Long before the ancestors of modern flowering plants diverged from their ancestors, ancient plants developed means that allowed them to tell time by the Sun, to sense the lengths of day and night. This told them when to grow and when to flower and when to rest. At that time, the climate was much warmer, and the present configuration of continents did not exist. There was only the single supercontinent Pangaea where the freezing temperatures that characterize Earth’s contemporary temperate zones, such as most of North America, were not a worry for those plants.
More recently (on a geological time scale), a couple hundred million years ago the supercontinent began to break apart, and flowering plants emerged. Continental drift along with a changing climate and altered radiation patterns promoted the diversification in these plants that led to modern flowering plants. In this new world, the ability to sense chilling temperatures and adapt to them had value. Those groups that adapted and coordinated their life cycles with the changing seasons thrived. Among these were the flowering plants, which make up close to 90% of all plant species on Earth and dominate every terrestrial environment except the northern coniferous forests.
Nobody knows for sure quite how plants sense cold. But what is known is that cold perception occurs in the bud, the shoot apical meristem, and that there are several distinct but related ways that flowering plants use cold to synchronize flowering with the seasons. Vernalization and breaking of bud dormancy are two of these. Vernalization is seen in the cereals such as wheat and barley and winter annuals such as beet, and breaking bud dormancy is seen in temperate fruits and nuts. Both occur after sufficiently long periods of low but above-freezing temperatures. And both enable plants to remain dormant and conserve energy during the colder months of the year and to begin flowering when it’s safe to do so.
Mechanistically and physiologically the two are quite different. Vernalization does not trigger flowering; it renders plants capable of producing flowers. Breaking bud dormancy occurs in already developed flower buds and allows flowering to proceed. In both, though, what finally initiates flowering is the more ancient machinery centered in plant leaves that perceives lengthening or shortening days and signals the flower buds that the time for reproductive growth has arrived.
If you live in any region with recognizable winter weather, Minneapolis–St. Paul (USDA growing zones 3b–4b) or even Atlanta (zone 7), the chances are pretty good that you don’t consider whether your plants are getting enough cold weather. You’re far more likely to worry about them freezing. But in some regions of the country, particularly where the climate is subtropical, too little cold weather is a genuine concern, particularly if you want to grow temperate fruits.
I have an on-going vernalization experiment with beet and chard. Both are winter annuals (or biennials); they are, in fact, the same plant that has been bred for different edible parts. Winter annuals complete their life cycle over two growing seasons with an intervening cold period (winter). To flower, they rely on a sufficiently long period of cold to help them coordinate flowering with the growing season and prevent flowering before spring has actually arrived.
I initially planted them not as an experiment but as dinner. I didn’t harvest them all, and after a winter on California’s central coast (which is subtropical), I expected the survivors to bolt and flower as soon as the days grew longer and warmer. They did not. I puzzled over it for awhile before realizing that the cold period had not long enough for vernalization to occur. They still don’t know that winter has come and gone, twice. They are eternally young.
Except by chance, as with my beets, most people will not encounter an example of vernalization as clear as this. But in temperate fruits and nuts (including, but not limited to, apple, pear, apricot, nectarine, peach, plum, and many berries—Rubus and Ribes, for instance, as well as almond, filbert, and pistachio), it’s all too easy in warmer climates to stumble upon plants that have not had a sufficient period of cold (or “chilling hours”).
Temperate fruits and nuts need between 100 to 1400 chilling hours to grow normally. Exactly what determines a “chilling hour” is not clear, but there are a number of empirical models that are used to calculate chilling hours (see for example click here). The basic idea is that flower bud dormancy is broken when a plant is subjected to sufficient increments of temperature between freezing and 45°F during November through mid-February; cool temperatures in December and January seem to be most important. Chilling hours are cumulative. Temperatures below freezing do not add to cumulative chilling hours, and daytime temperatures >60°F reduce them. After the minimum chill requirement has been met, the plant is released from bud dormancy, although it may not begin to grow actively until temperatures warm up later in spring.
Minimum chill requirement is genetically determined and varies with species as well as with cultivar (lots of research money is devoted to breeding low-chill cultivars). Without sufficient chilling, flowering is delayed and haphazard. Leaves may not emerge normally or fully and may grow in whorls at the tips of branches, leaving the rest of the stem bare. Lower buds may eventually flower, but fruit set may be reduced or nonexistent. When the plant should senesce, it may retain its leaves and struggle on. Buds for the next year’s fruit may be weak and limited. Fruit quality and quantity will be reduced.
At the end of the day, as with any worthwhile endeavor, anticipation and planning are fundamental in horticulture. In areas where insufficient chilling hours are of concern (USDA zones 9 and 10 for certain), be mindful of your plantings and be sure that what you plant will meet its minimum chill requirement. Nurseries in these areas generally provide this information; but if not, it’s easily found online. Be aware that even small residential yards have microclimates, and that some places will be cooler than others. Cold air sinks while warm air rises. A wall that receives afternoon sun will retain and radiate heat. Next to it is a poor choice for planting a fruit tree with a high chilling hour requirement.