How to drought proof your garden

The nutrients that the plants need are only available when they are dissolved in water, so water not only provides moisture, but food as well. No matter how much organic matter is contained in a soil, it only becomes ‘digestible’ to plant roots when it is in soluble form. Water is the lifeblood for plants.
The average Australian uses over one million litres of water each year – a tiny amount through the tap and a huge amount in the provision of food, power and consumer goods, which is called embodied water.

Of the one million litres of water consumed each year, 70% of that water is applied by farmers irrigating crops and pastures to grow our food (fruit, vegetables, meat and milk). Another 20% is used by coal-fired power plants and other industries to provide consumer goods so that 90% of our water use is embodied in the goods and food we buy. The amount we directly pay for washing and indoor use is about 6% and a further 4% for gardening. So far our governments have only applied restrictions to gardeners, ignoring the 96% of use that is beyond gardening.

When gardeners grow their own food at home, we estimate that water use declines by 89% because home grown fruit and vegetables are vastly more efficient in water use compared to those grown on farms. It takes 100,000 litres of water to produce just 1kg of beef, compared to 48 litres for a mixture of vegetables. Highly organic soils hold up to five times as much water as heavily ploughed, heavily fertilised, chemically grown food.

At our Heronswood garden, where we grow vegetables, fruit and flowers, we have done extensive trials on the amount of water we use and the area needed to become self-sufficient in food.

During the hottest summer in the eleventh year of the longest drought, we found we needed to apply 570 litres of supplementary water for every square metre of productive vegie garden from December through to April, which is about 150 days. Our daily application of water ranged from 4 to 8.3 litres per square metre each day.

The average rainfall days in Melbourne over this summer period (from December to April) is 48 days, with an average rainfall during this period of 263mm (total).

Calculating your water requirements

Vegetables and fruit are thirsty plants that will not yield or thrive without 850mm+ of water during the growing season. In our experience, we believe 1,255mm of water is required during the summer period, or 1,255 litres per square metre.

To calculate how much supplementary water you need, see the table opposite. You can also look up your annual rainfall on the Bureau of Meteorology website.

Capture water – don’t waste it

Your home has a huge catchment area available for water storage – its roof. The average four bedroom home has a roof catchment area of 200m², which is generally all the supplementary water required by your fruit and vegetable garden.

Depending on summer rainfall and the size of your water tank, even during the dry months you can collect enough water to irrigate your garden when rainfall is not predicted. Based on our calculation of 1,255 litres per square metre over summer, most areas of Australia can irrigate at least 10 to 12m² of vegetable garden – that’s enough food to feed a family of four!

Basic calculations

Collection capacity of house roof:

• Typical four bedroom house roof is: 10m x 20m = 200m²
• For Melbourne gardeners annual rainfall is 650mm, so potential roof harvest is: 200m² x 90% efficiency x 650mm = 117,000 litres

Size of storage tank needed:

• If gardeners need between 4 and 8.3 litres of water per square metre per day over the December to April growing period, use the size of your vegie garden to calculate how much water you need to capture and store.
• An average 10m² vegetable garden requires 8.3 litres per day (over 150 days), or 12,450 litres. See table below to find out the ideal water tank required for your garden.

_{Source: Bureau of Meteorology 2018.  * Limited by the size of your tank and the direction of rainfall.}