Building a Rainwater System for Herb Pots

Why rainwater is worth the effort for herbs

Herbs in pots lead a tough life. Their roots are confined, their compost dries out far faster than garden soil, and the water you give them is often the one thing you can't control. Tap water in much of the UK is hard, alkaline and dosed with chlorine and fluoride. Mediterranean herbs such as rosemary, thyme, oregano and sage evolved on thin, free-draining, slightly acidic ground, and they never see a chlorinated supply. Rainwater is soft, close to neutral or gently acidic, free of the additives, and — crucially in summer — already at air temperature. Cold mains water in July gives roots a shock that shows up as sulking growth and yellowing lower leaves.

There's a practical angle too. If you're on a meter, every can you draw from the butt is a can you aren't paying for, and a single downpipe can collect a surprising amount. One millimetre of rain falling on one square metre of roof yields about a litre. A modest shed roof of six square metres will hand you 60 litres from a ten-minute shower.

Choosing a butt and siting it sensibly

For a collection of herb pots, 100 to 200 litres is the sweet spot. Anything smaller empties in a dry fortnight; anything larger becomes a job to move and a heavy thing to site. A 200-litre butt full of water weighs over 200 kilograms, so it needs a firm, level base — paving slabs, a small concrete pad or a purpose-made stand. Never balance one on soft ground or a wobbly deck.

Position matters as much as size. Put the butt as close to the downpipe as you can, and as close to the pots as you can manage. If you have a choice of downpipes, pick the one nearest the herb corner rather than the one that collects the most water; carrying ten litres across the garden twice a day is the quickest way to abandon the whole idea. Allow 30 to 40 centimetres of clearance beneath the tap so a watering can fits comfortably, and keep the lid on — it keeps out leaves, discourages mosquitoes and stops children and pets treating it as a plaything.

Connecting to the downpipe

You don't need to reroute the downpipe, and you shouldn't cut it off entirely, or heavy rain will simply flood the wall. A diverter kit does the job neatly. The principle is straightforward: a section of the downpipe is removed, the diverter is fitted in its place, and a flexible pipe carries water through a hole in the butt. Once the butt is full, the water backs up and carries on down the downpipe as normal.

• Measure twice before cutting the downpipe, and check the diverter sits comfortably above the top of the butt.
• Drill the inlet hole near the top of the butt with the size of hole saw the kit specifies — usually 25 to 32 millimetres.
• Fit a gutter guard or leaf trap above the diverter, or you'll spend autumn scooping debris out of the water.
• Add an overflow pipe near the top and lead it to a drain, a soakaway or a thirsty border — never let it run down the house wall.
• Check whether your local authority has any rules about rainwater harvesting and where overflow may discharge.

Fitting the tap and running a short hose

Fit the tap as low as you can while still leaving room for a can underneath. A standard ½-inch tap with a bulkhead fitting is ideal: a rubber washer inside the butt, another outside, a firm hand-tighten and a quarter turn more. Wrap a little PTFE tape around the thread to stop drips. If the butt has a threaded boss ready-made, so much the better.

From the tap, run two to four metres of hose with a snap-on connector. Keep it short. Gravity gives you very little pressure, so a long hose flops about and delivers a dribble. A gentle rose or a short soaker line laid across the pots works well; a pressure-hungry spray gun won't. If you'd rather not bother, a nine-litre can filled at the tap is perfectly good technology and far less to go wrong.

Using stored water well in dry weeks

The skill is watering deeply and less often, so roots travel down rather than sitting in a shallow, damp crust. Push a finger two centimetres into the compost before you water; if it's cool and moist, wait. Water in the early morning or evening, when evaporation is lowest.

Be realistic about how far a butt will stretch. A 15-litre rosemary in July may want a litre or more a day, so a 200-litre butt is roughly a fortnight's supply for a modest collection. A few habits make it go further:

• Group pots together so they shade one another's compost.
• Top-dress with gravel or bark to slow surface evaporation.
• Line terracotta pots with a plastic sleeve, or choose glazed containers — unlined terracotta wicks water away alarmingly fast.
• Prioritise the thirsty herbs. Basil, mint, parsley and chives drink freely; thyme, sage and rosemary would rather be a little dry than waterlogged.
• Top up with tap water when the butt runs low, but let it stand overnight in a can so the chlorine disperses.

Keeping the system clean through the year

Clear the gutters and check the diverter each autumn, and drain the butt down to a quarter before the first hard frost — a full butt that freezes can split. Every year or two, empty it, scrub the inside with warm water and a splash of white vinegar, rinse thoroughly and let it dry before refilling. Skip the bleach if the water goes anywhere near edible leaves.

Set up properly, the whole system takes an afternoon and then quietly looks after itself. From April onwards, you'll be watering your basil with soft, tepid rain while the tap stays off — which is better for the plants, better for your bill and quietly satisfying in a way that a hosepipe never quite manages.