The Physical and Chemical Environment of gardens
By Steve Head Reviewed by Ken Thompson
Gardens deep in the countryside don’t have any special temperature differences, but gardens in built up areas experience the “heat-island” effect. Hard surfaces like concrete and tarmac, and in particular buildings, heat up in the daytime, and lose their heat slowly at night. The activity of people - transport and businesses, house heating and air conditioning are also sources of heat. London for example, can be 5°C warmer through the winter than its surroundings, and gets many fewer frosty days. This allows London gardeners to grow tender plants outside, which would otherwise need greenhouses or coastal gardens in Cornwall or the Scillies. We don’t know yet what general effects this temperature hike has on garden wildlife, but research has shown effects on insects and lawns. Temperature differences will also be found geographically from north to south, and in gardens on hills, where for every 100m altitude increase, average temperatures drop half a degree centigrade.
Rain generally falls on gardens in the same way as the surrounding countryside, but air rising from big cities can create dry “rain shadows” in downwind suburbs. Where gardens differ from natural habitats is in their owner’s ability to beat drought by watering. At least half of garden owners water their plants, and this allows them to keep plants healthy (and available to wildlife) during dry spells when they would otherwise wilt or grow very slowly. This must have a beneficial effect on most garden wildlife, as will the many gardens with ponds and bog-gardens, which in really dry spells with hosepipe bans, can be the only really vibrant parts of the garden.
One definitely negative aspect of most gardens is noise and disturbance from traffic, people and pets. Some of our rarer birds are very intolerant of disturbance, and could never nest in any but the biggest, deeply rural gardens. Some of our familiar songbirds seem to have adapted to disturbance and tolerate lots of noise, but have been found to have to sing louder, or in higher notes, to make themselves heard and maintain their territories. Dogs and cats, particularly the latter, are a significant source of disturbance by day and by night. This is one of many areas where gardeners could make useful observations and help our understanding of garden ecology.
Another negative aspect of urban gardens is the high level of light pollution, such that some urban children have never seen the stars. Wildlife is definitely affected, with birds singing earlier in the year and later into the evening. Many insects are attracted to light, and some killed by exhaustion, or eaten by bats, which find street lights a benefit.
Pollution and chemicals
Pollution is nothing like as bad in British cities as it was fifty or more years ago, but we still suffer from traffic fumes and some industrial air pollution. We know this has significant effects on plants in controlled experiments, but remain ignorant of how much effect it has in garden wildlife in different built up areas. One other issue here is the tendency of many gardeners to over-fertilise their soils. This isn’t a great problem, but it can cause poor water quality in garden ponds, and it certainly makes it easy for grass and other weeds to out-compete slower growing and delicate flowering plants.