Collecting and Using Rainwater in the Garden

Watering & Weather

Jul 22

Collecting rainwater can reduce demand on mains water, wells and other local supplies. It also provides a useful reserve for dry periods and slows the amount of stormwater leaving roofs during heavy rain.

The value of a collection system depends on local rainfall, roof area, storage capacity and the length of the dry season. A small water butt can be useful, but it should not be expected to supply a large garden through a prolonged drought.

Understand How Much Water You Could Collect

One millimetre of rain falling on one square metre of roof produces approximately one litre of water before losses.

For example, 10 millimetres of rain falling on a 50-square-metre roof could provide up to approximately 500 litres. The amount actually collected will be lower because of splashing, evaporation, leaks and water retained on the roof or in the gutters.

This calculation helps you compare the potential supply with your available storage. It may also show that a small container will fill rapidly during a storm but empty quickly in dry weather.

Consider Your Local Rainfall Pattern

Annual rainfall totals do not tell the whole story.

A region may receive a reasonable amount of rain over the year but experience long, dry summers when garden demand is highest. Other areas receive regular rainfall throughout the growing season and need less storage.

Look at when rain normally falls, how long dry periods last and whether rainfall tends to arrive as gentle showers or intense storms.

Local averages are useful for planning, but weather varies from year to year. Build a system that is practical for your garden rather than relying on a perfect seasonal pattern.

Choose a Suitable Collection Surface

Roofs are the most common collection surfaces because gutters can direct a large volume of water into storage.

The roof and guttering should be made from materials suitable for the intended use. Water can pick up dirt, bird droppings, moss, chemicals and residues as it moves across the surface.

Avoid collecting water from roofs treated with unsuitable chemicals or made from materials that could contaminate the supply. If you are uncertain about a roofing material, seek guidance from the manufacturer or an appropriate local authority.

Greenhouse and shed roofs can also provide useful collection areas, provided the materials and gutters are clean and suitable.

Keep Gutters and Filters Clear

Leaves, moss and other debris can block gutters, downpipes and filters. Inspect and clean them regularly, particularly after storms or autumn leaf fall.

Fit a simple screen or filter to prevent larger debris entering the storage container. The design should allow water to continue flowing safely if the filter becomes partially blocked.

A first-flush diverter can discard the initial water from a rainfall event, which is likely to carry the greatest amount of dust and contamination from the roof.

Filters and diverters work only when maintained, so include them in regular garden checks.

Select Safe Storage

Water butts are suitable for smaller gardens and greenhouse roofs. Larger tanks provide greater resilience where there is enough collection area and rainfall to fill them.

Use a container designed for water storage or one whose previous contents are known to have been safe. Do not repurpose a tank that previously held fuel, pesticides, industrial chemicals or unidentified substances.

Large volumes of water are extremely heavy. Place every container on a firm, level base capable of supporting its full weight.

Secure storage against accidental movement or tipping. Covers should prevent access by children and animals while keeping out leaves and other debris.

Keep the Storage Covered

A close-fitting cover reduces contamination, limits algae growth and prevents animals from falling into the water.

Mosquitoes can breed in standing water. Screen every opening, inlet and overflow with fine mesh where mosquitoes are present.

Inspect the mesh regularly for holes and make sure the lid remains secure after filling, cleaning or maintenance.

An opaque container also limits light and reduces algae growth. If the tank is translucent, placing it in shade or using a suitable external covering can help.

Provide a Safe Overflow

Every storage container needs an overflow for times when rainfall exceeds its capacity.

Direct overflow away from buildings, paths and neighbouring property. It should not create erosion, waterlogging or an unsafe slippery area.

Where conditions allow, overflow can be directed towards a rain garden, swale or planted area designed to accept occasional additional water. The receiving soil must be able to absorb it safely.

Check local rules before altering drainage or directing large volumes of water across land. Requirements vary between countries and local authorities.

