By Belinda Castles
Researcher | Kairangahau
If you’re a keen gardener or plant lover, you’ve probably got potting mix or compost in your shed or on your shopping list.
But bagged and bulk loads of potting mix and compost can contain Legionella bacteria. This bacteria can cause Legionnaires’ disease when the soil particles are breathed in. The symptoms can be severe and life threatening.
Karen had Legionnaires’ disease in 2024. She got the disease from potting mix but admits she did things wrong while handling it.
“I was aware of the risks so was wearing gloves. But I wasn’t wearing a mask. The potting mix was quite dry, and I tipped it from a height. About 5 to 7 hours later I became very unwell. The next day I went to my doctor and a blood test confirmed I had Legionnaires’ disease. Although my severe symptoms improved after a week, it took a few months to fully recover.”
Karen doesn’t buy potting mix anymore. She knows Legionella bacteria occurs naturally in soil and now always wears a mask if she is turning her compost over.
Nikki told us when her aunt was in her late 50s, she became ill and hospitalised. A few days later she died. The family was told she had contracted Legionnaires’ disease from the potting mix she had been using.
New Zealand has a high incidence of Legionnaires’ disease compared with other countries. The number of cases usually fluctuates between 150 and 250 each year. In the past year, there have been 159 hospital admissions recorded.
Typically, there’s an increase in cases in spring, associated with increased gardening activity brought on by warmer weather. Many cases report the recent use or exposure to compost or potting mix.
Tests confirm presence of bacteria
To understand the extent of the risk, we tested samples of 20 potting mix and compost products at accredited lab PHF Science for the presence of Legionella bacteria.
Of the 20 samples we tested, 4 had low levels of the bacteria.
PHF senior scientist, David Harte, said while it was encouraging that the levels of bacteria found were very low, our test is a snapshot in time.
“If the test was repeated using different bags of the same potting mix or compost, we are likely to get different results. Some of the products that didn’t contain the bacteria in Consumer’s test, contained Legionella in previous public health surveillance tests the lab has undertaken.”
Mr Harte said consumers should consider that there is a risk of any potting mix or compost mixture containing the bacteria regardless of the brand.
“No composted organic waste material can be regarded as ‘safe’ or ‘free’ of Legionella bacteria. It can be found in any potting mix or compost because it lives inside organisms like protozoa and nematode worms, which naturally thrive in rich organic soils. Although Legionella usually make up only a tiny part of the microbes present, these host organisms help them survive for long periods. That means it’s important to handle compost and potting mix carefully and take safety precautions to reduce exposure.”
Because Legionella can be potentially present in any compost or potting mix products, we’ve decided not to publish the name of the specific products in which our tests detected the bacteria.
Warning labels and signs
All the products we tested had warning labels about the risk of Legionnaires’ disease. They also all had instructions for using potting mix and compost safely.
When we purchased the products, we noticed all the stores had warning signs about the risks of using potting mix and compost, which was good to see. But some were more prominent and informative than others.
Bunnings displayed a small sign about reading product labels and wearing appropriate equipment when using gardening products.
California Garden Centre had a sign with tips for keeping yourself safe from Legionnaires’ disease.
Mitre 10 signage advised shoppers to read health and safety panels on products. It also had a warning sign about the risk of Legionnaires’ disease, instructions for using potting mix and compost safely, and a reminder about having a face mask and gloves
The Warehouse had a large red information stand. The stand included information about the risks and instructions for using the products safely. The stand stated the precautions were in accordance with Health NZ Te Whatu Ora guidelines.
We also asked the main retailers about their store policy with regards to in-store warnings.
Mitre 10 told us all stores must follow a national signage policy. At a minimum, magnetised warning signs are displayed at eye level in every third bay that contains these products. The signs outline the risks and provide safety advice, including about recommended protective equipment and handling tips. Mitre 10 team members are trained so they can advise consumers about safe practices. Compliance is reviewed every year as part of the company’s store audit process.
Foodstuffs told us all products of this type that it sells, including Pams Value potting mix, have safety messages on the packaging. Foodstuffs does not have a policy regarding point-of-sale warning posters. These products are kept separate from food to avoid cross-contamination.
Bunnings told us it displays ‘gardening safely’ signs in-store as a reminder to customers to read product labels and wear appropriate equipment when using gardening products. The company routinely reviews packaging on garden care products to ensure it’s clear for customers. It also has masks, gloves and other personal protective equipment available to buy.
The Warehouse told us it refreshed its point-of-sale safety information earlier this year. These messages are prominently displayed in all stores that sell these products.
King’s Plant Barn, Oderings Garden Centre, Palmers Garden Centre and Woolworths did not respond to our request for comment. The Oderings store we visited in Havelock North had a large information stand with brochures about how to prevent Legionnaires’ disease.
What is Legionnaires’ disease?
Medical officer of health for the National Public Health Service, Dr Cheryl Brunton, said Legionnaires’ disease is a type of pneumonia caused by Legionella bacteria. It can be severe and in some cases life threatening.
“Legionnaires’ disease is more common and more serious in older people, particularly smokers, and people with poor immunity or who have a chronic illness, However, healthy young people have also died from Legionella pneumonia.”
Symptoms are similar to the flu and include:
a dry cough
high fever
chills
shortness of breath
muscle aches
headaches
diarrhoea
vomiting
stomach pain
excessive sweating.
Legionnaires’ disease may also cause mental confusion and disorientation. The illness worsens over several days. The infection is not contagious.
If you have any of these symptoms and have recently been handling potting mix or compost, see your usual doctor or healthcare provider as soon as possible. Tell them you have been in contact with potting mix or compost.
Other sources of Legionella bacteria
Legionella bacteria are also found in high numbers in warm water that is left sitting and becomes contaminated. This makes spa pools and cooling towers on top of buildings ideal places for the bacteria to grow.
In New Zealand, cases of Legionnaires’ disease have been linked to spa pools. To reduce the risk, it’s important you keep your spa water clean.
Health NZ recommends that hot water systems have the water heater thermostat set to a minimum of 60oC to prevent the growth of the bacteria.
9 tips to safely use potting mix and compost
Read the warning labels on the packaging for potting mix and compost.
Work with potting mix or compost in a well-ventilated outdoor area.
Wear a well-fitting face mask that covers your nose and mouth. Ensure the mask fits close to the skin with no gaps. Gaps caused by facial hair or wrinkles can compromise how effective the mask is. An N95 or respirator mask is best.
Wear gloves when handling potting mix or compost.
Open potting mix or compost bags carefully using scissors. Open the bag away from your face.
Reduce dust by dampening down the potting mix or compost before using it.
Don’t eat, smoke or vape while handling potting mix or compost.
Use a low-pressure hose when watering gardens and compost.
Wash your hands thoroughly after handling potting mix or compost and before touching your face or removing your mask.
About our test
We tested one sample of each of 20 products in an accredited laboratory for the presence of Legionella bacteria.
All products had warning labels about the risks of using potting mix and instructions on how to use the product safely.
Health New Zealand | Te Whatu Ora funded this article and our test of potting mix and compost as part of its ongoing efforts to raise awareness about Legionnaires’ disease.
