This is a complex and variable topic because each garden will have different requirements. However there are many elements that apply across the board regardless of the conditions under which you are gardening
Water is without a doubt one of our most if not our most precious resource, it should be used sparingly with thought and care. When using water its worth remembering that we have a finite supply which future generations may wish to enjoy.
The keystone plants in your garden that is the large trees and shrubs should flourish with no irrigation once they are established.To plant trees and shrub that will require water for the entirety of their existence is condemning them to a short life, at some stage someone will neglect to irrigate. unless you live in extreme aridity there are plant for your environment.
, The length of time it takes to establish a large plant varies from species to species, climate to climate but between 3-5 years a safe bet. Don’t expect big plants to settle in quickly. Here in Olinda which is by Australian standards a cool moist climate I have a 3 year policy meaning that the trees and shrubs I plant will be nurtured for 3 summers after which they’re on their own. If they don’t survive then I’ll try something else. There are exceptions to the 3 year policy for example some of the fibrous rooted shrubs such as Rhododendron take much longer to establish.
How to minimise watering your garden.
The most efficient way to minimise watering is the extensive use of organic material. There is no doubt in my mind that the frequent application of mulch will dramatically improve soils ability to retain moisture. The type of mulch used plays a critical role. Soft mulches such as pea straw or sugar cane should be reserved for annual crops such as vegetables after which they are dug in and a fresh layer applied. For garden beds with more permanent plantings a mulch which contains wood is a much better option. These hard mulches should still be applied annually to a thickness of 10-15cm and between applications Autumn leaves, compost or other organic materials the can also be spread. Developing a thick layer of rich humus takes many years and requires commitment and resources however if you have a longterm interest in your garden this is the way to go. Soil rich in hummus not only holds moisture but is also easier to re-wet if it dries out.
Positioning plants in your garden to minimise water usage
Its always a smart move to group plants according to their water requirements, so for example you wouldn’t put Salvia and Astilbe in the same bed but instead group the Salvia with Agastche and Echinops. All of us at some stage want to grow plants that will struggle in our climatic zone this is just human nature. The best plan for this is to have one bed dedicated to plants with high water needs and install an underground drip system. Some knowledge of the plants you want to grow and the habitat in which they have evolved to grow will help to understand their water needs. When planting a shrub border in the initial stages there will be large spaces between each plant. As well as a thick mulch some soft fast growing ground covers will prevent Evaporation while the shrubs establish. I quite often use smaller grasses to fill this role because they generally fill in quickly and are easy to remove/transplant down the track.
What are the best cost effective watering systems
If you want to install permanent in ground watering then I find drip systems laid in a grid just underneath the mulch to be efficient and cost effective. Sprinklers and risers both of which water above the ground and lose a lot of water to evaporation and they also tend to extend their reach beyond the required zone. Underground drip systems deliver water only where its needed, they are extremely accurate in their distribution and there is no evaporation. It is inevitable that you will put the spade through it so keep plenty of joiners on hand. When using a drip system deep soaking occasionally is better than small amounts regularly. An occasional deep soaking will encourage the plants to get their roots down deep. As with pots garden soil is best dried out between watering. When soil drys it aerates stays porous and light. This is an important process for good soil health
When planting trees and shrubs that will only require water during their establishing phase I make a wall of earth around them about 500mm diameter and 100mm high. The wall means you can quickly tip in a bucket of water and walk away, you can mulch over the wall because the depression will remain under mulch. Alternatively sprinklers that throw a circle of water work quite well. These simple devices have no moving parts so last forever and its easy to control the radius using water pressure. Its good practice to lightly fork the soil/mulch surface prior to using the sprinkler this will enable the water to soak in quickly rather than run off.
Design your garden to suit the climate you live in, think outside the box when it comes to the use of plant species. My last garden in the inner west of Melbourne faced north so was hot and at times dry. I sheltered the entire back yard with Eucalypt leucoxylon rosea dwarf, these little trees cast dappled shade and don’t use vast amounts of water. Beneath them I could grow many of the more tender smaller plants I love.