Microclimates can be affected by a number of factors;
Topography: Hills can intercept rain and slopes facing west or south-west (Britain only) are usually wetter than ones with other aspects.
The higher the garden is above sea level the cooler it is, and probably more exposed.
Dips, dells and hollows can influence exposure, but they can also act as collection points for cold air on frosty nights. Nearby hills can also act as sources of cold air. On frosty nights cold air forms on cooling objects such as trees, sinks to the ground and literally flows downhill to collect at low points which are known as frost pockets (see below).
Frost pockets: A garden in a frost pocket is at significant risk of late and early frosts and has a shortened growing season between the first and last frosts. Early flowering plants, especially fruit trees, are often unrewarding to grow in frost pockets.
The cold air sometimes collects behind barriers across the slope such as fences or hedges. Making gaps in the barrier can allow the air to drain away and alleviate a frost pocket.
Aspect: South-facing areas get more light and warmth than those with other aspects; north-facing areas get the least. A slope will accentuate this, and also enhance drainage. The effect of a south-facing area is emphasised if there are fences or walls to trap and reflect warmth. However, the north-facing side of a fence or wall is also of value as it has a more equable climate without extremes of either hot or cold, and is favoured by some plants such as hardy fuchsias, hellebores and Japanese anemones.
South-facing walls also heat up in the day releasing warmth through the night preventing frost and promoting ripening of fruits.
Exposure: Wind is more damaging than other forms of plant stress in Britain. Shelter from wind whether by nearby high ground, trees, hedges, fences or buildings is highly significant in promoting plant growth. Gardens on exposed hilltops or west-facing slopes, seaside locations or in ‘wind tunnels’ between hills are most likely to have microclimates dominated by their exposure, and most likely to benefit from introducing shelter.
Good barriers slow down rather than block wind and growing conditions are improved on both sides of the shelter. Shelter can improve growing conditions for up to 30 times the height of the shelter. The closer to the shelter the more pronounced the effect, but the greater the potential for shade.
On a smaller scale, corridors between buildings can be very unrewarding to plants as the air flow not only dries and cools plants but the flow in such places is turbulent with gusts that are especially damaging. Solid fences also cause turbulence and this not only shortens the life of the fence but also stresses plants and reduces the enjoyment of the garden by users.
Containers in particular are vulnerable to drying and wind damage, especially when sited in exposed positions; roof gardens and windowboxes for example.
Shade: Where trees, hedges, walls and fences cast shade, conditions approximate to woodland. This is very favourable for woodland plants such as mahonia and pieris. Walls and fences have the advantage of lacking roots and therefore soil is relatively moist compared to soil beneath trees and along hedges. However, in shaded gardens some gardening styles, lawns or late summer flowering herbaceous perennials might be relatively unrewarding. Managing tree canopies and tree density is often required.
Paths, patios, decking and other paved areas can become unsightly and slippery in shaded areas, so that alternative paving must be used.
Drainage: Low-lying areas, especially if near ditches, brooks or ponds, can have water levels that come close to the surface. If this persists all year then bog and water gardens are likely to be highly successful.
More commonly, however, water levels fall during summer but remain wet for much of spring and autumn as well as winter. This combination of wet and dry soils make such areas very difficult to manage with only a limited range of plants – dogwoods and elders for example – tolerating these conditions. Drainage is the best remedy, but raised beds are more practical in many cases.
Modern house construction methods can result in degraded subsoil and broken drainage systems. New gardens might have severe waterlogging problems even though not in low-lying areas. Drainage is again the best remedy.
Drought: Walls keep off rain causing a mini-rainshadow up to a metre (yard) wide from the base of the wall. In this zone the soil will be particularly dry and plants may fail to thrive unless watered.
Trees have roots that can spread up to three times the height of their canopy, and their canopy too intercepts rain and increases evaporation of rain so that only heavy rain penetrates to the soil below. For example, a large eucalyptus in a small garden will impose drought and shade for a long distance into adjacent plots.