The main source of water for plants grown in open ground is the rainfall. A big part of rainfall is lost through evaporation and run-off, but any moisture that soaks into the soil is absorbed by soil particles or held as a thin film around them. Water and essential nutrients are then extracted from the soil by plants roots hairs. For optimum growth, plants need a steady supply of water. In reality, rainfall is variable in regularity and also in quantity.
In badly drained soils, a build-up of water leads to waterlogging. Most plants will survive for a short period to an occasional heavy downpour, but where waterlogging is prolonged, roots may die through asphyxiation. Exceptions are specially adapted plants such as marginal aquatics, swamp cypress and willows. Any other plants will fail to establish on permanent waterlogged sites, unless drainage is improved.
Plants development is more often restricted by too little available water in the soil rather than by too much. Drought is a common problem that occur during the summer, when the temperature and sunlight are at their maximum levels. The first outward sign of drought is plants wilting. Plants functions slow down until more water is available again and water loss by transpiration is reduced by partial closure of stomata in the leaves.
Plants that originate in regions with low rainfall, where water is available only from time to time and in low quantities, often have special adaptations like: hairy, sticky, glossy, spiny, narrow or fleshy leaves, that help them to reduce water loss through transpiration. Cacti and succulents have water-storing tissue in their leaves, stems or roots, that help them survive long periods of drought.
Soil and water
In areas with low rainfalls we need to apply various techniques to increase the amount of water available to plants from the soil. We have to remove weeds constantly so that they will not compete for water with our plants. We will have to much our plants to reduce water loss and increase the humus content of the soil by digging in organic matter. Choose planting sites for your plants in open positions, away from buildings, fences and trees, as these often produce areas of rain shadow.
Torrential rain may damage soil structure but the worst effects can be avoided by gardening on well-drained sites. In places where this is not possible, you can improve drainage by deep digging or by installing artificial drainage aids.