Climate and Weather Effects on the Garden
Gardeners, like farmers, are notorious for their pre-occupation with weather although this is perfectly understandable because it is so unpredictable. A basic understanding of the climatic features that make up the weather can help us organise and plan garden activities to cope with the unexpected, but for all plants, the key climatic factors are the same: precipitation, sunshine and wind.
I hardly need to stress the importance of water to plants. Approximately 90 per cent of the protoplasm of actively growing plant cells is water and most of the total plant is protoplasm. It is in aqueous solution that nutrients are taken up from theand it is also in water that they and the chemicals manufactured by the plant itself are moved from one region of leaf, shoot or root to another. Deprive a plant of water and it will die from a combination of starvation, structural instability and general biological malfunction. Most of the water is taken tip from the soil and the amount of water present at any particular time depends on the soil’s physical characteristics, the prevailing temperature, the drying effect of the wind and, of course, the volume of precipitation that has fallen upon it. Precipitation is a useful word for all the forms in which water falls to earth. Most, of course, falls as rain, although drizzle, dew, fog, hail, snow, sleet and hoar frost also contribute small amounts. There are obvious regional variations in annual rainfall but in general, the nearer you are to the sea and to high ground, the more rain you will have.
In Britain, this rain is due mainly to warm westerly or south-westerly winds passing over the Atlantic Ocean and causing water to evaporate. When this moist air reaches the high land of the west and central parts of the country, it rises, cools and expands. The moisture condenses, clouds form and rain falls. This is called relief rain.
Cyclonic rain is rather different and is produced in the low pressure systems that you see depicted on weather maps. Here, warm and moist air rises above cold and dense polar air moving in the opposite direction. As with relief rain, the rising moist air expands and cools, but with the significant difference that cyclonic rain can occur over low as well as high ‘round and that, as low pressure systems, unlike mountains, are constantly moving, outbreaks of cyclonic rain are much harder to predict.
Finally, there is the least predictable of all, convectional or thunder rain. Localised heating of the earth gives rise to powerful upcurrents of warm, moist air in which large raindrops form, buoyed up by the rising air. The very heavy, short-lived rainfall of thunderstorms can have considerable impact in gardens, however, because of its sheer physical force; flowers and vegetation generally are beaten down or broken and the soil may become compacted and capped.
Lack of rain, a drought, is of course, rather more important than an excess. It is because of the unpredictability of drought that you should plan for water shortages carefully. I’ll come to this shortly but first a word about snow. The amount of moisture contributed in this way is never excessive as approximately 25cm (1in) of snow represent only 2.5cm (1in) of rain; and most, of course, falls in winter at times when plants have little use for moisture. Nonetheless, odd as it seems, a blanket of snow, although itself frozen, will protect plants and soil – a covering of about 7.5cm (3in) will usually prevent them from freezing. When falls of snow are very heavy, however, its sheer weight will break branches of shrubs and trees; particularly those evergreens that present a large surface area on which the snow-can settle.
Making the Most of Rainfall
Rain never seems to fall when it is wanted and most gardeners have to supplement natural rain with hand watering or irrigation. Unfortunately, the time you most need water for the garden is the time when water companies are liable to restrict its use, so it makes sense to plan ahead. Water butts will conserve lain water during the summer months, but you should use this mainly outdoors as greenhouse and indoor plants may suffer from the micro-organisms it will accumulate. Concentrate on watering where it will yield most benefit, except in the most long severe drought, establishedwill survive so use the water elsewhere. Water plants when the parts that you most want are maturing: flowers at flowering time and vegetables and fruit as the edible parts develop. Recently planted trees, shrubs and perennials will need watering during dry spells for the first couple of years, but once their roots have developed they can be left for longer periods. Plants in containers, however, will always need regular watering. The improvement of the moisture retentiveness of soil is also extremely important.
Sunshine – Temperature
All life on earth depends on solar radiation; radiation from the sun. Less than hall of the sun’s radiation reaches the earth’s surface, the remainder being lost, mainly by reflection back into space. For plants and gardens, there are two important components of the radiation: the shorter wavelength visible light, and the warming, longer wavelengths tending towards the infra-red. Although neither penetrates the soil, the surface warming effect of the sun’s heat is conducted into the top few centimetres of soil and the re-radiation of this into the air is a very important factor in dictating the temperatures that plants experience. The temperatures that occur in any region depend mainly on the angle at which the sun’s rays strike: at the Poles, where this angle is shallow, the warming effect is much less than at the Equator where the sun is higher in the sky. In most countries there can be considerable north-south differences in temperature.
All plants have their own maximum and minimum growth temperatures and, of course, they are higher for tropical than for polar species. Although all green plants grow in the same, basic, biochemical way, it is clear that mechanisms have evolved to permit particular species to survive satisfactorily in extreme conditions. Nonetheless, survival isn’t the same as active growth and almost all plants grow little below 5°C (41°F) or above 25°C (77°F) and grow best between 10°C (50°F) and 25°C (77°F). This explains why plants in very cold or very hot climates (alpines andfor instance), are usually slow growing.
Most of the long-wave radiation leaving the earth’s surface during the night-time cooling period doesn’t pass back into space, for it is largely retained through the absorbing properties of the atmosphere. With clear skies, however, at a time of year when the compensation of day-time warming by the sun is low. The heat loss from the earth can be sufficient to cause temperatures at ground level to drop below freezing and so form a frost. This type of frost is called a radiation frost and when it occurs in spring, it is crucial. The dates of the first and last frosts of the year are among the most important in any gardener’s calendar.
