Harvesting Rainwater for the Garden

Why Rainwater?

Mains water in the UK and many developed countries is chlorinated, fluoridated, and in hard water areas is high in calcium and magnesium. While not harmful to most plants, these additions are unnecessary and in some cases — notably for acid-loving plants like blueberries and rhododendrons — genuinely detrimental. Rainwater is naturally soft, slightly acidic, and free of chlorine. Plants, given the choice, prefer it.

Beyond water quality, collection reduces the environmental impact of irrigation: mains water requires energy to pump and treat. Harvested rainwater is free at the point of collection and uses no grid energy.

Basic Water Butt Collection

The simplest rainwater harvesting system is a water butt connected to a downpipe from a roof:

1. Install a downpipe diverter kit (widely available from garden centres). These automatically divert flow to the butt when it is not full and return it to the drains when full.
2. Raise the water butt on blocks to allow a watering can to fit beneath the tap.
3. Cover the top with a lid or fine mesh — to prevent evaporation, exclude debris, and deny mosquitoes a breeding site.
4. A standard 200-litre water butt will fill rapidly in moderate rain. Linking two or three butts in series greatly increases capacity.

How Much Water Does a Roof Collect?

A 1m² of roof collects approximately 0.9 litres per mm of rainfall. A typical house roof of 100m² receiving the UK average rainfall of 700mm per year could theoretically collect 63,000 litres — far more than most gardens need for irrigation.

Larger-Scale Storage

For serious kitchen gardens or allotments, a 1,000-litre IBC tank (intermediate bulk container) — available cheaply second-hand from agricultural suppliers — provides genuine storage capacity. These can be plumbed with a simple gravity-fed drip irrigation system.

Water Hygiene

Standing water in open butts can become anaerobic and smelly, particularly in warm weather. To prevent this:

• Keep the butt covered
• Clean it out annually
• Add a small amount of diluted hydrogen peroxide if water becomes stagnant
• Use water regularly, avoiding long periods of stagnation

Note: collected rainwater should not be used for drinking or cooking. For irrigation of vegetables, it is safe and beneficial.