Water is the major constituent of cell sap and is a vital element for plant health. It is essential for the process of photosynthesis, whereby water and carbon dioxide are converted into food and transport nutrients for the plant. A good water supply is also needed for respiration, seed germination and the development of healthy roots, shoots, foliage, flowers and fruits.

When rain (or water) soaks into the soil, it is absorbed by soil particles and then extracted from the soil by plant root hairs, along with any dissolved nutrients.

Ideally, plants should receive a steady supply of water; however, as rainfall (especially in the UK) can be unpredictable and variable, this is rarely the case unless they are grown in a strictly controlled environment.

Too Much Water (Waterlogging)

Poorly drained soils can lead to a build up of water that can be extremely harmful to a plant's health. Waterlogging can lead to asphyxiation of the plant's roots, causing them to die, and unless the site is installed with improved drainage, most plants will fail to establish.

However, some plants are able to survive such conditions, including marginal plants and trees such as willow (Salix) or the deciduous conifer - swamp cypress (Taxodium distichum). This is one of the major trees of the Florida everglades, which is also fully hardy here in the UK.

Too Little Water (Drought)

Too little water can be more damaging to a plant's development than too much. Although drought is technically a prolonged period without rain (usually more than 2 weeks), plants will begin to suffer long before this; plant functions will slow down, transpiration is reduced and the plant will begin to wilt.

Most plants will need extra watering during hot, dry summers, especially if they are not fully established. However, if drought is a regular occurrence, you may want to consider planting cacti and other succulents that store water in their leaves, stems or roots which enable them to survive long periods of drought.

Be wary of positioning plants close to a wall or fence; ground on the leeward side (the side that is sheltered from the wind) receives much less rain than ground on the windward side. This is known as a 'rain shadow'. Unless you are looking for a very dry area in which to position plants, avoid putting root systems closer than 45 cm (18 in) to the base of the wall.

Improving the Moisture Content of Soils

To improve the drainage of soils:

• Install artificial drainage.
• Double dig the soil, incorporating organic or inorganic matter to help bind the soil particles into crumbs.
• Consider building a raised bed.

To increase the amount of water available to plants:

• Dig in organic matter such as compost to improve the soil structure.
• Add a mulch layer, such as a 15 cm (6 in) layer of coarse bark or a plastic sheet held in place with pegs.
• Remove weeds (this reduces the competition for water and nutrients).
• Position plants away from areas of rain shadow.