Yes - farms, nurseries, market gardens and garden centres carry a Legionella duty, even though the subject usually gets written up as if it only touches office blocks and hospitals. The risk is not your crops or your livestock. It is the people who breathe in water that some piece of kit has turned into a fine spray.

Legionnaires’ disease is a lung infection you catch by inhaling small water droplets carrying the bacteria, which is why systems that produce a spray or aerosol are the ones that matter [1]. So on a growing or horticulture-retail site the question is narrow and practical: which bits of equipment take warm, possibly stagnant water and throw it into the air near a worker or a customer? Misting units over fresh produce, propagation-house foggers, welfare showers and stored irrigation water are the usual answers. Livestock drinking troughs are not.

The risk is the spray, not the soil

Anyone who controls premises has to assess and manage the risk of exposing people to Legionella from a water system - the Approved Code of Practice L8 sets that expectation [2], and the technical guidance HSG274 fills in the detail [3]. None of it concerns plant health or animal welfare; those sit under the separate rules you already meet.

That reframing tells you where to spend effort. Your “people at risk” are employees, contractors, and - at a garden centre or farm shop - members of the public, some older or with weaker chests, walking right past your displays. The job is to find where water becomes breathable mist near those people, and to keep the water behind those points clean and cool, or properly hot.

Garden centres and farm shops: the produce-misting unit

The misting bars that keep salad leaves, herbs and bedding plants looking fresh are the part of a horticulture-retail site that most closely resembles a recognised outbreak source. HSG274 Part 3 deals with exactly these “other” aerosol systems - humidifiers, water features and spray units - separately from ordinary hot and cold water [3]. A produce-misting unit is not the same thing as an HVAC humidifier hidden in the ceiling; it sits at face height and sprays a fine mist a foot or two from a shopper, which is the distinction drawn out in Misting systems and humidifiers: Legionella in unexpected places.

The hazard is the small reservoir behind the nozzles. Many display misters recirculate water from a sump that sits at room temperature - squarely in the range bacteria like - and the spray is fine enough to reach the lungs. This is the garden centre produce misting Legionella route, and it is easy to miss because the unit reads as a fridge accessory rather than plumbing. Keep the reservoir fed with fresh cold water rather than topped up and left, drain and clean it on a written schedule, and descale the nozzles. Where a unit can be plumbed to a short cold mains run instead of a standing sump, that is the better design.

Polytunnels, glasshouses and irrigation

Inside a propagation house, foggers and misting lines raise humidity by filling the air with fine droplets - and a worker pricking out seedlings breathes that air all day. Treat the polytunnel misting risk like any other indoor aerosol: know where the water is stored, keep it moving and cool, and clean the nozzles. Warm water standing in black irrigation pipe coiled in the sun is the classic amplifier.

Open-field spray irrigation is a different and generally lower human risk. Droplets thrown across a field disperse and dry in open air and sunlight, and there is rarely anyone standing in the plume for long. The horticulture water hygiene concern shifts upstream, to where the water is stored before it is pumped: header tanks, lagoons, rainwater-harvesting butts and IBCs. Stored, un-chlorinated water warming through a summer is where amplification happens - the same principle covered in Garden hoses and water butts: the home Legionella risk. Drip and trickle irrigation barely aerosolises and sits low on the list, while overhead spray near a pick-your-own row or a packing line, where people work, is where irrigation Legionella stops being purely an outdoor question.

Welfare facilities: where farm water risk really concentrates

On a working farm the highest people-risk is often the most ordinary kit: the welfare block. Showers, washbasins and mess-room taps - frequently fed from a private borehole or a storage tank, sometimes in a seasonal cabin used only at harvest - tick every box for stagnation and lukewarm water. A shower is an efficient aerosol generator, and one used twice a week by contractors is exactly the little-used outlet that needs flushing. The wash-down hoses and yard taps belong in the same assessment; their outdoor side is covered in Outside taps, hose union taps and garden hoses: the outdoor Legionella risk. Stored hot water around 60C, roughly 50C at the tap and cold below 20C are the usual targets, but your risk assessment confirms the figures for your site [4]. This is the heart of farm water Legionella control, and the part most likely to be missing on a holding that has never written anything down.

