The house where we’re staying is located on California’s central coast, which has a subtropical, Mediterranean climate characterized by warm, dry summers and mild winters. It never gets cold here, and in my limited experience along the coast, it never really gets hot; the air temperature is rarely above 75°F. But the sun is intense. Standing without shade directly in the rays of the sun even in late March (let alone early July), your brain seems to boil.
This is an ideal climate for most plants and crops—everything from cut flowers to premium wine grapes to citrus and avocado thrives here. But I planted beet (‘Detroit Dark Red’) and chard (‘Fordhook Giant’) last spring, vegetables that are quite happy in any midwestern or northeastern garden. Here, they never grew to a size worth harvesting. Both cultivars I’ve successfully grown more than once (elsewhere); the poor growth I think was the result of a cool, foggy summer and a garden plot poorly placed in the shade of a nearby tree. With the advent of spring, the days have grown longer and warmer and the sun hotter, and I have expected my beets and chard to bolt, go to seed, and die. They did not and have not. This puzzled me and worried me the way the memory of a mostly forgotten dream lingers and comes back at intervals.
Then it came to me. Many tropical plants flower more or less continuously and their seeds fall to the ground and germinate essentially immediately. This makes good sense in the tropics where the growing season is in effect the whole year long. But temperate plants would not long persist following this strategy and have evolved mechanisms that allow them to tell time by environmental cues. These cues help them coordinate flowering with the growing season and prevent flowering before spring has actually arrived. My beets and chard are still waiting; they never got the message.
Part of my perplexity was that I’d forgotten that beet and chard are the same plant, so why would they not behave in the same way? They differ at the subspecies level only (beet—Beta vulgaris subsp. vulgaris and chard—B. v. subsp. maritima); their different forms reflect breeding that emphasized different plant parts—roots for beet and leaves for chard. More to the point, though, they are winter annuals, or biennials, and complete their life cycle over two growing seasons with an intervening cold period (winter). To flower, they need to experience an adequate period of cold exposure. In this climate, they have not had that and they will not get.
This period of cold exposure would have assured my beets that spring had indeed arrived and that it was safe to flower. It would have induced the physiological process that we call “vernalization”. Vernalization occurs at low (but above-freezing) temperatures, with an effective temperature range that depends on plant species typically between 36 and 50°F. But this can vary. Some grasses are vernalized by temperatures as low as 22°F, and olive can become vernalized at temperatures as high as 55°F. The duration of the required cold period also varies with species but is commonly 30 to 90 days long. Vernalization does not cause flowering; it makes plants ready to flower. Other cues such as lengthening or shortening days are needed to actually bring about flowering.
Vernalization is a metabolically active process that can be disrupted or reversed by periods of warmth. But once it has been established, vernalization represents a cellular change, or a “memory of winter”, as it’s been colorfully called, that is irreversible and stable throughout the life of the plant. This memory dies with the plant and is not passed on to the seed of the next generation. If it were, beet and other winter annuals would not long maintain that growth habit but would become summer annuals and complete their life cycles in the same season that their seed germinated.
The difference between summer annuals and winter annuals, though, is not always hard and fast. There are species in which both growth habits exist. Beet is one of these, and the genetics of the beet biennial growth habit had been worked out by 1936 and shown to be conferred by a single recessive locus. It would be possible to select for genetically identical plants that would differ at this locus only and would behave as winter annuals at one location and as summer annuals at another with a different climate.
An interesting sidebar to vernalization is the derivation of the word, which is attributed to Ukrainian–Soviet agronomist Trofim Lysenko (1898–1976). Lysenko became prominent after the famine and paltry crop harvests that resulted from the forced peasant collectivization in the Soviet Union in the early 1930s. Two types of wheat exist, one with a summer annual growth habit (spring wheat) and the other with a biennial habit (winter wheat). In years with little snowcover, winter wheat crops, sown in the fall, are often severely damaged. Lysenko claimed to have developed a new procedure for cold treating winter wheat that would significantly increase yields as well as cause winter wheat to behave as spring wheat. Furthermore, he falsely claimed that the summer annual trait that his treated winter wheat exhibited was carried over to subsequent generations of seed. Lysenko coined a word for his process that is based on the Russian word for spring wheat; the translation is the English word “vernalization”.
Lysenko’s contention that the environment, rather than genetics, shaped the nature of his wheat fit well with Marxist principles. He became popular with the Soviet press and Stalin appointed him head of the Academy of Agricultural Sciences of the Soviet Union. Under Lysenko, Mendelian genetics was denounced as a bourgeois science and officially abandoned. Scientists who did not subscribe to Lysenko’s ideas, many of which today seem frankly nutty, were censured, sent to the Gulag, and/or executed. Ultimately, Lysenko was debunked, but that did not occur before Soviet cereal production had dropped sharply and an entire generation of Soviet crop breeders had been lost.
Back to my beets: it’s my guess that they would live forever as vegetative plants, their development arrested; it’s the equivalent of eternal youth. But with almost two growing seasons and a “winter” to bulk up, I think they’ll be ready to eat before long.