Recycled or 'grey' water should be used with care, but can be useful in times of water shortages
Plants can be watered with shower, bath, kitchen and washing machine water (from rinse cycles), collectively referred to as ‘grey’ water. It varies in quality and may contain contaminants such as soap and detergent. Fortunately, soil and potting composts are effective at filtering them out, and the residues can sometimes act as a mild fertiliser.
To minimise bacterial growth, grey water should be saved for only 24 hours, unless filtered through a reed bed or professionally designed system. It is best applied by watering can; grease and fibres can clog irrigation systems.
There should be no problem with small-scale, short-term use of grey water to tide plants over in summer drought. An exception is on edible crops, due to the risk of contamination from pathogens in the water.
In recent RHS experiments with a few popular border perennial plants and sub-shrubs, we found that applying the grey water for several weeks over the summer did not significantly affect plant function. However after 6 weeks some plants (e.g. Stachys byzantina) benefited from a rinse through with tap water as they were beginning to show signs of salt stress.
Long-term, extensive use or permanent altering of indoor plumbing should not be attempted without expert advice.
Softened tap water and dishwasher water are less useful. Salts used in them can damage soil structure, particularly if rich in clay. This said, short-term use of softened water should not cause serious damage and may be worth considering in an emergency.
Read more on collecting, storing and reusing water