Two things in an ordinary back garden can grow Legionella and then spray it into the air you breathe: a hose left full in the sun, and a water butt of warm, standing rainwater. Neither is cause for alarm, and the everyday risk to a healthy adult is low. But “low” is not “nothing”, and the habits that keep it low cost nothing either.

Most of what circulates about garden Legionella is a muddle of half-truths. Here is what is myth and what is real.

Myth: rainwater in a butt is clean, so it is safe to spray

Reality: collected rainwater is untreated. A water butt is usually open or loosely lidded, sits in the warmth, and gathers leaf litter and organic matter washed off the roof — the kind of nutrients bacteria feed on. Through summer the water drifts into the roughly 20–45°C band where Legionella multiplies most readily, and it can sit there undisturbed for weeks [1]. The water is fine for what a butt is for: watering at the roots with a can. The risk only appears when that water is broken into a breathable mist — which is exactly what a pressure washer or a pump-up sprayer drawing from the butt does [4][3].

Myth: you can only catch Legionnaires’ from big systems like cooling towers

Reality: cooling towers and large hot-water systems are the classic high-risk sources, but the disease is caught the same way wherever it starts — by inhaling fine droplets of contaminated water that carry deep into the lungs [2][3]. A spray nozzle on a sun-warmed hose, or a pressure washer fed from a warm butt, produces precisely that kind of aerosol at close range. Scale, not some special property of industrial kit, is mostly what separates the garden version from the headline one.

Myth: the danger is drinking from the hose

Reality: swallowing the water is not how Legionnaires’ disease is caught — you get it by inhaling the aerosol, not by drinking [2][3]. Drinking from a hose that has stood in the sun is unpleasant for other reasons, but it is not the Legionella route. The risky moment is the spray: that first burst of warm standing water out of a trigger gun or lance, atomised into a mist right where you are standing.

Myth: this is a problem for businesses, not households

Reality: there is no household legal duty here, and the odds of harm to a healthy person from a domestic hose are genuinely low — this is good practice, not compliance [2]. The duty-holder version (employers, landlords, grounds and allotment operators) is a separate matter, covered in Outside taps, hose union taps and garden hoses: the outdoor Legionella risk. But “low risk” and “no risk” are not the same, and some of the people standing in that garden mist — older relatives, smokers, anyone with a weakened immune system or a long-term lung condition — are more susceptible than the average [2].

Why the myths stick

They stick because everyday experience seems to back them up: millions of people use hoses and water butts every summer and almost none come to harm, so “it is obviously fine” feels true. It mostly is. What the everyday picture hides is the narrow set of conditions that shift the odds — warm water, left standing, then deliberately atomised near someone vulnerable. The mildness of the usual outcome tells you nothing about the state of the water in one particular butt during one particular hot week.

The seasonal habits that keep the risk low

You do not need a risk assessment for your own garden. You need a few warm-weather habits [1][3][4]:

  • Drain the hose after use rather than leaving it full and coiled on hot ground, and store it empty and out of direct sun.
  • Run the first burst from a hose or sprayer to waste — at the ground, away from faces — before you spray anything near people.
  • Water from a butt at soil level with a can where you can, rather than pumping it out through a fine spray.
  • Keep the butt lidded to cut light, warmth and debris, and empty and clean it out from time to time.
  • Take more care on behalf of anyone vulnerable in the household: keep them clear of the spray, and do the spraying yourself.

When water in the garden warrants a doctor

The garden risk is low, but the illness is the same one, so the symptoms to watch for are the lung ones: a cough, breathlessness, fever and aching muscles developing within roughly two to ten days, sometimes with confusion [2]. If a chest infection like that takes hold and is getting worse rather than better, contact NHS 111 or your GP, and mention any spraying of stored or standing water — it is the kind of detail that helps a clinician [2]. Call 999 for severe breathing difficulty.

This is general guidance for a domestic setting, not medical or legal advice. If you store and spray water at any scale beyond a back garden, or you let a property to tenants, the picture moves toward the managed-risk advice in the companion pieces rather than the household habits above.

What to do this weekend

Walk the garden and look at two things: where the hose lives between uses, and what state the water butt is in. If the hose is lying full in the sun, drain it and rehome it in the shade; if the butt is warm, open and full of debris, lid it and give it a clean. Then make “first spray to the ground, away from faces” the household habit for the rest of the summer. That is the whole job. For the deeper outdoor and stored-water detail a managed site needs, Rainwater harvesting and greywater systems: Legionella risk and control goes further than a back garden ever has to.

Sources

[1] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [2] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [3] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm