Garden Irrigation: Water and Watering
Water is essential for plant growth, because not only is it the main constituent of the protoplasm of plant cells, but because there must be sufficient moisture in thefor plants to maintain a through-flow of water and the nutrients dissolved in it. While much research has been performed on the amount of water required to produce commercial crops, until recently little thought was paid to the needs of the amateur gardener. Now gardeners find their watering habits coming under close scrutiny as water charging through metering becomes more widespread and water shortages are experienced in many parts of the world. As a result, the benefits of research for commercial growers are becoming more widely available to gardeners.
Watering can be a chore, so even if water is in plentiful supply, it is still worth assessing how you water your plants to see if the task can be done more efficiently and if new watering equipment could make your life easier. You should aim for the majority of your established garden plants to obtain most of their moisture from the soil with only some supplementary watering during dry spells in the growing season. But to achieve this, you need to determine which plants will benefit most from extra water, when to apply it and how much to give.
If you live in a low-rainfall area or you have a soil that is light and free draining, drought problems are likely to be frequent. In the medium to long-term, you should set about systematically improving the soil’s moisture retentiveness by adding organic matter regularly. In the shorter term, applying the organic matter as a mulch (provided this is done when the soil is already moist) will help. But even more immediate benefit can be achieved by growing plants better able to tolerate dry conditions. Species from the Mediterranean (such as many) or South Africa (many summer annuals and tender perennials, like pelargoniums) make good choices. And any plants with grey or silvery foliage are drought tolerant, the colour being due to numerous tiny hairs that restrict water loss from the leaf surface.
Keeping down weed growth can ensure that more water in the soil is available for your plants, and only grow as many vegetables as you really need because much water is wasted by loss through their foliage.
How much water and when
Although all plants need water, it is only at certain stages of growth that their water demand is critical. As a general guideline, I suggest that you concentrate first on annuals, such as bedding plants and vegetables and then on fairly shallow-rooted perennials like rhododendrons and soft fruit. Apply water when the object of their cultivation is beginning to mature: lettuces as they begin to heart up, potatoes at flowering time (when their tubers start to swell) andas buds burst, for example. During these periods, you should aim to supply about 25 litres of water per square metre (5 gal./ sq. yd). Or, alternatively, apply 15 litres per square metre (3-1/2gal./sq. yd) once a month or 10 litres per square metre (2-1/4gal./sq. yd) once a fortnight. New plants will need watering until their root systems are established and they can obtain moisture from further down in the soil. Plants in containers will always need regular watering, the smaller and more exposed the container, the more it will lose water and hanging baskets may need watering twice a day in summer.
Many gardeners spend much time, effort and expense in watering their lawns but despite turning brown in periods of drought, grass has remarkable powers of recovery so, with the exception of newly-laid or sown lawns, lawn watering in most years is in fact wasteful. Where it is practicable, try to water plants in the evening as this gives the moisture a chance to soak into the soil surrounding the roots before the sun’s heat evaporates it. When pouring water from a watering can or hose, allow water to soak into the soil before adding more. Simply flooding water around the base of a plant will cause water to run off away from the plant and can wash away soil and expose surface roots.
Garden watering systems
While watering cans still look familiar, other garden watering equipment has changed beyond recognition in recent years. But before investing in expensive watering equipment, check with your local water company for any regulations or restrictions relating to their use. Unattended watering equipment, in particular, is regulated in some areas and, to use it. You may need a permit or licence or be required to have your domestic water supply metered. And in times of water shortage there may be additional restrictions on the use of hose pipes and sprinklers. The mains water pressure in your area can affect the functioning of certain types of sprinkler and in high-pressure areas, less well made types of tap and hose connector can leak or burst. In Britain, the mains pressure varies from 30 lb/square in (211 kg/square cm) to about 130 lb/square in (914 kg/square cm). All watering systems are immeasurably easier to use if you have an outside tap with a screw-thread but, by law, they must be fitted with a back-flow prevention valve to avoid contamination of the water supply.
