Bathwater is to become the new norm for the garden, as people look for more sustainable ways to tend to their lawns and plants.

The Royal Horticultural Society (RHS) predicts greywater – recycled from household sinks, baths and washing – will be a go-to source of water for gardeners by 2035.

While the UK charity has historically encouraged a switch from mains to rainwater, a growing shortfall of the resource means is also looking at how used household water can be recycled for use in gardens when required.

This is because rainwater stores are more easily exhausted during drier, summer months, and water butt capacity may not always be sufficient to meet demand.

Dr Nicholas Cryer, senior water scientist at the RHS, says, “The greenest approach to watering your garden is to minimise its use entirely through clever planting and good soil care, with rainwater harvesting the next best thing.

“But with summers predicted to become hotter and drier and the need to remedy a growing water deficit, we need to be more creative in how we maintain our green spaces. Single use products are now rightly frowned upon and we should consider water the same way.

"The estimated 60 litres of greywater produced per person within our homes each day can be recycled in our gardens.”

The RHS is already investigating the role greywater will play in maintaining its own five gardens, and the estimated 23 million domestic gardens in the UK, by conducting research into which plants and substrates are most effective in water cleaning. One of the advantages of using greywater is that it is generated each day, so large storage tanks or reservoirs are not needed.

Its research also involves removal of potential pollutants and making the movement of water from house to garden more practical. Rain gardens are also being created across RHS sites as a way to hold water for longer, manage rainwater run-off, minimise flood risk, and act as sponges to retain water for use during dry spells. Rain gardens such as the Dry Garden at RHS Garden Hyde Hall require no watering once established.

Fundamental to all rain gardens is healthy, deeply aerated soil which readily absorbs water, and then balances storage and deep drainage of that water, says the RHS.

The microbes found in healthy soils improve the filtering capacity of soils by digesting organic matter from any greywater applied to it such as detergents, skin and sweat, and enhancing soil structure.

Gardens showcasing clever rainwater management feature heavily at the 2024 RHS Chelsea Flower Show, from 21-25 May, including in the Water Aid Garden which contains a rainwater harvesting pavilion designed to slow flow, collecting and storing it for irrigation and filtering it for use as drinking water.

In the National Autistic Society Garden, rainwater is channeled away from the main terrace via a 'waterfall roof' feeding into a mossy dell which acts as a swale during periods of high rainfall, holding rainwater until it can drain away into the subsoil.

Grabbing greywater for garden? The RHS advises:

Plants can be watered with shower, bath, kitchen and washing machine water (from rinse cycles).

Greywater can be used in the short term on ornamentals but not edible crops.

To minimise bacterial growth, greywater from sinks and baths should be stored for no more than 24 hours.

Greywater should be applied by watering can as pollutants in the water can clog irrigation systems.