Plants in the wild have adapted to the soil and climate of the regions in which they grow. If you want to grow plants where they do not originally belong, you have to try and provide the conditions they are used to or they will die. Many plants from the tropics or the deserts will not survive out of doors in a temperate climate. Other plants may be half-hardy and able to withstand a certain degree of cold but will be killed by a hard frost.
In warm temperate regions such as the Mediterranean, plants have learned to grow where the soil is poor and there is a lack of moisture during the summer months. Often they have silvery leaves, the silver due to tiny hairs, which help to protect the plant from the sun.
Plants in the tropics have a plentiful water supply and heat all year round and so grow non-stop; buds, flowers and fruit all out at the same time on the same plant. Succulents and cacti, on the other hand, are used to being in dry-as-dust deserts. They have adapted their stems as plump reservoirs for water and reduced their leaves to spines so that they lose very little moisture through the pores.
Lush tropical plants
Between the very warm and the cold, frozen regions, lie the temperate zones where the majority of plants are deciduous. Plants that have evolved in temperate zones have learned to cope with the wide variation of conditions in different seasons, having adapted to grow when the weather is warm and become dormant when it is cold. Deciduous trees drop their leaves and hibernate in winter, evergreens pause in their growth and perennial plants die down completely, sheltering their buds under the ground and not pushing up new shoots until the following spring.
How hardy a plant is depends on the lowest temperature it will have to endure. In the USA, where severe winters are common, plant hardiness zones (zones of consistent annual average minimum temperature) have been mapped out by the Arnold Arboretum of Harvard University.
Zones are numbered and a plant might be described as ‘Hardy to Zone 9’. This would mean it would survive an annual average minimum temperature of between 6° and 1°C (43° to 34°F). A similar map has been compiled for Europe.
Within each zone are areas with milder or more severe climates. Local conditions can vary considerably and altitude is an important factor. For every 100m (330ft) upwards, the temperature drops substantially. Unexpected frosts may kill the new shoots of plants that have survived a severe winter, while dormant and even hardy plants may be vulnerable to frost damage.
Frost can turn winter seed heads and stems into magically mysterious and attractive shapes with sugar icing coatings.
Several things can make a difference to an individual garden. The climate in a city can be much warmer than the surrounding countryside, allowing more tender plants to be grown. Aspect is important, too, i.e. whether a garden is facing north or south, or whether it is at the top or bottom of a hill.
South-facing slopes are much warmer than north-facing ones and will bring on growth early in spring. In a hollow, there is always a risk of frost. The stillness of the sheltered air contributes to the risk and what seems to be a sheltered corner of a garden can be far from sheltered in reality. If wind meets a solid wall, the compressed gusts have very high speeds and may damage plants.
European style garden