Is water butt water safe to use on vegetables?

This question gets two confident and opposite answers online, and the sensible position is between them. Rainwater is better for plants than tap water in several respects, water butts are strongly worth having, and there is a genuine if modest hygiene consideration for crops that go straight into a salad bowl. All three things are true at once.

Safety

Step by step

1. Understand where the risk comes from.The rain is clean. What it picks up is not: bird and animal droppings on the roof, debris in the gutter, and whatever has been growing in the butt itself. That washes into the butt and, in warm weather, multiplies in still water. The concern is bacterial contamination transferred to the edible part of a plant, not chemical contamination.
2. Use it without concern on most of the garden.Ornamentals, lawns, hedges, shrubs, trees, fruit bushes, and any vegetable that will be cooked before eating are all fine. Cooking deals with the organisms involved. This covers the large majority of what a garden needs watering.
3. Be more careful with anything eaten raw.Salad leaves, spinach eaten raw, strawberries, radishes, herbs — anything going from the ground to the plate unheated. Use mains water on these where you can, particularly in the last weeks before harvest, and particularly if there are gulls or pigeons roosting on the roof.
4. Water the soil, not the leaves.This does most of the work on its own. Contamination reaching the root zone is filtered by the soil; contamination sprayed onto a lettuce leaf sits there until you eat it. Base watering is better practice for disease and evaporation reasons anyway.
5. Keep the butt covered and dark.A tight lid keeps out leaves, debris, animals and light. Light is what allows algae to grow, and algae plus warmth plus organic debris is what turns a butt smelly and green. A covered butt also stops mosquitoes breeding and prevents a child or an animal falling in.
6. Use it and refill it rather than storing it all summer.Fresh water cycling through is much better than a butt that sits full and warm for two months. Using the stored water regularly, rather than saving it for an emergency, keeps it in better condition.
7. Fit a first-flush diverter or clean the gutters.Most of the contamination arrives in the first minutes of rain, washing accumulated debris off the roof. A first-flush diverter sends that initial flow away. Cleaning the gutters annually achieves a good deal of the same thing for nothing.
8. Clean the butt out periodically.Empty it, scrub out the sludge in the bottom, rinse, and refill. Once a year is enough for most, and it makes a marked difference to smell and to what is in the water.
9. Wash the produce regardless.Washing vegetables before eating is sound practice whatever you watered them with, since soil itself carries organisms. It is the simple final step that makes the whole question a small one.

Tips

• Rainwater is genuinely better than tap water for ericaceous plants — blueberries, rhododendrons, camellias, azaleas — because it is not alkaline. For those it is worth the butt on its own.
• Water from a butt fed by a shed or greenhouse roof is generally cleaner than one fed by a house roof with more bird traffic.
• Never use butt water in a pressure washer, a mister or any device producing a fine spray. Aerosolised stagnant water is the legionella route, and that is the one genuinely serious risk in this area.
• A butt that has gone green and smelly is still usable on ornamentals and cooked crops; it just tells you the lid is not sealing or it needs a clean.
• Keep butts raised enough to get a watering can under the tap, and connect several together so you capture the whole roof rather than overflowing after the first hour of a storm.

Common mistakes

• Using butt water overhead on salad crops — It puts any contamination directly onto the part you eat raw. Base watering removes almost all of that concern at no cost.
• Leaving the butt open to the light — Light drives algal growth, and an open butt collects debris, drowns animals and breeds mosquitoes. A tight lid solves several problems at once.
• Using stored water in a pressure washer or mister — Fine aerosols from stagnant water are how legionella reaches the lungs. This is the one use to rule out entirely.
• Saving butt water for an emergency and never using it — Water that sits warm and still for months is in far worse condition than water that is used and replaced. Use it routinely.

Questions people ask