The Beginner’s Guide to Rainwater Harvesting for Gardens
Why should we collect rainwater?
Rainwater is an amazing resource and is the secret of all life on land.
Plants need water year-round but garden plants not adapted to dry conditions will need most in hot weather, after dry spells, and in windy weather (which many do not realise, particularly after laying a new lawn).
Additionally plants will need the most water in a garden just after they have been planted - as their roots haven’t grown through the soil to seek water for themselves in deeper and moister soil below the planted level. Our guide to how and when to water a new garden offers more detail on this.
Plants grow better with rainwater as well - especially ericaceous plants (that prefer acidic soil) as rainwater has a lower pH than mains water and does not contain the soluble minerals that affect the water’s pH levels and can therefore affect nutrient take up by the roots of some plants.
Do we really need to collect water?
A combination of climate change causing changing weather patterns and population growth is leading in many places to a lack of water, especially at certain times of the year. Mains water therefore needs to be used with more discretion than ever before and that is why harvesting the rainwater that would otherwise be lost from the roofs of our homes is crucial. Indeed it has been estimated that at least 24,000 litres of rainwater a year is a conservative estimate of the water that could be harvested from a house roof in the UK.
It is certainly the case that potable water (water treated for drinking by humans) uses significant energy and resources to produce and to simply then spray this onto the garden is not a sustainable approach even in the short term.
How can we collect rainwater?
On a smaller scale water butts attached to the downpipes from the house, shed or greenhouse can collect water for later spreading via watering can. This is the most common form of rainwater harvesting and is easy to install even for a DIY system.
On a larger scale however rainwater harvesting tanks - with pumps and electronic irrigation systems (or just outdoor taps) - can be installed, but installing them underground involves the disruption of a large hole needing to be dug as tanks can range in size from a small 1,500 litre capacity to over 15,000 litres (and these large tanks can be 3.00m deep and 2.60m in diameter!).
How can we use the rainwater we collect?
Collected rainwater can be used on newly planted and established plants growing in your garden. However for the keen gardener raising seedlings in the greenhouse, rainwater stored in butts will not be used for their seedlings. This is because water butts can risk the spread of fungal plant diseases (often from rotting leaves that make their way in via the gutters). It is therefore essential to clean out the water butt annually as part of your garden care routine.
To minimise the chance of water butts causing issues though they should be sited in the shade (so they don’t warm up in sunlight and cause bacteria to breed at accelerated rates) and water from them should not be spread as a fine mist but instead via a watering can or hose on a coarse setting (as almost all domestic gardeners will anyway).
With under-ground tank systems, the water in these is cold and away from sunlight so they are much less likely to develop problems. They will need filters to keep out debris and a pump to bring the water to the surface though as a minimum. It may then be as simple as attaching a tap and running a hose with a sprinkler attached from the tap, or more complex systems with an electronic timer may be considered which will run multiple watering zones of pop up sprinklers and drip hoses - allowing the right amount of water to be applied to each bed or specimen plant.
Using grey water or not?
Waste water from our homes is known as grey water - this is the waste water from our sinks, baths, showers and washing machines (but never, ever water from a toilet or septic tank).
Grey water can be used in the garden in dry spells to keep plants alive - although it should never be used for edible crops. Indeed the gradual build up in the soil of chemicals from using grey water (various soaps, detergents and bleaches for example) will impede plant growth and root development (at least until the rain washes the chemicals away from the plants immediate environment). In particular though it is important to remember that grey water should be used as it is produced and never stored for a later date - otherwise it starts to smell and can develop harmful organisms. Personally I see grey water as simply a way to keep most of your plants alive in a drought and not a solution to keeping your garden watered. Indeed it is far better to properly plan your garden so that the plants within it are adapted to the levels of water that can be expected throughout the year - as this saves the need to water in all but the most extreme of conditions.