Keats's season of mists and mellow fruitfulness is here. It is the time of year When yellow leaves, or none, or few do hang/ Upon those boughs which shake against the cold.
From now on, the temperature will sometimes slip below freezing point on a cold night, and plants have evolved many different ways of protecting themselves from the worst ravages of these winter frosts.
Very low temperatures affect plants in much the same way as icy conditions cause problems with domestic plumbing: water expands as it freezes, and as it does so it is liable to shatter the cells of plants containing it, just as water turned to ice may burst domestic pipes.
Trees and other plants survive the cold by developing very clever strategies for doing so. Many trees, for example, adopt an attic strategy: they provide themselves with lagging jackets in the form of thick and craggy bark, whose insulating properties protect them from the worst effects of falling temperatures.
Some plants, on the other hand, have what we might call a motoring solution: they add an anti-freeze to their hydraulic systems in the form of salts, sugars or other chemicals that lower the freezing point of their internal fluid.
And yet others exploit the fact that water finds it difficult to lapse into the solid state unless it contains specks of impurities on which the ice can grow. Some plants maintain their water "supercooled" as a liquid well below the nominal freezing point by ensuring that their internal plumbing is kept scrupulously clean.
Broad-leaved trees shed their leaves in wintertime - a familiar phenomenon in this country - but before dispensing with their leaves they extract the nutrients, and store them in the stems and buds for later use.
Another way of coping with the ice is to avoid the problem altogether. Annual plants just fade away and die, and leave the problem of survival to their seeds.
The seeds themselves are dehydrated, so there is no water to expand and damage them, and they can survive for months, or years, or even centuries, until charged with moisture and triggered into growth again by rising temperatures.
And the rhododendron has managed to adopt this strategy while staying alive. It uses a "freeze-dry" technique, whereby as the temperature falls with the approach of winter, the sap inside the plant is siphoned out, leaving dehydrated tissues that can survive the very coldest of conditions; the following spring water is pumped back into the leaves, and growth resumes.