Some gardens are more prone than others to spring frosts: a dry, free-draining soil, for instance, is a poorer conductor of heat than a wet one and there is less transfer of heat from reserves at depth in the ground. So, dry soils are more likely to give rise to radiation frosts. A good cover of vegetation, such as grass, can also limit the amount of heat loss and shrubs growing in lawns are less liable to spring frost damage than those in bare soil. Overhanging trees can also limit the loss of heat upwards and serve to protect the plants beneath them.
Of course, frosts also occur throughout the winter, and not always when the sky is clear. Most winter frosts arise when cold air is transported over land but as plants are pretty well dormant at this time, these are or much less significance to gardeners than radiation frosts.
We need to distinguish between ground frosts and air frosts. As ground frost arises when the temperature at or just above ground level has fallen to 0°C (32°F), but this doesn’t usually cause much harm to plants. An air frost, however, which occurs when the temperature is 0°C (32°F) or less at a height of 120cm (4ft) above the ground, can be much more serious. Even so, air frosts aren’t generally damaging to plants during the winter when they are dormant, but spring air frosts, occurring after growth has begun, can be disastrous.
The mechanism of frost damage to plants is complicated and no-one fully understands it, nor indeed, what determines hardiness. It seems to be related, however, to the ease with which water can be drawn out from plant cells and frozen in the spaces between them; and to the rapidity with which subsequent thawing takes place. A slow thaw, allowing the cells to rehydrate gently, is much less damaging than a rapid one.
Tender plants are those that can only be grown outside in the period between the last and the first frosts. Once frosts do occur, these plants need protection, either in a greenhouse, a conservatory or under cloches. Heavy plants in large containers that can’t be moved may be wrapped in insulating material such as bubble polythene film. Container-grown plants, even if they are normally hardy, may need protection from severe frosts as their roots will be more vulnerable to damage in a small container than in the open ground. Plants that are marginally hardy should be protected by mulching witharound their crowns in autumn, although, when practicable, it makes sense also to lake and overwinter these under cover as back up if the parent plant fails. Spun fleece is a relatively new product that gives quick and easy frost protection for gardens. It comes into its own for those unexpected late spring frosts when any bedding plants or vegetables already outside can be quickly covered up with a double layer overnight.
Sunshine – Light
Light from the sun allows photosynthesis to take place and so enables plants to manufacture food and develop. It is not always true. However, that the more light a plant receives, the better it will grow. As with temperature and other factors, each type of plant has an optimum light requirement. Many. or mahonias that are naturally shade species won’t thrive, even in our latitudes, when planted in full sun. The fact that different plants have differing requirements for, or different tolerances of, light intensity is something that gardeners should appreciate when making mixed plantings. Very high light intensities can also be damaging, and many gardeners will have seen the symptoms of greenback on tomato fruit that are growing too close to greenhouse glass.
Another, rather different role for light lies in the phenomenon of day length, the fact that the daily light period varies during the year. Special chemicals in plants are receptive to this variation and when certain thresholds are crossed, internal processes in the plant are triggered, most importantly that of flower initiation. This is called photoperiodism and it is of most importance to commercial growers who adjust the length of the period of artificial illumination to induce flowering in plants likeor poinsettias at all times of the year. The retention of leaves by urban trees growing close to street lights is a common example of another photoperiodic effect.
Making the Most of Sunshine
To make optimum use of sunshine, especially the warming infra-red component, we need to capitalise on something called the greenhouse effect. At its simplest, this occurs when relatively short wavelength radiation passes through a surface or layer, strikes a reflecting surface and is re-emitted as longer wavelength radiation that is unable to pass back out again. The greenhouse effect we have heard about in the media relates to the trapping of warmth in this way al the earth’s surface because of chemicals in the atmosphere that prevent it from being re-emitted. Do remember, however, that the greenhouse effect itself is vital; without it life on earth couldn’t survive. The concern is strictly with an enhanced greenhouse effect leading to global warming.
This phenomenon is used in real greenhouses, and also in cloches, polythene tunnels and similar garden structures. Solar radiation passes through the glass or plastic, is re-emitted in the form of long wave-lengths which can’t pass back through the glass and is, therefore, trapped. We tend to refer to any greenhouse that depends on the sun’s warmth alone as an unheated greenhouse to differentiate it from one in which some form of artificial heat is supplied in winter.
Artificial heat may be supplied minimally, to maintain the temperature at just above freezing so that non-hardy plants may be kept over winter, or, more elaborately, to enable warm climate species to be grown actively all year round.
The final climatic factor to consider is wind; the movement of air over the earth’s surface. Although wind al any one place may blow from any point of the compass, there are general, global wind patterns.
I’ve already mentioned the importance of wind in carrying rain but dry wind is equally significant in gardens. It is the medium for the dispersal of pollen, of many seeds (including), insects, spores of pathogenic fungi, salt spray from the sea and, on occasion, of noxious artificial chemicals. The drying effect of wind on leaves results in the evaporation that in turn draws water up plants from the soil but in more extreme circumstances, the drying effect can be great enough to have damaging consequences if the water can’t be replaced sufficiently quickly.
Wind is also a powerful physical force and can be seriously damaging in gardens; gales may smash greenhouses, uproot trees or. At least, break branches, whilst lesser strengths can rock shrubs to and fro or remove fruit from trees. I am convinced that almost every garden would benefit from some wind protection.
Exposed sites can be made more favourable for plant growth if a wind-break is erected to provide. Some form of shelter will also help to protect structures such as greenhouses, cloches and arches which can be vulnerable to wind damage. Research into the best types of wind-break has shown that the most efficient is one that is 50 per cent permeable (half solid and half holes) rather than entirely solid, while a harrier 2m (6-1/2ft) high has been demonstrated to protect a garden 20m (60ft) wide. Temporary wind-break netting is invaluable for giving a hedge or individual shrub a better chance to establish but for a more permanent harrier you should consider the relative merits of fences, walls and hedges.