icy weather is sure to have an impact on our plants. Reports have been coming in of some varieties thriving far above their expected Hardiness range, due no doubt to the nearly two decades of relatively temperate winters we have enjoyed. The USDA even recently updated their plant hardiness zone map to reflect the changing climate. Yet this year’s winter is no powder puff, and it might have put a lot of those zone-challenging plants down for the count.
But look on the bright side: all this cold weather will ensure that our plants have gotten PLENTY of time to vernalize. 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 good crops.
When cold weather comes on, first the plant hardens to protect itself from freezing, drawing all soluble water deep into its tissues and leaving on the surface the proteins and plant alcohols that form a natural antifreeze. As winter wears on, several metabolic pathways change their function to prepare the dormant plant for the blooming/fruiting phase, and the process is governed by three different genes known as VRN1, VRN2, and FT (VRN3). Plants are so in tune with the cycle of the seasons that they actually remember winter ( Sung and Amasino 2004): 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 their life cycle.
The internal workings of the plant are complex, but the conditions to produce vernalization are simple: cold weather and less sunlight, just like nature provides in the winter (usually). This process can be artificially induced with refrigerators—the first famous example of which is Trofim Lysenko making his winter cereals behave like spring cereals in 1927, which led to a life-saving harvest when the Soviets adopted the practice in 1928.
Artificial vernalization was an agricultural breakthrough that has since been applied to all types of growing. Thanks to 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 many biennials into annuals, and convince many slow-to-flower perennials to flower the first year. Artificial vernalization is related to forcing: in artificial vernalization, we provide cold and dark conditions to simulate winter and tell a plant to prepare to bloom; in forcing, we provide warm and sunny conditions to simulate summer and tell a plant to bloom.
In the South, artificial vernalization lets us grow plants with higher chilling requirements, like Tulips and many other bulbs. A plant’s chilling requirement is expressed in terms of “chill hours”, or the number of hours the plant needs to spend between 32 and 45 degrees Fahrenheit to properly vernalize that year. Plants that originate from warmer climates might require very few chill hours, and chilling 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.
Fewer perennials need much vernalization, but the list includes plants like Campanula and Aquilegia. 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 bit of cold—though, of course, not too much. The plants that are most notorious for requiring vernalization are bulbs. Most quality bulbs come from the Dutch bulb market, with ancestry coming in colder climates, like Holland or the Netherlands. Luckily, bulbs are easy to pull up every year, because many of them benefit from some time in the refrigerator if you are trying to grow them on the southern side of their hardiness range.
Assuming next winter is not so cold (and fingers crossed, it isn’t), you might need to artificially vernalize any plants that require more chilling hours than your area is likely to provide. For shrubs and trees you have already planted, it is not very practical to refrigerate, but for bulbs, the process is pretty simple and painless. 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.
Then place the bulbs in a box (cardboard is good) with some newspaper or vermiculite, and place the box in the refrigerator. The right moisture level is important—plants do “drink” while they are dormant, and too much water can promote rot, so some air flow is important, but at the same time, you do not want your bulbs to dry out, thus the importance of wrapping them in some medium that will hold their moisture well. Being dark, still, and around 40 degrees, the average refrigerator is perfect for producing the winter-like conditions needed for good vernalization. Check on the bulbs about once a month to make sure they aren’t rotting (too wet) or drying out. And be sure not to keep any fruit in that refrigerator. This part is important! 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 out as you normally would, and enjoy the full beauty of your properly-wintered flowers!
Artificial vernalization can keep your garden beautiful, but it is also handy for controlling the timing of houseplants. Many bulbs can be grown indoors and forced to bloom at the time of your choosing. The most popular example of this is Amaryllis, which is tender in most gardens, but is commonly used as a houseplant because of the huge, beautiful red blooms that it can be forced to produce during the holidays.
If you are growing from seed to plant out later in spring, you might consider vernalizing seeds now. With seed-starting systems (like Park Seed’s Bio Dome), you can prepare your whole flat and refrigerate it, so that all you have to do in the spring is pull it out to grow in your house for a while, then plant it out later.
But before you go and buy an extra refrigerator, do some research on your particular variety. Many new cultivars have been carefully bred and selected for a lower vernalization requirements, so they might still flower or fruit beautifully even in warmer regions.