Plants Can't Come In From The Cold . . .

My "Nature Notes" column for the month :-

If you had to spend even a day or so standing outside your house without moving, and without any clothes, at this time of the year, I wonder how long you’d last? Plants have to do it all winter long. The mild weather before Christmas encouraged spring flowers like celandines and even daffodils to flower, while some of last summer’s wayside flowers, for example yarrow and hogweed, could still be found along the hedgebanks. But since then, we’ve had something more like winter - so how do our countryside and garden plants cope?

Frost and cold can be pretty bad for a plant. Ice crystals forming inside the plant’s cells can cause an expansion that quite simply bursts them open. Water freezing between the cells will effectively dessicate the plant, drying it out. Alongside this, the cold leads to a decrease in enzyme activity and affects the way membranes and channels work, and this too can cause severe harm to the plant.

Some plants do simply die, of course. Annual plants survive only the one year; once their seeds are set they die, and it’s the seed that will ensure the continuation of the species over winter. Perennial plants do survive winter, but the exposed plant above ground may well not. Many such plants will have underground storage organs: tubers (e.g. potatoes), or tap roots (e.g. carrots, parsnips - and dandelions). Others have tangled networks of underground roots (e.g. stinging nettles), or produce bulbils, bulbs and corms (e.g. celandines, onions, daffodils or crocus). Food reserves are built up through the summer and autumn, and these sustain the plant in winter, and when spring comes are used to grow new leaves and shoots above ground. Perennial plants can do this year after year, though of course they will often also produce large numbers of seeds each year.

But some cold-tolerant plants can survive freezing temperatures. There are several ways in which they do this. A decent layer of snow can itself help to insulate the plants beneath it, while trees and other woody plants are protected by the bark and the woody outer layers beneath it. That’s rather like lagging your water pipes. The leaves and needles of evergreen trees have a waxy layer that helps protect them.

Some plants accumulate solutes like sucrose in their cells. This tends to build up naturally in response to the shortening of days, and will depress the freezing point of water, rather like the salt and grit on our roads. This is effective down to about 20 degrees Fahrenheit. There are also “antifreeze” proteins that can help prevent ice crystals forming in the spaces between cells, and so-called “dehydrins”, proteins that bind water molecules and help stabilize cell membranes. Plants may need to experience several days of cold weather before a freeze for these to be produced, so even hardy plants may be damaged by a sudden sharp freeze. But even plants that seem wrecked by winter can still rejuvenate from buds at or below ground level, when spring at last returns.