Wash water on the garden

If your garden is suffering during a dry spell you might be tempted to use water from your washing machine.

But there are health issues in collecting and recycling grey water (that’s the wastewater from the sink, bath, shower and washing machine but not from the toilet or bidet). If you’re diverting the grey water to a tank you should ask your local council if you need a consent. You won’t need one if you’re simply collecting the water in containers to water the garden straightaway.

Chemicals are also an issue. The components of wash water most likely to cause problems are:

• salinity/sodium (the sodium in salts harms plants and affects soil structure)
• phosphorus (when this gets into waterways it can cause excessive algal growth)
• high pH (laundry detergents that have a high pH can harm plants).

Our test

To test our grey water, a specialised lab analysed water containing the manufacturer’s recommended amount of detergent dissolved in the combined wash and rinse water that’s used in a standard cycle (120 litres for top-loaders; 60 litres for front-loaders).

We wanted to find which detergents had:

• relatively low pH (less than 10.8)
• less than 20g of sodium per wash
• less than 1g of phosphorus (phosphates) per wash.

Products that met all these criteria have at least one tick in the “grey-water suitability” column of our test results table. (Those with less than 10g of sodium per wash have an extra tick – which gives them two ticks in total. Those with less than 5g of sodium per wash and a pH below 10 have an extra two ticks – three ticks in total.)

All detergents in this test contained less than 1g of phosphorus per wash.

Combined wash and rinse water from detergents with at least one tick in our table is suitable for intermittent use on a garden or lawn with an area greater than 150m². It’s safe to use the rinse water from any of these products on the garden (except on acid-loving plants such as azaleas and camellias).

Don’t use grey water on anything you may eat raw – such as lettuces.

More information

• Recycling wash water: www.smarterhomes.org.nz

Report by Bev Frederikson.