Livestock drinking water: keep it in proportion

Drinking troughs and nipple drinkers are low on the human-risk scale. Animals drink the water, they do not inhale a mist of it, and the open-air aerosol off a trough is slight. Do not pour the budget into testing trough water for the herd’s sake. What does deserve attention is shared storage: if one header tank feeds both the cattle drinkers and a welfare shower, you assess that tank because of the shower, not the trough. On mixed sites with kennels, a grooming room or animal hydrotherapy, the people-facing aerosol kit is its own subject, set out in Legionella in veterinary practices, kennels and animal facilities.

What nobody tells you about spray on a growing site

Here is the counterintuitive part. The dramatic outdoor sprinkler throwing a plume across a field is usually a smaller human risk than the modest misting cabinet humming over the bagged salad indoors. Open air dilutes and dries the droplets and sunlight degrades the bacteria; still indoor air a few feet from a customer’s face does neither, and the indoor mister is fed from a small warm sump that may never have been cleaned. The misting-linked cases described in the outbreak literature are the enclosed retail and humidifier kind, not the open field.

Two things follow that nobody puts on a poster. First, those plug-in display misters and propagation foggers almost never make it onto the asset register, because they read as appliances rather than water systems - so they fall outside the cleaning schedule by default. Put every reservoir, sump and nozzle on the list explicitly. Second, the moment your water comes from a borehole, a spring or a rainwater butt instead of the mains, you lose the small chlorine residual that mains water carries, so stored private water can amplify faster. Growers on their own supply have to treat storage and turnover as a control in its own right, not an afterthought.

A quick walk-round to start with

Use this on a clipboard or your logbook app and record a date and initial against each line:

  • List every point where water leaves as a visible spray near a person - produce misters, propagation foggers, welfare showers, wash-down lances, overhead irrigation by a packing line.
  • For each display or fogging unit, find the reservoir or sump, and put draining, cleaning and nozzle descaling on a written schedule.
  • Map welfare outlets and any seasonal cabin, and flush little-used showers and taps on a routine you can evidence.
  • Identify your water source and storage - mains, borehole, tank, butt or IBC - and record how stored water is kept cool and turned over.
  • Name someone responsible for the records, even on a small holding.

Keeping it honest

None of this replaces a competent, site-specific risk assessment, and it is not legal, veterinary or design advice. A market garden with a borehole, a propagation house and a public farm shop is a very different assessment from a single arable holding with one welfare cabin - the systems you hold, and the figures and frequencies that fit them, come from your own assessment by someone competent to judge them. Treat the prompts here as a brief for that work, not a substitute for it.

Frequently asked questions

Does open-field sprinkler irrigation spread Legionnaires’ disease?

The human risk from open-field spray is generally low: droplets disperse and dry quickly in open air and sunlight, and people rarely stand in the plume. The bigger concern is the water before it is sprayed - stored, un-chlorinated water in tanks, butts or IBCs warming through summer - and any spray close to where staff or the public work.

Are the misting units over vegetables in my farm shop a real risk?

Yes, they are a recognised aerosol source, distinct from the air-handling humidifier in the ceiling. They sit at face height and spray a fine mist near customers, fed from a small reservoir that often sits at room temperature. Keep that reservoir fed with fresh cold water, clean and drain it on a schedule, and descale the nozzles.

Do cattle troughs and animal drinkers need Legionella controls?

Drinking troughs are low risk to people, because the water is drunk rather than inhaled as a mist. Keep effort proportionate. The exception is shared storage: if a header tank feeds both animal drinkers and a welfare shower, assess and control that tank because of the shower.

Your next step this week

Walk the site once and mark every place water becomes a spray near a person - the produce mister, the propagation fogger, the welfare shower, the wash-down lance. Hand that list to whoever keeps your water records, or start the record if there isn’t one, and put the produce-misting reservoirs at the top. They are the item most likely to be aerosolising near the public and least likely to be on anyone’s cleaning schedule today.

Sources

[1] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm