How to Harvest Rainwater in Your Vegetable Garden

No matter where you live your kitchen gardening efforts are subject to the vagaries of weather. The clued-up grower has a number of means to smooth the peaks and troughs of rainfall, though sometimes even the most prepared of us can find the unpredictable nature of the elements a serious challenge. Here in the UK the southern half of the country has experienced two consecutive winters of abnormally low rainfall. The result was the official declaration of drought in many counties – swiftly followed by the wettest April on record!

Of course, for many of our North American, Australian and South African readers being cautious with water comes as second nature. Hot summers and long periods of raincloud-denuded skies have taught these gardeners to treat water as a precious resource. It’s only when something taken for granted becomes in short supply that you really begin to appreciate its value and the penny’s beginning to drop for us Brits.

Free from the Sky

Fresh, treated mains water isn’t all that great for the environment – or the garden. For a start it has to be extracted from somewhere. Then it has to be purified then stored, before finally being pumped into our homes. This requires (a) a lot of energy and (b) an inordinate volume of chemicals to treat the water. Extracting and storing this water often disrupts natural ecosystems and/or lowers water tables, which in turn can threaten fertile farmland. And when it does finally get to us, our fruits and vegetables don’t much care for it, preferring instead the natural balance and purity of highly oxygenated rainwater.

Rainwater isn’t metered, it’s free from restrictions placed on mains water and, most crucially, our plants love it! It makes sense, therefore, to collect some of this natural resource for use on our crops. Even in areas of low annual rainfall it’s surprising just how much water it is possible to collect from a roof surface, particularly an expansive one such as the roof that covers your home.

Working out this potential requires a simple calculation. If you are metrically minded, start by multiplying the area of your roof in metres squared by the average annual rainfall for your area in millimetres. Multiply this by 0.75 to account for evaporation and rain bouncing off the roof and you have the number of litres per year you could be collecting. For imperial units, multiply your roof area in square feet by the annual rainfall total in inches. Now divide by 12 to convert to cubic feet and, as above, multiply by 0.75 to account for water loss. To convert cubic feet to US gallons multiply by 7.5 or for imperial gallons by 6.2.

No Ifs, Plenty of Butts!

The figure that pops out from your calculations will be a large one! The question now is how to catch and store it. For gardeners the best method is, of course, the water butt. These can be fitted to intercept a downpipe coming off a roof. A rainwater diverter will do as it says – divert the rain running down the pipe to your water butt. But don’t stop at one water butt; if you have the space link two or more together so that as one reaches capacity the water overflows to begin filling the next. Modern water butts don’t have to look utilitarian. There are plenty of eye-catching butts that are features in their own right (and you don’t read that in a gardening blog very often!).

Every roof surface is a potential source of water. If you have a greenhouse or shed secure guttering to roof edges and have this feed directly into a butt. Open-ended barrels and containers can be used in this instance but shade the water by covering it with fine-gauge shade netting. This will keep the water cool, reduce contamination, stop insects from breeding in it and prevent it becoming clogged with green algae. Covered barrels are essential for safety where younger children live or visit.

You can be imaginative with water what containers you use to collect rainwater – beer barrels given a second life to make an attractive showpiece, old bulk containers begged free from caterers and restaurants, or metal drums acquired from factories and recycling centres. Just about any water-tight container will do, so long as you can cover the open end to keep the water inside clean. Buy a water butt tap kit to fit at the base of the container for drawing off the collected water.

Using Rainwater

Whenever possible locate water butts and other rainwater storage containers uphill from the growing area. This way gravity will do the work of distributing it through any irrigation system you decide to connect to it. By the same token raise butts up on purpose-made bases, breeze blocks or slabs to allow easy access to the tap at its base. Make sure the base is level and supportive as the container will get very heavy indeed when full.

While collected rainwater is a lot better for plants than mains water, there are very minor risks to human health. Bird contamination from gutters, along with other detritus will find its way into your store of water. This needn’t be a problem so long as caution is exercised when eating salads and other crops recently watered with collected rainwater. Use rainwater to water the soil, not the plant and take care to thoroughly wash your produce after harvest.

Going Further with Collected Rainwater

Heavy water users can exhaust a series of water butts within a matter of days, particularly during hot weather. There are two options to further bulk up the amount of water you have available. The first is to install ever-bigger storage containers. Thousand gallon plus tanks can be fitted under a driveway, out of sight but offering an almost limitless supply of water, while evaporating away the worry of your stash of rainwater running dry.

The other solution is to use a proportion of household ‘grey’ water – that is waste water collected from baths, showers and kitchen sinks. This is well worth exploring and further information on this can be found in our article on Using Greywater. You can also reduce the amount of water your plants require by adopting Dryland Gardening techniques.

Whatever water you manage to save or reuse will equate to fewer gallons taken directly from our mains supply. Growing food should help to reduce our environmental impact and harvesting your own water will bring you another step closer to truly growing in harmony with nature.

By Benedict Vanheems. Photo of at top of page credit: Lee Jordan