Botulism: Make Wild Plant Pestos Safely

Last updated: 3 Nov 2026 with new references and official recommendations.

L’wild garlic is one of the most popular wild plants for making pestos. It’s an excellent way to enjoy its flavours, as well as those of ground elder, stinging nettle and other edible wild plants. However, certain precautions should be taken, as preparations made with fresh plants preserved in oil may pose a risk of botulism.

If you like making preserves of plants in oil, you should know that there is a risk, although rare, of producing an extremely dangerous toxin: botulinum toxin.

For many years, I made pestos without worrying about it because, like many people, I was unaware of the problem. I don’t even mention it in the first edition of my book on edible wild plants, as I knew little about the subject when I was writing it. Today, it seems important to me to share this information, especially as many of us prepare jars to keep for several months or give to our loved ones.

Fortunately, there are simple ways to reduce this risk, including freezing.

Botulism and botulinum toxin

Botulism is an illness caused by a toxin called botulinum toxin. It is one of the most toxic substances known: according to the Swiss Federal Office of Public Health (FOPH), less than a millionth of a gram of type A toxin can be enough to kill a person weighing 70 kg ^[1].

This toxin is produced, among other things, by a bacterium called Clostridium botulinum. It occurs naturally in the environment, particularly in soil and aquatic sediments. It can survive there for a long time in the form of spores, resistant structures capable of withstanding unfavourable conditions ^[1][2].

When conditions become favourable, the spores can germinate and give rise to active bacteria, which multiply and produce botulinum toxin. So dormant spores do not produce the toxin directly. Wild plants growing in contact with the soil may carry spores. If these plants are placed in an oxygen-poor environment, such as a preparation containing oil, the bacterium can grow and produce the toxin when the other conditions are also favourable ^[2][6].

Fortunately, botulism remains rare. In Switzerland, the FOPH generally records one or two cases a year. In mainland France, 122 cases of human botulism, of all forms, were reported between 2018 and 2024 ^[1][3].

After toxin is ingested in food, symptoms usually appear within 12 to 36 hours, sometimes sooner or later. They may include:

• dry mouth;
• blurred or double vision and impaired eye movements;
• hoarseness or difficulty speaking;
• difficulty swallowing;
• muscle weakness that may progress to paralysis of the respiratory muscles.

Botulism is a medical emergency. If these symptoms appear after eating a preserve or pesto, seek emergency medical care immediately ^[1][2].

A case of botulism linked to wild garlic pesto in France

In September 2024, five people were hospitalised in Indre-et-Loire after eating homemade wild garlic pesto preserved in jars. The National Reference Centre at the Institut Pasteur confirmed the presence of Clostridium botulinum in the pesto in question. Investigations also revealed that the sterilisation process used for the preserves was not under control ^[4].

This case is a reminder that the risk, although rare, directly concerns preparations made from wild plants.

Conditions favourable to bacterial growth

The bacterium responsible for botulism grows under certain conditions that are useful to know about:

• Absence of oxygen: Clostridium botulinum grows in an anaerobic environment, meaning without oxygen. Oil can create such an environment around plant material.
• Presence of water: the bacterium needs available water to grow. Fresh plants contain a lot of it. Thorough drying reduces this risk, but using dried plants alone does not guarantee that they can be safely stored in oil.
• Temperature: some strains can grow slowly at temperatures as low as 3 °C. Refrigeration therefore limits the risk, but does not allow for indefinite storage. At −18 °C, however, bacterial multiplication and the production of new toxin are prevented.
• Acidity: a pH below 4.6 normally prevents the growth of C. botulinum. But for an acidified preparation to be safe, the required acidity must be reached in time and throughout all the plant material, not just in the liquid surrounding it.

These conditions explain why pesto made from fresh plants, which are rich in water, and containing oil should not be considered a shelf-stable preserve at room temperature ^[2][6].

What can you do to prevent botulism?

A few simple precautions can reduce the risk:

• Use correctly identified, clean and healthy plants: wash them and work with clean utensils and containers. Washing reduces dirt, but does not guarantee that spores will be removed.
• Eat promptly: if you make pesto for a meal, eat it without leaving it at room temperature for a long time.
• Refrigerate immediately: if you are not eating it straight away, store it at 4 °C or below.
• Freeze for longer storage: this is the simplest way to keep homemade pesto for more than a few days.

In its publication Herbs and Vegetables in Oil, revised in January 2025, the Oregon State University Extension Service explicitly recommends keeping homemade pestos in the refrigerator for no more than 4 days, or freezing them for longer storage ^[5].

This recommendation applies to homemade pestos containing plant material and oil. As a precaution, it can be applied to wild-plant pestos made in a similar way; it is not experimental validation of every wild-plant pesto recipe.

Can you tell if pesto is dangerous by its smell?

According to the Swiss Federal Office of Public Health, you should avoid eating preserves with bulging lids or jars that are not properly sealed ^[1].