Link Water Butts Carefully

Several water butts can be connected to increase capacity. They may be linked at the top so each fills in sequence or lower down so their water levels rise together.

All containers must stand securely on level bases. Use fittings designed for the containers and check regularly for leaks.

Make sure the overflow remains large enough to cope with heavy rain after additional storage is connected.

More containers provide useful capacity only if the roof and local rainfall can refill them.

Use Rainwater Where It Is Most Valuable

During dry weather, prioritise seedlings, recent transplants, containers, greenhouse plants and crops at important flowering or fruiting stages.

Apply the water directly to the soil around the roots. This reduces waste and limits contact between stored water and edible leaves or fruit.

Drip irrigation, soaker hoses and watering cans can all distribute collected water effectively. Check that pumps, filters and outlets are working evenly.

Stored rainwater should not be treated as drinking water unless it has been collected, treated and tested through a system specifically designed to make it potable.

Use Stored Water Safely on Food Crops

Rainwater collected from roofs is not sterile. It may contain microorganisms and contaminants from birds, animals, roofing materials or dirty gutters.

Apply it to the soil rather than spraying it over edible leaves, especially crops eaten raw. Avoid contaminating the harvested parts of the plant and wash produce appropriately before eating it.

Use particular care close to harvest and with low-growing salad crops. If the collection surface or stored water may be contaminated, do not use it on edible crops.

Local public-health or water authorities may provide guidance suited to your area and intended use.

Prevent Stagnation and Odours

Stored rainwater may develop an unpleasant smell if leaves, grass or other organic material enter and decompose.

Keep gutters, filters and containers clean. Remove accumulated sediment when necessary, following the container manufacturer’s instructions.

Do not add garden chemicals in an attempt to keep untreated irrigation water fresh. Preventing light, heat and debris from entering is generally more appropriate.

If the water appears heavily contaminated, smells unusually foul or contains dead animals, do not use it on edible crops.

Prepare for Winter

In cold climates, freezing water can damage taps, pipes, pumps and containers.

Follow the manufacturer’s winter instructions. Some systems must be drained or disconnected, while others are designed to retain water during freezing conditions.

Make sure downpipes can carry water safely after a collection system is disconnected. Do not leave water trapped in exposed hoses or fittings that could split when frozen.

In mild climates, winter may be the main period for refilling storage before a dry growing season. Make sure the system is clean and ready before seasonal rain arrives.

Use Pumps Safely

Larger tanks may require a pump to move water through a hose or irrigation system.

Choose equipment designed for the intended use and protect electrical connections from water. Follow the manufacturer’s installation and maintenance instructions.

Never enter a water tank. Confined tanks can be dangerous and should be inspected or cleaned only using safe methods or appropriate professional help.

Check hoses and irrigation outlets for leaks so collected water is not lost before it reaches the plants.

Combine Collection With Water Conservation

Rainwater harvesting works best alongside healthy soil and efficient watering.

Add mature compost, keep the soil appropriately mulched, control competing weeds and deliver water slowly to the root zone. Shade and wind protection can also reduce demand during extreme weather.

Do not use collected water simply because it is available. Check whether the soil genuinely needs it.

Groundwater from a private well is also a shared and limited natural resource. Follow local rules and conservation guidance, particularly during drought or when groundwater levels are falling.

Monitor the Weather and Your Supply

Check how much water remains before hot or dry conditions arrive. A simple gauge or visible level indicator helps you plan which crops can be supported with the available reserve.

Your Growing Companion provides location-aware weather information that can help you anticipate rainfall and dry periods. Forecast rain is never guaranteed, so combine it with observations and realistic storage planning.

Keep a simple note of how quickly your storage fills and how long it lasts. This will show whether additional capacity would genuinely help or whether reducing garden demand would be more effective.

Rainwater collection cannot remove every water shortage, but a clean, secure and well-planned system can make the garden less dependent on other supplies and ensure that rainfall is used more carefully.