Water conservation
Economical watering
Watering techniques
When to water
Soil water
Drought-resistant gardening
Drought-resisting plants
Hosepipe bans and restrictions
Water - using, saving and collecting
Warmer, drier summers are on the cards, and with dry winters reducing the amount of water stored in UK reservoirs etc, there's only one logical outcome - water shortages and hosepipe bans for gardeners. So what can you do to ensure your garden grows in summer and you reduce your water dependency?
Water is an essential requirement for plant growth. It is a limited and valuable natural resource, which may become scarce during periods of low rainfall. The RHS urges gardeners to make economical use of water by understanding the needs of plants and taking steps to reduce the loss from plants and the soil.
Water supplies
Plants need most water in hot, dry (and windy) weather when water companies are least able to meet the demand and often ban the use of hosepipes and sprinklers. Garden water use is estimated at less than 3% of the annual water consumption of an average household. At peak times however as much as 70% of water supplied is used in gardens. Water supply records indicate that peak demand begins in the evening after two weeks without rain in summer. This surge in demand can lead to water companies being forced to deplete groundwater and streams, which can cause serious environmental harm.
The cost of this peak demand has to be born by water users. In the wider interests of the environment and for the use of water in the garden to be acceptable to other water users, gardeners should use mains water as sparingly as they can.
Water storage
Rainwater can be collected, and stored in tubs and other containers, from the roof of dwellings, garages, greenhouses and other structures.
Water butts are designed to collect water from either open or closed down pipes. Closed drain pipes can be easily tapped into with a rain water diverter kit. Similar kits are also available to connect several butts together, ensuring no water is lost even when the first container is full.
Local councils and DIY stores are good places to try for conventional plastic water butts. It is easier to access the water if the butt sits on a stand, either ready made or from bricks, and is fitted with a tap at the base. Butts moulded to look like beehives or terracotta urns are a more attractive option, as are recycled wooden barrels (the wood of which may have shrunk when dry: fill the barrel with mains water initially to swell the wood and make it watertight).
Even in dry districts 24,000 litres (150 water butts) could be collected from the roof each year. However, most water falls in winter, and would have to be stored for use in summer. An average planted area of 200sq m (about the size of an average garden) evaporates about 4,820 litres (about 30 ordinary water butts) every 10 days.
There are about 18 weeks from May until September when plants' needs exceed rainfall. At first, the shortfall is met from soil reserves but these may peter out by July leading to about six weeks when watering is needed, equating to up to 180 water butts!
Adding storage to existing houses would use up much garden space or be very expensive. But, as climatic change models suggest an increasing proportion of rain will fall in winter it may become cost effective to build in a rainwater storage when constructing new homes.
This usually involves sinking a large tank somewhere in the garden, water from which can be pumped out and used for the garden or for domestic tasks such as flushing toilets. The UK Rainwater Harvesting Association website has downloadable information and has contact details for member companies by their locality.
Domestic wastewater may also be used. Household soaps and detergents are harmless to plants but bleaches, disinfectants and stronger cleaning products must not be used on plants although they will be harmless on bare soil. In containers alternating wastewater and mains or rainwater is prudent. Domestic gardening is unlikely to lead to pollution of ground water or streams and ponds when using waste water. However, gardeners should be aware that careless discharge of contaminated water could cause serious damage.
The key to keeping gardens water needs met lie in plantings that need little water but still do the job of making an attractive garden, conserving water in the soil, storing as much water as possible in the soil and eking out the mains supply with stored and waste water where possible.
Water conservation
Hedges, fences and other shelter reduce the evaporation due to wind and provide shading to help reduce water loss.
Raising the height of mowing lawns reduces water loss from grass as the foliage shades lower leaves and the soil.
Mulching with a layer of organic matter or gravel, at least 5cm (2in) thick or using opaque mulching sheets reduces the loss of soil moisture from the soil. Mulching also encourages good root development in the upper levels of the soil increasing the area of soil that the plants can explore for moisture. Little water is lost by water rising to the surface from deeper levels so mulching only conserves moisture in the upper layers. This may amount to as much as the equivalent of 20mm (0.75in) of rain. Deeper water will be conserved whether mulches are used or not.
Hoeing to make a finely divided surface layer or ‘dust mulch’ is ineffective at conserving moisture loss and may actually increase loss of moisture. However, hoeing, or better from a moisture conserving point of view, using weedkillers, to remove weeds is vital as weeds are very effective at depleting soil moisture.
Similarly cultivation in spring and summer will lead to severe soil moisture loss.
Economical watering
Established plants do not generally need watering in Britain but their growth is usually improved by watering when they are under drought stress. Newly sown or planted areas are vulnerable and their watering needs should be a high priority.
Edible plants
Fruit and vegetables will usually crop adequately without watering but the quantity and quality of produce is greatly improved by watering at times when drought stress would affect the part of the plant that is gathered. When it's the leaves that are harvested, the crop should never be short of water. If watering is needed give a single, thorough watering about two weeks before harvest.
Ornamental plants
Most established ornamental trees and shrubs have such wide-ranging roots that they are drought proof and need no irrigation. Herbaceous perennials often need watering for their best performance in summer dry spells, but drought-proof borders can be attained by selection of suitable plants and management techniques. The Dry Garden at RHS Garden Hyde Hall and the Bicentenary Glasshouse Borders at Wisley are examples of this.
Lawns
Watering lawns requires great quantities of water, and this is a questionable use of a scarce resource for other than quality lawns. In dry periods mowing should be less close and less frequent as an alternative contribution to keeping the lawn attractive. Lawns usually recover when the autumn rains return.
