Well, here we all are again, recovering from the usual festive craziness, and the world has turned: for us in the northern hemisphere at least, the days are getting longer …
A couple of walks around and about the village, noticing many plants “asleep”, but some, like winter jasmine and the occasional hellebore, obviously very much “awake” and in flower, set me thinking: why do plants hibernate? Why do some appear not to? Why do some, indeed, flower in the depths of winter? How do plants cope with cold temperatures? These are questions that are easily asked, but have extremely complicated answers, so please excuse any technicalities in what follows – they really are unavoidable …
In our temperate climate, with its changing seasons, many plants need to undergo a process called cold acclimation, allowing them to increase their tolerance to freezing through gradual exposure to increasingly cool conditions. This involves intricate changes, starting at the genetic level, especially the [technicality alert] “C-repeat Binding Factor/Dehydration-Responsive Element Binding 1” family of genes - CBF/DREB1 for short - which are the master regulators of the whole process. They also change their metabolism (by accumulating natural “antifreeze” chemicals like sugars and heat shock proteins), and mechanisms in their cells (for example, by moving water to the spaces between cells so that cell membranes are not shattered when it freezes).
Plants sense lowering temperatures by changes in the fluidity of those membranes, which triggers “calcium signalling”, that in turn activates genetic responses in the plant to produce the afore-mentioned antifreeze. They also sense the shortening of day-length through special protein photoreceptors called phytochromes (which deal with the red end of the spectrum, including light humans cannot see) and cryptochromes (concerned with the blue end of the spectrum, including ultra-violet).
Evergreens clearly respond differently. Many have darker green leaves, which improves the absorption of any sunlight available The stomata (pores) in their leaves, through which they breathe, are often sunken into the surface, reducing water loss. Furthermore, those leaves (and especially the needles of conifers) often have a waxy cuticle, which further reduces the amount of water lost through transpiration, vital when the ground is frozen and water uptake difficult or impossible. This last also prevents the photosynthesis by which all plants live, and under such conditions evergreens can become completely dormant – they really do fall asleep.
Plants that actually flower at this time of year often undergo another process called “vernalisation”, in which they need a period of freezing to kickstart their life-cycle. This is true of many spring bulbs, and remember how quite a lot of seeds respond well to being put in a fridge for a while (in nature, this is called “cold stratification”, and prevents germination when the resultant young plant would not survive). Many winter flowers are also woodland plants, having adapted by evolution to low light conditions in the understorey, and able to complete at least their flowering before the new leaves of the next spring close the canopy above them. The hanging flowers of snowdrops and hellebores have even evolved to help protect their pollen from harsh winter weather. Insects that pollinate early flowering plants survive low temperatures, and are tempted out by the least glimmer of sunshine. We have all witnessed a bumble bee blundering around in the snowdrops sometime in January, and there are at least six British moth species that have cryoprotectant antifreeze in their bodies. Early flowering also means less competition for the few pollinators that are around, and some early flowers (like hazel catkins) need none at all, relying on the more primitive mechanism of pollination by wind, which has existed for at least 300 million years.
Perhaps the most extraordinary fact about all the above is that plants just get on with it, quietly and “under our noses”. We may notice such results as the glories of autumn colour in trees and shrubs (partly caused by them reabsorbing nutrients into their branches and trunks). Perhaps we sense the special stillness of a wood in winter, where trees and undergrowth are as dormant as any hibernating animal that they may be concealing, while anyone’s spirits are raised by the sight of a snowdrop or an early crocus pushing up through snow.
The garden “holding its breath” in winter is a commonplace of garden writing, and indeed it rests, it sleeps, but plants will always “know” how and when to start the next year's growing cycle, not least by reversing the mechanisms outlined above. Here I have only scratched the surface of these extraordinary botanical processes, but they constitute a miracle of nature for which we can all be truly thankful.