Watering

When to water

Gardeners can inspect the soil at a spade's depth. If the soil feels damp there is unlikely to be any need to water, but if it is dry, then watering is probably required for some plants.

Gardeners should be aware that clay soils can feel damp even when all available water has been used and that sand soils can feel dry even if some water is available. The only way round this is to develop experience in matching the observed state of an individual garden’s soil to the growth rate of the plants. Wilting is usually preceded by changes in leaf position and darkening of leaf colour.

How to water

It is better to water the garden before drought really sets in, to keep the soil moisture levels even and avoid soil moisture deficits building up.

Once drought has set in, it is futile to try and remedy this by light watering over a wide area. Light watering may encourage surface rather than deep roots, leaving plants more susceptible to drought. Instead, make a single thorough watering of the plants that are suffering. Try to water in the cool of the evening or the very early morning so that less water is lost immediately to evaporation.

Watering should never be carried out where drainage is poor, as adding water will do more harm than good, roots being very susceptible to airless conditions, particularly when the soil is warm in summer.

How much water to apply

Light sandy soils need watering more frequently than heavy soils, but less water can be applied at each watering. Heavier, clay-based soils can be watered less frequently, but need heavier applications of water because they hold more water within their structure. 

A clay soil in which plants are wilting might need 81 litres per sq metre (17.5 gallons per 10 sq ft) and a sandy soil in which plants are wilting might need 60 litres per sq metre (12.2 gallons per 10 sq ft).

In practice, gardeners are unlikely to regularly let the soil get so dry that plants are wilting, so less water is required. Water can also be saved by applying it to the base of the plant rather than over a wide area. As a general guide, up to 24 litres per sq metre (5.2 gallons per 10 sq ft) every seven to 10 days will be sufficient to maintain plant growth.

Methods of watering

Sprinklers: These have only limited use in gardens, mainly to water the lawn where this is essential, and to raise the moisture level of unplanted areas.

Hoses and watering cans: Most garden watering should be aimed specifically at the stem bases beneath the foliage canopy, leaving the surrounding soil dry. This helps to limit weed problems and ensures all the water goes where it is needed.

Seep hoses: These hoses or pipes with holes in them deliver water accurately to established plants and plants in rows. They can be hidden beneath soil or mulch, which also avoids evaporation losses. They work best on heavy soil where the water spreads further sideways as it sinks than on lighter soils.

Automated irrigation systems: To save time and labour, watering cans and hoses can be replaced by drip or trickle irrigation systems. Only the root zone or top 60cm (2ft) of soil should be wetted - water that penetrates deeper will be inaccessible to most plant roots. Suppliers can advise on installation of these systems. See for example, Access Irrigation, Garden Systems, or LBS Horticulture.

Tips for economical watering

Know your plant's watering requirement;

• Established trees and shrubs do not generally need watering, as they have such wide-ranging roots that they are drought-proof. But their growth may be improved by watering when they are under drought-stress
• Trees and shrubs planted less than five years ago have increased water requirements and may suffer drought-stress without watering
• Newly sown or newly planted areas are very vulnerable to water-stress, and watering these should be high priority
• Herbaceous perennials often need watering to boost their performance in hot, dry spells. Plant choice is crucial if you want to achieve a drought-proof border. See our advice on drought-resistant plants for more on plant choice
• Edible produce yields and quality are greatly improved by watering at times when drought stress would affect the part of the plant that is gathered. Leafy crops such as lettuce and spinach should never be short of water. Onions require little or no watering. Most other crops need watering at sowing and transplanting time, and then again as the fruits, roots or tubers are developing. It is also a good idea to give a single, thorough watering about two weeks before harvest
• Lawns require great quantities of water for thorough irrigation, and this is a questionable use of a scarce resource for any other than high quality lawns or sports turf. Instead of watering in dry periods, feed the lawn well and mow less closely and less frequently. Brown patches usually recover when the autumn rains return
• Mulching with a layer of organic matter or gravel at least 5cm (2in) thick, or using opaque mulching sheets, reduces moisture loss from the upper layers of the soil. This may amount to as much as the equivalent of 2cm (0.75in) of rain
• Removing weeds is vital, as weeds use up valuable soil moisture reserves
• Planting new plants between autumn and spring gives them the best chance of growing roots before dry weather begins