All plants need water, but vegetables, fruit, containers, wall plants and new plantings have the greatest need for water. Established plants need about 2.5cm (1in) of water every 10 days in dry spells.
You can check application rates with rain gauges for sprinklers and measuring jugs for other irrigation systems.
It is difficult to water slowly enough with hoses or watering cans for water to sink in. Leaving hoses to trickle, leads to overwatering when they are forgotten. ‘Ponding’ around subjects or sinking a pipe or sawn-off plastic bottle will help speed up watering and get the water to where it is needed - down at the roots.
Sprinklers deliver water slowly, but much may evaporate in flight, and in windy conditions spread is uneven. Also areas that don’t need water such as paths, get a soaking too.
Sprinklers - rotary or reciprocating - have a place for watering
in newly planted areas and dealing with lawns, but need frequent
moving to avoid overwatering. Refinements include:
Travelling sprinklers, which move under their own power.
Pop-up sprinklers that automatically rise, sprinkle and then
retreat below ground to allow the mower to pass over them.
Mini-sprinklers, which are especially useful beneath tree
and shrub foliage, avoiding wetting foliage and giving a good
coverage over the whole root zone. They are cheap and can
be left in place.
Pop-up and mini sprinklers can be run from a timer, allowing
irrigation in the early hours when the air is still and cool,
reducing evaporation losses.
Delivering water slowly along a leaky pipe or from a weeping emitter is accurate and ideal for established plants and ones in rows, delivering water to just where it is needed. They can be hidden beneath soil or mulch, which also avoids evaporation losses. These work best on heavy soil where the water spreads further sideways as it sinks than on lighter soils. They lend themselves to automation using a timer.