Watering techniques
Sprinklers have a limited use in gardens - to water lawns where this is essential and to raise the moisture level of unplanted areas. But the aim of garden watering should be to apply water only at the stem bases beneath the foliage canopy leaving the surrounding soil dry. This limits weed problems and ensures all water goes to where it does most good.
To save time and labour, watering cans and hoses can be replaced by drip or trickle irrigation systems. Fine droplets are best for watering, as coarse droplets damage soil structure slowing ingress of water and preventing seedlings emerging.
Light soils need watering more frequently than heavy soils, and because heavy soils hold more water heavier applications of water can be given. Only the root zone or top 60cm (2ft) of soil must be wetted - water that penetrates deeper will be inaccessible to plant roots.
- Buy and plant new plants in autumn and spring so they get the best chance of growing roots before dry weather begins.
- Feed lawns and plants with fertiliser so that they make the best use of what water there is in the soil by having the resources to grow new roots.
- Invest in (more) water butts if you have space; even a summer storm will usefully top up water butts.
- Go easy on hanging baskets and small pots in summer; large containers are easier to keep watered and, plant for plant, use less water.
- Invest time is eliminating weeds as soon as they show themselves in spring - weeds suck valuable moisture out of the soil.
- Mulching has some effect in reducing moisture loss from the soil, but more importantly controls annual weeds and reduces the need for hoeing to control weeds; disturbing the soil leads to increased loss of soil moisture.
- Don’t dig soil after March, to avoid losses of water to the air.
- Choose drought-resistant plants - including vegetables: peas and broad and runner beans need more water than French beans; beetroot, chard and carrots are drought resistant compared to lettuces and spinach; cabbages need much less water than Brussels sprouts and cauliflowers; sweetcorn and pumpkins can get by with no watering.
- Pelargoniums and petunias do better in dry conditions than begonias and fuchsias. Try to buy plants not yet in flower - once flowering begins root growth is sacrificed to flower production and without watering plants will wither.
- Sow and turf new lawns as soon as possible in spring (or, better still, do it in autumn) so they get established before hot, dry weather strikes. Without sprinklers it is hard to keep alive lawns laid in June.
When to water
Most water needed by plants is used to replenish losses from leaves. Leaves have openings or stomata to allow carbon dioxide used in photosynthesis to enter from the atmosphere. However, water is lost from the leaves through the stomata. This is replaced by water from the roots. In effect, the evaporation from cells within leaves sets up a suction that pulls water from the soil via the roots and conducting tubes in the plant to the leaves. When water becomes short the stomata close and without carbon dioxide photosynthesis stops and plant growth ceases.
As the plants abstract water the soil moisture content declines. The difference between field capacity (see soil water below) and current soil water status is called the soil moisture deficit. For the best plant growth irrigation should aim at replenishing the soil moisture deficit, but no more or the excess will drain to waste.
Commercial methods of measuring soil moisture deficit from soil or meteorological data to detect when the soil water levels need replenishing are inapplicable to gardens.
However, 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. Leaving some vegetable plants unwatered can delay maturity and may actually usefully spread the harvest period or season of interest. Gardeners should be aware that clay soils feel damp even when all available water has been used and sand 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 gardens soil to the growth rate of the plants. Wilting is usually preceded by changes in leaf position and darkening of colour.
Once lack of moisture has been identified, water may be applied to restore the soil to field capacity. A clay soil in which plants are wilting might need 81-litres per sq m and a sandy soil 60-litres per sq m (or nine or seven watering cans) respectively. In practice gardeners are unlikely to let soil get this dry and water will usually be applied at the base of the plant rather than over a wide area greatly reducing the amount needed.
As a general guide up to 24-litres per sq m every seven to 10 days will be sufficient to maintain plant growth.
In practical terms it is best to water some of the garden before drought really sets in to keep the soil moist and avoid soil moisture deficits building up, in case water restrictions are imposed later, and because most gardeners will not have the resources to water the whole garden in a severe drought.
When the garden is affected by drought it is futile to try and remedy this by light watering over a wide area. Light watering may encourage surface rooting leaving plants more susceptible to drought. The water and time should be used to make a single thorough watering.
Try to water in the cool of the evening or very early morning. That way less water will be lost immediately to evaporation.
Soil water
When water falls on the soil it enters the space between the particles that make up the soil. Water does not move down into the soil until all the pores above the descending wetted area have been filled. Therefore light watering merely wets the surface layers. It is not worthwhile just making the soil damp; it has to be thoroughly wetted for the water to travel down to the roots.
After watering or rain stops the water travels by gravity downwards while air enters the partially emptied pores. Some water is retained around each soil particle by surface tension and eventually drainage ceases. The soil now holds the maximum amount of water that it can. In this state, called field capacity, the soil contains air, water and nutrients and is in the ideal condition for plant growth.
As roots remove water the layer around each particle becomes thinner and surface tension increases making it more difficult for the plant to extract the water, leading to wilting, called permanent wilting point, where plants are unable to abstract water for growth.
The texture of soils influences how much water they can hold. Coarse sand soil might hold the equivalent of 50mm of rainfall in the top 60cm of soil, while a clay soil might hold 105mm of rain. Hence soil that contains 50mm of rain has 81-litre per sq m of water available to plants.
Some soil water is so tightly held to soil particles that plants cannot extract it. Clay with many small particles retains most water explaining its cold, wet nature.
Watering should never be carried out where drainage is poor - adding water will do more harm than good, as roots are very susceptible to airless conditions when the soil is warm in summer.
Soil texture can be altered by adding coarse sand or fine gravel, but because much material is needed to permanently change the soil texture it is not usually as effective as adding organic matter. Organic matter, garden compost for example, increases the moisture-holding capacity of the soil by about 50mm of rain in the first year after application.
Further information
Guy Barter