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If only we could forecast the weather accurately, gardening would be much less of a chancy occupation. We would know exactly when to bring in or cover up all those half-hardy plants that we left out to enjoy the sunshine in early spring. And how many times have you watered the garden well only to find that it poured with rain during the night? But what chance have we amateurs got when the professionals can't get it right. There are a few old sayings which seem to work, but only in the short term. Like 'Rain before seven, fine before eleven'. This is often, though not always, true, and has a factual basis. Most of the rain that falls during the night is a kind of frontal rain. Since frontal rain usually lasts for no more than three or four hours in any one place, if it rains before 7.00am. it should be dry by 11.00am. Then there is 'A sunshiny shower won't last half an hour'. Showers form when small pockets of air rise and make convection clouds. If the clouds are big enough to make rain, but the sun is shining, they must be quite a way apart and the shower should soon pass. But I suppose the best known of these old sayings is 'Red sky at night, shepherd's delight. Red sky in morning, shepherd's warning'. This too is based on fact. A red sky at night means that the sun is setting through clear skies which will be with us next day. A red sky at dawn means the sun is shining on cloud heading towards us. But could we possibly use plants to help us predict the weather? The proverbial bit of seaweed is supposed to be a good forecaster of rain, but flowers are probably a lot more sensitive, and many of them may grow in your garden as weeds or garden plants. If the scarlet pimpernel, also known as the poor man's weather glass, closes its flowers in the daytime it signals rain, and if they open they tell of fair weather. There are many other forecasting plants that close their flowers at the approach of rain, such as the African marigold, bindweed, pot marigolds, the dandelion, and ox-eye daisy. There is a scientific basis for this apparent second sight. Flowers are desperate to avoid getting their pollen wet, which might otherwise sprout prematurely, so they have developed a keen sense of moisture which forecasts wet weather. When the humidity rises to something like 80% the flowers close. When the humidity drops the flowers re-open. This is why hay fever is always worst on dry days. But, unfortunately, while approaching rain may cause flowers to close, flowers closing does not necessarily mean that rain is approaching. It could be that night is approaching, for some flowers close when the light intensity drops. There is one plant I have heard of that is supposed to be better at weather prediction than any met man or woman. Abrus precatorius is a member of the bean family that grows wild in India. It has purple flowers and bright scarlet, white or black seeds. These seeds are used widely by Buddhist monks to make rosaries, which probably gave the species its name 'precatorius' comes from the latin 'precator', one who prays. On any normal day it can be seen rising and lowering its branches, and folding and unfolding its leaves. If you touch a leaf the entire branch will fold up. The plant also folds up at night or when rain or hail touch its leaves. The plant became known as the 'weather plant' in 1887, when Joseph Nowak applied for an English patent to use it as a weather forecaster. Even though his patent application failed he claimed that if young leaves from the north side of a plant showed unusual movements then thunderstorms were supposed to arrive from that direction within two days. If the leaf stalk moved the branches vigorously up and down then an earthquake was predicted. In reality, reading leaf movements is tricky since plants are sensitive to their own rhythms, the quality of daylight and temperature and they close at dusk, so cannot be relied upon to forecast the weather. Plants react to weather in all sorts of ways, and the droughts we have suffered in southern England during recent summers has tested some of our plant life to the limit. Plants need water to live. They take it up through the roots and lose it through the leaves. As soon as there is insufficient water round the roots the plants trigger off alarm signals, telling their leaf pores, officially known as stomata, to close to reduce evaporation and prevent wilting and slowing down growth in readiness for the tough times ahead. The marram grass growing on sand dunes can have an exceptionally dry life, which they can cope with by rolling their slender leaves into tubes to protect the stomata from the drying effect of the wind. Air-plants have moisture sensitive plugs attached to their leaf pores. When the weather gets really dry, the stalks contract so the plugs are pulled down to seal over the stomata. In wet weather, when it is safe to re-open the pores, the plugs do so. Freezing weather causes the same problems for plants as it does for household plumbing: when water freezes it expands and when the ice melts it contracts. That expansion and contraction can cause plant cells to burst, and the sharp ice crystals can cut the walls of cells with the same devastating result. Hardy plants have invented an ingenious range of strategies for bypassing frost damage. The most obvious way is with insulation, like the bark on trees. They also employ antifreeze like sugars and salts, and even the same glycol antifreeze used in car radiators, which keeps their freezing point below 0°C and stops ice crystals from forming. But perhaps the most bizarre method for protection against the cold is employed by the rhododendron. The buds of the plant literally freeze dry themselves before winter. As soon as it gets cold the sap inside the bud is siphoned out, leaving behind dehydrated tissues which can survive below -30°. The following spring the buds rehydrate and the buds open. Damn clever these rhododendrons. Happy gardening. Bill Hutchings |
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page last updated 27 AUGUST 2000 |
