Saving water – how much water can your garden store?

In our last article we looked at measuring how much mains water you use in the home and how much is left for the garden. One of the most important aspects of creating a sustainable, resilient garden is reducing mains water use.

It is best if plants can be watered naturally through rainfall, with no additional inputs required. If extra irrigation is needed, harvesting rainwater from roofs and other surfaces is much kinder on your plants than using tap water. Not only will this reduce pressure on the mains water supply, but rainwater has a more balanced pH and is free from the chemicals found in treated water which harm the all-important soil microbes, your underground workforce.

Most plants, even those that can thrive without ongoing irrigation, need watering during their establishment period. In the studio we calculate the water needs of a design proposal by looking at both the establishment irrigation and the ongoing irrigation requirements. By working out how much rainwater can be collected from surfaces and how much is needed to maintain the garden, we design on-site storage capacity (such as water butts, attenuation tanks, ponds and lakes, and dipping ponds) to match water requirements, so the garden does not rely on mains water to thrive.

We have created a simplified version of our irrigation calculator that you can apply to your own garden or site. The calculator will help you decide how much storage you need to water your garden from rainwater alone. It will also help guide you in your choice of planting types when planning a water-efficient garden.

To use the calculator you will need to measure or estimate the areas (in m2) of the different types of planting in your garden (or proposed design). To fill in the rainwater harvesting section you will need to measure the area of the roofs you can collect water from, from sheds and greenhouses to main houses and barns. You can measure areas yourself on-site or approximate from Google Earth using the area measurement tool.

Existing trees should not need irrigation, but if you are planting new trees, you should enter the number of these into the calculator. Typically a newly planted tree will require 70 litres per week for the first two summers after planting and in prolonged dry periods if you are on well-drained soils. On our projects we use tree irrigation bags, such as Treegator bags, in this establishment period.

To receive a copy of our irrigation calculator (an Excel spreadsheet), please join our emailing list below and you will be emailed a download link.




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    First image © Marianne Majerus. Second image © Jason Ingram