Gray water is the collective term for water from the bathroom (shower and bathtub) and kitchen (washing machine rinse cycle only, as softened tap water and dishwasher water are less useful as they contain salts that can damage soil structure). This means gray water may contain contaminants such as soap and detergent because of where it's coming from.
According to the horticultural experts at the RHS, gray water should be saved for only 24 hours to minimize bacterial growth, unless you're using a professionally designed system.
'If you're keen on the idea of reusing bathwater for plants, consider installing a gray water system designed to collect and filter water from various household sources, including bathtubs,' says Itamar Ben Dor, founder of Green-Life. 'The water is treated to remove contaminants before being used for irrigation. Gray water systems ensure that the water reaching your plants is free from harmful chemicals.'
Check local regulations regarding gray water use, as restrictions or guidelines may exist, especially during drought conditions. 'Some areas have strict laws regarding the utilization of non-potable water for irrigation,' says horticultural expert and garden planner Diana Cox. 'If it is permitted, a simple valve system can redirect bathwater to an outdoor storage tank for later use in your garden.'