27 February 2012 By Marina Christopher
To coincide with the coldest fortnight of the winter my central heating packed in and I was left without a way to heat the house.
I thought I was a hardy perennial but soon realised what a tender little soul I was as I clothed myself in more and more layers and curled up in a tight ball wishing for hibernation or oblivion. My brain deteriorated to standby and for several days the kittens and I sulked as the house got colder and colder and temperatures outside dwindled to -9C. Kind friends brought heaters and shivered companionably with me but I was cold, so cold, until the plumbers came to sort out the problem.
It was a salutary lesson – I had no idea that feeling so cold would cause me such inability to think straight and make me so miserable. How I sympathised with those poor souls in Europe where temperatures were plummeting to -30C; I know I could not have survived.
But what of our plants? How do they cope in the cold?
Those of you who grow dahlias know that one hard frost can reduce foliage to a black sticky mush by the following day whereas other plants appear to come through a severe chilling without a blemish. Grass turns straw coloured and brittle at the ends and when winter has passed can be raked or mown to leave fresh growth at the base. The grass family are unusual in that their growing point is around ground level, which allows them to be grazed (or mown) without damaging further growth. In tender plants such as the dahlia or potato, although the top vegetation is destroyed they will overwinter as tubers as long as they can be kept frost-free. Hardy perennials such as the monkshood (Aconitum sp.) and bulbous buttercup (Ranunculus bulbosus) also opt for an overwintering tuber but in their case they are able to withstand occasional but not prolonged freezing of their tubers.
Deciduous trees (those that lose their leaves) and many hardy perennials annually lose their foliage protecting them from the worst onslaught of the cold weather. Trees present winter with a tough exterior of bark often with a corky layer, which is a good insulation against cold. Perennials meanwhile, retire underground where there is less risk of freezing.
How do evergreen plants cope with cold?
Those plants that are evergreen or maintain some foliage have to deal with cold and frost in different ways. Growth tends to be negligible during winter so there are few cells which are growing and dividing. Freezing temperatures will cause ice crystals to develop around the plant cells where there is a layer of water. The cells have more dissolved salts and sugars in them so their freezing point will be lower than the water-filled spaces around them so they will not be damaged.
As cold increases, more water passes out of the cell making the salts and sugars in the cells more concentrated and consequently lowering the temperature at which the cell will freeze but at the same time dehydrating it. That is why many plants can look dry during winter. It is also the reason why several winter flowering plants such as Camellias should be sited away from early morning sun. Slow warming of the plant allows the ice around the plant cells to unfreeze releasing water that the cells can reabsorb slowly. If the thawing is too rapid the cells are unable to take up water quickly enough and are damaged resulting in browned flowers and burnt foliage.
Conversely seeds in the cow parsley and buttercup family require cold, frosty conditions to stimulate germination. After freezing cold conditions they shed their nice warm seed coat and emerge into the world. Completely barking mad!
My central heating restored, outside temperatures have soared to 17C during the day. Birds are singing, bees are on the wing and I have seen my first bat of the year flying at dusk. Time for me to stir my stumps and get on with sowing seeds and welcoming the new season.
Phoenix Perennial Plants, Paice Lane, Medstead, Nr. Alton, Hampshire, GU34 5PR.
Early openings 2012: Friday & Saturday 2-3rd March and 16-17th March (11-4)
Reopen Friday & Saturday 30th March – 27th October 2012