Having made it through January, I’m wishing for the swift arrival of spring. After a long winter (they always feel long) seasonal change starts to loom. Yet I remind myself why, for many of the plants and crops we love, a sustained cold period is essential.
While little plant growth takes place in winter, important biological processes happen in this stillness. For a great number of plants that are able to survive cold weather, a good period spent below a certain temperature is key to their flowering in spring – a process called vernalisation.
The exact temperature and duration each plant needs to spend below it depends on the species and even the variety, although most of the fruit trees we grow outdoors in the UK have this “chill requirement”. For example, apple trees need to be exposed to between 700 and 1,000 hours below 6C (43F) but above freezing. Interestingly, plant breeders are developing varieties that have a far lower winter chill requirement so that apples can be grown in warmer climates than the UK’s, which is naturally ideal for them.
Another crop that requires vernalisation in order to develop optimally is garlic. A period of cold weather below 6C for about eight weeks triggers a seed clove to produce a number of cloves as it grows. This is why we sow our garlic from early winter through to January. If you’re lucky enough to live somewhere where the temperature doesn’t drop low enough for long enough to be sure your garlic will develop a bulb with numerous cloves, you could put your seed garlic in the fridge, stored in a paper bag, to mimic the vernalisation process.
For seed savers, vernalisation is a key part of the journey of biennial crops. Carrots, beetroot and parsley all produce vegetative growth in their first season, when they tend to be harvested and consumed. But if you want to replenish your seed stock from these plants, they will need to make it through winter and into spring, where vernalisation will ensure that they flower, poised to produce the seeds.
There’s another magical thing that certain plants do when the mercury starts to drop, and that is protect themselves from the cold. They do this by breaking down starch into sugars, which are more able to resist the cell-wall busting effects of frost.
This is a bonus for us eaters as it makes these vegetables even more palatable. So if you’re ever trying to convince someone of the tastiness of leeks, kale, carrots, beetroot or turnip, offering up a platter of vegetables that were exposed to the cold before being picked will make a more compelling case.
