Water: collecting, storing and re-using
Water is a limited and valuable resource, but is also essential for plant growth. To make the most of water, collect what you can rather than use mains (tap) water. Store and use it safely.
Timing: All year round
Difficulty: Moderately easy
Why collect and store water?
Plants need the most water in hot, dry and windy weather, which is usually when water companies are least able to meet demand. Plants are most vulnerable to shortages of water when they are first planted when their roots have not yet established into the deeper, moister layers of soil. Newly planted shrubs, trees, herbaceous perennials, seedlings, annuals and vegetables will all need watering to be managed carefully until their roots establish. Once plants have established, keeping your soil healthy by adding organic matter should help to minimise the need for watering.
Water resources in the UK are under pressure from climate change and population growth. Gardeners should therefore use mains water as sparingly as they can.
Even in dry districts, 24,000 litres (5280 gallons or 150 water butts) could be collected from the roof each year. However, most water falls in winter, and would have to be stored for use in summer. During a dry spell, at first the shortfall is met from soil reserves, and then from deeper ground water as the soil can act as a wick to draw water upwards.
Our mains tap water in the UK is of the highest quality, but plants don’t need perfectly clean water. As gardeners, we can help to avoid hosepipe bans in the future by using our water resources wisely. A significant amount of energy and treatment are used to provide safe water to our homes so using stored rainwater or grey water in your garden also lowers your carbon emissions.
Rainwater is also better for your plants as it often has a lower pH. The minerals that are sometimes found in mains water, especially in hard water areas can raise the pH of your root zone, which can affect the nutrient availability. Rainwater is especially good for ericacious plants such as azaleas.
Rainwater can be collected from the roofs of homes, garages, greenhouses and other garden structures as long as they have gutters and a down pipe that enters the drain at ground level. If all of your rainwater goes to a soakaway rather than the foul sewer you may be entitled to a discount from your water company. If you don’t have room for a water butt, you can still increase the water holding capacity of your soil by adding organic matter either as a top dressing, mulch or digging it in.
Water butts are not the only way you can collect the rain. You can get an estimate of how much mains water could be saved in a typical garden by following this link and pledging to make the switch from mains to rains.
Water butts with rain water diverters are designed to collect water from the down pipe and still let the overflow enter the drain or soakaway.
You can avoid the water becoming smelly or carrying diseases if you clean water butts annually. Collected water can be used on established plants rather than seedlings, because of the risk of fungal plant diseases. If multiple butts are used, rotate the use of each one to keep stores fresh. They can be joined together by either a water butt connector kit or a siphon.
Local councils and DIY stores are good places to purchase basic plastic water butts. It is easier to access the water if the butt has a tap at the base and sits on a stand, so that the watering can will stand on the ground under the tap. Stands can be either ready-made or improvised with a pile of bricks, but it’s important to make the base firm and level in case it topples, and use the lid supplied to stop wildlife falling in, prevent algal growth and discourage mosquitoes. More expensive butts moulded to look like beehives or terracotta urns are an attractive option, as are recycled wooden barrels. Slimline models are available for narrow spaces.
Climatic change projections suggest an increasing proportion of rain will fall in winter, so it may become cost effective to build in rainwater storage when constructing new homes. This usually involves sinking a large tank somewhere in the garden, pumping water out for use in the garden or for domestic tasks such as flushing toilets.
The UK Rainwater Management Association can advise on purchase and installation of rainwater harvesting systems.
Re-using grey water
Domestic wastewater (known as ‘grey water’) may also be used in the garden. This may be from the kitchen, the washing machine or baths, basins and showers. 'Black water' from WCs should always be consigned to the sewerage system and never used in the garden. Water from septic tanks is best not used either.
Household soaps and detergents are harmless to plants, but water containing bleaches, disinfectants, dishwasher salt and stronger cleaning products should not be used, as they can harm plants and even damage soil structure if used long-term on soil.
It is prudent to alternate containers used for wastewater and mains or rainwater, to prevent build-up of potentially harmful residues and bacteria. It is also sensible to avoid using grey water on salads and other produce to be used without cooking.
Grey water should be used as it is produced and storage avoided. If left potentially harmful organisms might multiply and it will certainly smell most unsavoury.
If plants are watered entirely with grey water during a very long dry spell, there is a danger that dissolved substances will build up in the soil, which can stop plants growing well. These will drain from the soil once there is a rain shower or if you alternate watering with stored rain water.
Minimising health risks from stored water
- Site the rainwater butt in the shade if you can, as the water will be kept cooler, reducing the growth of bacteria
- Keep the guttering clear of debris as the organic material can encourage the growth of bacteria in the collected water
- Use collected water in a watering can or a coarse spray setting on a hose; a fine spray setting will generate fine aerosol particles which can be carried on the wind and breathed into the gardener's lungs and cause infection. Using a pump to transfer water encourages aerosols which are best avoided
- As with all gardening activities, good hand hygiene will help protect you from infection
- Use the water frequently, as replenishing the water with freshly collected rainwater helps to dilute the stored water and keep the water clean
- If the water becomes smelly, you can still use it in a watering can. There may be a build-up of debris collected in the bottom of the butt. Emptying and cleaning it out, and checking if gutters need clearing is a better solution than using chemical additives as the chemical will be lost once the water is used but the cause of the problem will still be there
- If you use hoses for any source of water, disconnect them when you are not using them so that the water will drain out and reduce the risk of the water warming up in the sun which encourages bacteria to grow
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.