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Recycling bathwater for watering gardens set to become mainstream

Greywater is expected to become a go-to source of water for the garden by 2035, predicts RHS Senior Water Scientist

As climate change leads to hotter, drier summers in which rainwater stores are more quickly used up, gardeners are expected to increasingly look for ways to keep their plants hydrated through the summer months while reducing their reliance on mains water.  

Many of us already turn to using some form of greywater for watering during the summer months, such as using washing up water, but recycling of household water from taps, showers and baths is expected to become mainstream practice over the next decade, according to RHS Senior Water Scientist Dr Nicholas Cryer.


A key role for greywater 

RHS Science, having grown its water research capabilities following the appointment of Dr Cryer and Water Reduction Officer Janet Manning, will explore how greywater use can be expanded to play a significant role in maintaining both its own five gardens and the estimated 23 million domestic gardens across the UK. This will be achieved by looking at how water can be practically recycled from sinks, baths and washing, and conducting research into which plants and substrates are most effective in water cleaning.

This includes researching plant and substrate combinations that support the right soil microbiology and plant functions to remove potential pollutants, and developing practical solutions for the movement of water from house to garden.

Household greywater is increasingly used for garden watering
The need to switch from mains water to rainwater remains crucial, but with Government targeting a 20% reduction in household and business water use by 2038, and a roadmap produced for water-efficient new homes that includes stepped reductions in average water use per person, demand for garden water will increasingly need to be met by other non-mains sources once rainwater butts have been exhausted. One of the advantages of using greywater is that it is generated each day, so large storage tanks or reservoirs are not needed.


Rain gardens help to manage unpredictable rainfall

Rain gardens are being created across the five RHS Gardens as a way to hold water for longer in the landscape, manage rainwater run-off to minimise flood risk, and act as sponges for water that is then stored ready for uptake 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. The microbes found in healthy soils enhance soil structure and improve the soil’s filtering capacity, such as by digesting organic matter (e.g. skin and sweat) in any greywater that may be used on the garden.
 
Gardens showcasing clever rainwater management feature heavily at the RHS Chelsea Flower Show this year, including in The WaterAid Garden, which includes a rainwater harvesting pavilion designed to slow down the flow of water, collecting and storing it for irrigation of the garden and filtering it for use as drinking water. In The National Autistic Society Garden, rainwater is channelled away from the main terrace via a ‘waterfall roof’. This feeds into a mossy dell that acts as a swale during periods of high rainfall, holding water until it can drain away into the subsoil. Rain is also channelled into areas planted with species that can cope with wet conditions.


Managing water in your own garden

RHS Senior Water Scientist Dr Nicholas Cryer 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.”


Tips for promoting healthy soil and effective greywater use

  • Your soil is your largest water storage reservoir. You can improve its structure and ability to hold water by adding organic mulch and minimising soil disturbance.
  • Healthy soil needs to be kept covered by plants that protect the soil and whose root systems improve aeration and stabilisation.
  • Greywater can be used in the short term on ornamentals but not on 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.


What is happening at RHS Gardens?

The RHS is investing in improving rainwater collection and storage to reach sustainability targets its Gardens. For example, all the rainwater that falls on RHS Hilltop at RHS Garden Wisley is now collected and directed into the purpose-built Clear Lake for storage and later use in irrigating the Garden. This initiative has already enabled Battleston Hill woodland garden to significantly reduce irrigation demand from other water sources.

Clear Lake at RHS Garden Wisley stores the rainwater collected from the roof of RHS Hilltop
Coupled with other water efficiency measures such as improved leak detection, use of precision irrigation methods and selection of plants that are ideally suited to local conditions, these water collection and storage developments will mean that across all RHS Gardens, an absolute minimum of water will need be taken from the outside environment by 2030.

The greenest approach to watering your garden is to minimise its use entirely through clever planting and good soil care

- Dr Nicholas Cryer, RHS Senior Water Scientist

 

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