Irrigation systems

Drip irrigation delivers a controlled trickle of water directly to the roots of the plant and avoids wetting the foliage, helping to control some diseases. Weeds are not encouraged and evaporation of water from the soil is minimised. The slow rate of application means that no water is wasted through run off.

The simplest form of drip or trickle irrigation is the soaker hose, which requires no installation other than being connected to the tap or the end of a hose. Water seeps slowly through its porous surface all along its length. It must be moved about the garden for complete coverage and requires the same timing precautions as sprinklers to avoid over-watering.

More sophisticated, semi-permanent dripper or trickle systems typically use 13 mm diameter polytube as feeder lines into which drippers or emitters are inserted. The drippers may have either a fixed flow rate or may be adjustable according to the water requirements of individual plants. Drippers may also be connected to lengths of flexible 4 mm polytube which is in turn plugged into the feeder lines, enabling plants some distance from the line to be watered effectively. Drip irrigation requires lower water pressures than sprinklers so a pressure regulator should be installed in the main line just after the filter. If a regulator is not fitted, the water pressure may cause drippers to blow out.

Both microspray and dripper systems are vulnerable to clogging from water-borne debris. To avoid repeated blocking, an inline filter should be installed just after the main tap.

Some local authorities require a backflow valve to be installed by a plumber before any irrigation system may be installed. This valve prevents water from the irrigation system flowing back into the household supply.

The simplest solution to watering the garden is a hand-held rose attached to the end of a hose. This may be all that is required for the watering needs of a small inner city garden with small beds and a few potplants. The next step is to a movable sprinkler attached to a hose and moved about the garden as required. Such sprinklers vary enormously in complexity and price from very simple sprays to complex mechanical ‘butterfly’ units with adjustable spray patterns. Sprinklers like this are easy to forget once switched on, so an inexpensive clockwork timer fitted between the tap and the hose is a worthwhile investment. This is not an efficient solution for irrigation because there is a higher potential for wind spray and evaporation and for this reason it has been banned under water restrictions in parts of the country. The ideal rate of application is about 12 mm per hour. At higher rates some water will not be absorbed by the soil and will run off to be wasted. To determine the flow rate of a sprinkler, check the level of water filling straight-sided containers such as cans placed at various points under the spray after 15 minutes.

Micro irrigation was developed in Israel to make the best use of a limited water supply in an arid climate, not unlike much of Australia. Miniature spray heads or jets are mounted onto rigid risers plugged into a network of black pvc irrigation tubing, known as polytube, as feeder lines, usually 13mm in diameter. The spray heads are available in a variety of spray patterns and spreads enabling efficient, even coverage only to those areas of the garden requiring watering. Like moveable sprinklers this irrigation is prone to wind spray and high evaporation losses and is not permitted under water restrictions in some parts of Australia.

Various forms of time controllers can be used to turn watering systems on and off and these vary widely in their complexity and expense. The simplest type is a clockwork timer which is fitted between the tap and the hose line or irrigation system, allowing up to two hours of watering before switching off.

More complex computerised units allow several independent lines to be programmed to turn on and off independently. The units control low voltage electric solenoid valves which open and close each line, and the more sophisticated of these devices allow each line to be operated independently several times daily over a period of a week or more.

A rainfall sensor connected to the timer should be used to cancel the system in periods of high rainfall when additional watering would be unnecessarily wasteful.