When purchasing a hose, opt for a double-wall, knitted type on a through-flow hose reel with snap-fit connectors. Consider an automatic metering device (attached to your tap) which will turn off the flow after a predetermined length of time or, more usefully, after a predetermined volume of water has been delivered. More sophisticated ‘watering computers’ are also available which enable you to programme your watering to turn on and off on a daily basis. Although perhaps of limited use in the garden itself, these are distinctly valuable for greenhouse watering (see below). Of the many attachments available at the delivery end, I find you really need one good, adjustable sprayer that offers a gentle spray as well as a strong jet, and a comparable, adjustable pattern sprinkler. Choose a sprinkler that will be appropriate to the size and shape of your garden: nothing is more wasteful or annoying than watering neighbours’ gardens as well as your own. In a large garden, it makes sense to fit a hose-end connector that automatically shuts off the water supply when you change hose-end appliances, so avoiding a long walk back to the tap. Where watering restrictions are in place, use a watering can to supply water to the plants that need most, and direct the water close to the plant root zone.
Greenhouse watering systems
Watering in the greenhouse has its own particular problems, the extra warmth combined with plants in small pots mean that watering must be done frequently, and a watering can may be hard to manoeuvre in confined space. The following types of irrigation system, ideally controlled from the mains by a ‘watering computer’, are worth considering if you are regularly away from home:
Trickle (or drip) irrigation: Water, from the mains or from a reservoir, is carried along a series of narrow plastic tubes, each ending in a nozzle through which water drips Each drip-nozzle supplies one plant pot. These systems can be tricky to assemble and need time and patience to set up but, once established, are useful for not only but for containers on or for hanging baskets. They also are very precise, with water being directly delivered to each pot.
Overhead (or mist) irrigation: These have similar water supply systems to trickle irrigation, but water is sprayed downwards from nozzles attached to the greenhouse roof. They are useful for propagation and are often installed over a sand propagating bench but great care is needed in greenhouses with mains electricity.
Capillary Watering: This depends on the ability of the compost in a pot to act as a wick. Drawing up moisture from wet matting through the potsholes. The plant pots are set out on capillary matting contained in shallow metal trays, the mailing being kept moist by contact with a water reservoir. I have found polyester matting 2-3mm (1/8in) thick gives the best results for 9cm (3-1/2in) diameter pots of soilless compost but pot size and compost type will affect the results. This is a simpler system than trickle or mist irrigation but it has its limitations. Using pots of different sizes or with different plants or composts can result in uneven watering and there must be good contact between the base of the pot and the matting. Algal growth on matting can be unsightly so an algicide may have to be used twice a year. Because there is restricted drainage, fertilizers (which still need to be applied by watering from above) can build up in the pots to damaging levels. As a precaution, an occasional thorough watering from above to flush out any build up of salts is advisable.
Rainwater can and should be stored whenever possible. One or more water butts can be positioned to collect water from buildings. Use this collected water for outdoor plants in, but not for as the algal and fungal and ‘wildlife’ that it contains may result in damage to young seedlings. During periods of water restriction, domestic waste water from washing up bowls and baths should be used in the garden although water containing bleach, chemical disinfectants or grease is best not used.
Increasingly, gardeners are turning again to the underground storage tanks, popular in Victorian gardens, and a number of systems are now available, including the use of permeableblocks to collect water hilling onto driveways. Inevitably, these are costly to install but worth considering for their long-term value.
Types of water
Tap water can be used for all plants as the chlorine in it does not affect plant life. Soft water has very few mineral impurities whereas bard water contains large quantities of calcium or magnesium salts. In an outdoor border or bed, hard water is unlikely to be problem for lime-hating plants but such salts can build up when plants are grown in containers. The result can be yellowing of the foliage and poor growth so to avoid this, use rainwater for lime-hating species. Chemically softened water has caused harm to some types of plant so should be tested on a few samples first.