Other signs should also raise the alarm: an unusual release of gas or liquid when opened, mould, an unusual change in appearance or a suspicious smell.

Warning: botulinum toxin has no smell or taste that would allow it to be detected. A pesto may therefore seem perfectly normal and still contain the toxin. If you have any doubts about how it was prepared or stored, do not taste it; throw it away ^[7].

Does cooking make pesto safe?

Botulinum toxin is heat-sensitive. According to the World Health Organization, it can be inactivated when the temperature at the centre of the food exceeds 85 °C for at least 5 minutes ^[2].

However, the toxin must be distinguished from the bacterial spores. Cooking that inactivates the toxin does not necessarily destroy the spores, which are much more resistant. Destroying spores in low-acid preserves requires a specific heat treatment validated for the product and its packaging.

You should therefore not rely on cooking to «rescue» pesto that has been stored incorrectly or looks suspect. If in doubt, throw it away rather than trying to make it safe to eat by heating it ^[2][7].

Sterilising jars in the oven is not a reliable method either: the temperature displayed by the oven does not guarantee that the entire preparation has received sufficient heat treatment. This method is not recommended by the National Center for Home Food Preservation ^[8].

Oregon State University also points out that there is no validated home-preserving process for making pesto shelf-stable at room temperature ^[5].

For homemade pesto, it is therefore best to opt for short-term refrigeration or freezing rather than trying to sterilise it.

Freezing: the simplest way to preserve pesto

Several of you have asked whether pesto can be frozen. The answer is yes, and this is the method recommended for long-term storage by the Oregon State University Extension Service ^[5].

At −18 °C, bacteria no longer multiply or produce new toxin. However, freezing does not necessarily destroy spores or any toxin already present before freezing. Pesto should therefore be frozen freshly prepared and handled correctly, not after being left at room temperature for too long ^[2][9].

This method preserves the flavour of pesto without needing to cook it.

How to freeze pesto

1. Make the pesto with fresh plants that have been correctly identified, cleaned and prepared.
2. Quickly divide it into small portions in freezer-safe containers or ice-cube trays.
3. Place it in the freezer at −18 °C without leaving it at room temperature unnecessarily.
4. Once the portions are frozen, you can put them together in an airtight container or bag. Label it with the date it was made.
5. Defrost in the fridge, then use promptly. Oregon State University recommends using defrosted stinging nettle pesto within 4 days at most ^[10].

Pesto can be kept this way for several months. Provided the temperature remains constantly at −18 °C, the length of time it is frozen mainly affects its flavour quality. Oregon State University even provides a recipe for stinging nettle pesto frozen in small portions, with a suggested storage time of up to one year ^[10].

This is the method I prefer for making large quantities of wild plant pesto and then enjoying it without cooking, for example on bread.

Lacto-fermentation: another option

Another preservation method is called lacto-fermentation. It relies on the growth of lactic acid bacteria, which gradually acidify the preparation.

When acidification is sufficient and properly controlled, it prevents Clostridium botulinum from growing. The microbiological reference threshold is a pH below 4.6. To provide an additional safety margin, Virginia Tech recommends aiming for a pH of 4.1 or lower for fermented vegetables ^[2][12].

How can you check the acidity?

You can check the pH with suitable test strips that can distinguish at least one tenth of a pH unit. However, plant pigments can make the results difficult to read, and many strips are too imprecise. A properly calibrated pH meter is preferable when you want to check the acidity of a ferment precisely ^[11][12].

The measurement must be representative of all the plant material, not just the brine. Acidification must also occur sufficiently early: a satisfactory pH at the end of fermentation would not eliminate any toxin that might have been produced earlier. It is therefore important to follow a tried-and-tested fermentation method, using appropriate quantities of salt, temperature and duration ^[2][6][11].

Can lacto-fermented plants then be put in oil?

This preparation is an option, but a pH measurement below 4.6 is not, on its own, enough to validate long-term storage in oil at room temperature. In particular, you would need to ensure that all the plant material has reached the desired acidity and that the entire process is properly controlled. Health Canada emphasises the importance of equilibrium pH in plant material preserved in oil and of validating the production conditions ^[6].

For a homemade preparation, I therefore recommend keeping lacto-fermented plants to which oil has been added in the fridge, without assigning them an unvalidated long storage life. Lacto-fermentation is an interesting option, but it requires more expertise than freezing fresh pesto.

In summary: how should you store wild plant pesto?

• Eat immediately: make the pesto with fresh ingredients and use it promptly.
• In the fridge: store it at 4 °C or below and use it within 4 days at most ^[5].
• For several months: quickly freeze freshly made pesto at −18 °C ^[5].
• After defrosting: keep it in the fridge and use it within 4 days at most, preferably sooner ^[10].
• In a jar at room temperature: do not use an unvalidated home process to make pesto shelf-stable outside the fridge ^[5][8].

These precautions allow you to enjoy the flavours of wild plants while limiting microbiological risks. They are particularly important when making pesto in large quantities or to give to friends and family.