Rainwater Harvesting for Small Gardens: A Simple Setup Guide

The first time you carry eight watering cans from the kitchen tap to the pots by the back step, you start to think about rain differently. A roof is a catchment. A downpipe is a tap that only runs when the weather decides, and a couple of hundred litres of soft, chlorine-free water can be the difference between a container garden that limps through August and one that keeps going. You do not need a buried cistern or a plumber. A barrel, a diverter, a level patch of ground and one careful afternoon will do.

What a small rain barrel actually does for you

Rain barrels are often sold as a grand gesture. In a small garden they are better understood as a modest one: a buffer that carries you across a dry week, not a reservoir that gets you through a drought. The arithmetic is simple and worth doing. One millimetre of rain falling on one square metre of roof gives you roughly a litre. A shed with a twelve square metre footprint, then, sheds about 120 litres in a 10 mm shower — enough to fill a small barrel in a single wet morning.

Sizing follows from that. A 200-litre barrel will fill in one decent rain event and send the rest down the overflow, which is why the overflow matters as much as the barrel. What you get in exchange is water that is soft, free of the chlorine and lime in most tap supplies, and usually at ambient temperature rather than cold from the mains. Container plants respond to that. So does a metered water bill.

Choosing a barrel that fits your garden

Most courtyard, balcony and terrace gardens are well served by one or two barrels of 100 to 200 litres. Two small barrels usually beat one large one: they are easier to move, easier to tuck behind a shed, and when one runs dry you still have a reserve. Measure the gap before you order anything, then check the details that decide whether a barrel is pleasant or annoying to live with:

• Capacity and footprint. A 200-litre barrel is typically about 60 cm across and close to a metre tall. Account for the stand as well as the barrel.
• Lid and inlet. A wide, removable lid lets you scoop out leaves and silt. A small inlet is fine if the diverter hose seals into it.
• Tap position and thread. Look for a threaded outlet near the base, usually ¾ inch, and confirm you can buy a replacement tap or adapter.
• Material and colour. Dark, thick-walled high-density polyethylene keeps light out, which slows algae. Wooden barrels look lovely but need a liner and more upkeep.
• Base shape. A flat bottom sits securely on a stand. Rounded or tapered bases need a shaped cradle.
• Frost behaviour. Rigid barrels tolerate cold better than thin collapsible ones, which should be emptied and stored before hard frost.

Siting it safely

Weight comes first

A full 200-litre barrel weighs around 200 kg before you count the barrel itself. That rules out decking, loose gravel and any patch that turns soft after rain. A level concrete slab, a paved corner or a firm patio is right. If the only flat spot is next to the house, keep a gap of at least 30 cm so a leak or overflow does not soak the wall.

Build a proper stand

Raising the barrel 30 to 40 cm on a sturdy stand buys you two things: enough head to fill a watering can or run a short hose from the tap, and a tap you can reach without kneeling. Use pressure-treated bearers or a purpose-made stand wider than the barrel base, and check it is level in both directions. A barrel that rocks will eventually split at the seam.

Plan the overflow before the first storm

Every barrel overflows, usually at the worst moment. Fit an overflow pipe near the top and lead it to a border, a soakaway or a drain — never against the house or onto a neighbour's path. A short length of hose angled into a thirsty shrub is the simplest solution, and it turns a nuisance into a small irrigation line.

Fitting a downpipe diverter, step by step

A diverter sends rainwater into the barrel and, once the barrel is full, lets the surplus continue down the drainpipe as normal. It also keeps leaves and grit out of your stored water. If you rent, live in a listed building or share a downpipe with a neighbour, check the position before you cut anything; rules on downpipe diversion vary, so a quick word with your landlord or local authority office settles it.

1. Measure the downpipe diameter and buy a diverter to match. Note the height of the barrel inlet so the diverter sits comfortably above it.
2. Stand the barrel on its finished stand in its final position and mark the inlet height on the wall. A dry run now saves a second cut later.
3. Mark the section of downpipe to remove, following the diverter's template, and cut it with a fine-toothed saw. Deburr the edges.
4. Clip the diverter body into place and refit the pipe above and below it. Check it is vertical and that the outlet faces the barrel.
5. Connect the diverter outlet to the barrel inlet with a short length of flexible hose, falling gently towards the barrel with no sagging loops where water can sit.
6. Fit the tap with a few turns of thread tape, hand-tighten, then a quarter turn with a spanner. Fit the lid and a fine mesh screen over any open inlet.
7. Test with a hose run for a minute. Watch for leaks at the tap and check that the overflow carries water away from the wall.

Tip: label the barrel clearly as non-drinking water, especially if children or guests use the garden.

Keeping the water clean and the setup working

Stored rainwater does not stay fresh indefinitely. Light and warmth grow algae; standing water and an open lid invite mosquitoes. A sealed, dark barrel with a fine mesh screen solves most of it. Empty and rinse the barrel once a season, and let the first few minutes of a heavy shower run to waste before you connect the diverter, since that is when most roof grit washes down.

In autumn, clear leaves from the diverter and inlet every couple of weeks. Before hard frost, disconnect the hose, drain the barrel to about a quarter full or empty it completely depending on the manufacturer's advice, and leave the tap open so ice has somewhere to expand. Water sitting in a hose over winter is what splits fittings.

Getting the water onto plants

Containers

For pots, a watering can filled from the tap is quicker and gentler than a hose, and a raised barrel makes filling easy. Water in the cool of the evening or early morning, and aim for the soil rather than the foliage. Rainwater's lack of lime makes it a good match for acid lovers such as blueberries, camellias and rhododendrons, which often struggle with hard tap water.

Borders and beds

On a border, use the barrel's tap with a short hose for a slow soak around the roots of new plantings, or fill a can and target the plants that wilt first. Freshly harvested rainwater is excellent for seedlings and newly transplanted shrubs, which appreciate the softness and the lack of chlorine.

What to avoid

Do not drink stored rainwater or use it for washing produce you will eat raw. If water has sat for weeks and smells sour, keep it for ornamentals and paths rather than for salad crops or seedlings. And if your roof has old lead flashing or deteriorated coatings, use the water on flowers and lawns rather than on vegetables.

Start with one barrel, then refine

Begin with a single barrel on a firm, level stand, connected properly and with an overflow that leads somewhere sensible. Live with it for a season. You will quickly learn how fast it fills, how far it goes in a dry spell, and whether a second barrel would earn its space. Most small gardens end up with two: one by the shed, one tucked behind a border, both quietly doing the work the weather provides. That is the whole trick — a small catchment, well placed, used with a little care.

Photo: Efrem Efre / Pexels