How long does mycorrhizal fungi last? Shelf life, storage and use-by dates
By Joe, Founder of Dr Forest · Oct 2026 · Edited 4 Oct 2026 · 5 min read
A bag of mycorrhizal fungi is a bag of living spores. Kept well, they last years. Kept badly, or made the wrong way, they can be finished within months.
In short
In good lab conditions, dried mycorrhizal inoculum has stayed useful for up to two years at room temperature and at least four years at fridge temperature. Products made mainly from chopped root last only a few months. Shop-bought products vary far more, and many tested products contained no living fungi even when they were within date.
The short answer
Mycorrhizal spores are alive but dormant. They do not die on a set date; their ability to colonise roots fades, faster or slower depending on how they are kept. These are the clearest figures from the research on arbuscular mycorrhizal fungi, the type most garden plants partner with:
| What and how stored | How long it stayed useful |
|---|---|
| Dried spore inoculum, closed container, 22°C | Up to 2 years with minimal loss1 |
| Air-dried lab inoculum (spores and root bits in growing medium), 5°C | At least 4 years1 |
| Dried root inoculum, 22°C | Under 100 days1 |
| Fungi in moist peat compost, constant 20°C | Very low infectivity after 1 year2 |
These are figures from a university lab storing its own high-quality inoculum. They show what is possible, not what any bag on a shop shelf will do.
Temperature and moisture
Temperature is the big one. In a year-long study of fungi mixed into peat compost, infectivity fell in every storage condition tested. At a steady 20°C, room temperature, it was very low after a year; at 5°C it held up best.2 A bag that has spent a summer in a hot shop window or a greenhouse is likely to have lost more than one kept in a cool shed.
Moisture matters too, though not as simply as ‘drier is always better’. In the university lab, inoculum is dried to below 5% moisture before it is stored.1 But in a study of three species stored for six months in soil at different moisture levels, spores kept in completely dried soil lost much of their infectivity, and the best moisture level differed between species.3 The practical point for a gardener is to keep the product as the maker supplied it: sealed, and away from damp.
Spores or root pieces?
Inoculum is made from spores, pieces of colonised root, or a mix of both. Root pieces can colonise quickly, but they do not keep: dried root inoculum lasted under 100 days at room temperature, against up to two years for dried spore inoculum.1
A good label tells you what the product contains and how many propagules (spores or other living units) there are per gram. If it tells you neither, you have no way to compare it with anything else.
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Why a missing date is a red flag
Quality across the market is patchy. In a 2025 Swiss study of 16 products sold in Europe, only three got any fungi into the roots, and none of the seven sold for home gardens did.4 A bag can look fine and contain nothing that works.
A use-by or manufacture date does not guarantee a good product. A bag with no date at all, though, tells you nothing about how long the spores have been sitting there or how they were kept. Before you buy, look for:
- A use-by or manufacture date. No date: ask the seller, or buy elsewhere.
- What it contains: spores, root pieces, or both.
- A propagule count per gram.
- The species list, so you can check it matches your plants. See which mycorrhizal fungi to buy in the UK.
How long Dr Forest powder lasts
Every pouch of Dr Forest Mycorrhizal Fungi Powder carries a printed date. Use the date on the pouch. The two-year and four-year figures above come from a university lab, not from a pouch on a shop shelf.
We buy it in small batches, with fresh stock coming in every month, so a pouch has not sat on a warehouse shelf before it reaches you. The blend contains 18 species: 9 arbuscular (endo) and 9 ectomycorrhizal (ecto).
How long spores last in soil
Once applied, spores do not need a root on the first day. In a laboratory study, germinated spores grew for about 15 days without a host plant, then shut down, and stayed able to infect roots for up to six months, although less well after three.5 That is why mixing powder with seed at sowing works. It is still best to put it where roots are, or will be within days. Method: how and when to use mycorrhizal fungi.
Storing an opened bag
- Reseal it after each use, squeezing the air out.
- Keep it cool. A cool cupboard, or a fridge. In the storage study, 5°C preserved infectivity best.2
- Keep it dry. Use a dry spoon. If it lives in the fridge, keep it sealed so condensation cannot get in when you take it out.
- Write the opening date on the bag, and use it before the printed date.
FAQ
How long do mycorrhizal spores last?
Dried spores kept cool and dry can stay viable for two years or more, and at least four years at fridge temperature. Products made from root pieces last a few months. Warm or damp storage shortens both.
Should I buy a mycorrhizal product with no date on it?
Be wary. Without a use-by or manufacture date you cannot judge how old the spores are. Ask the seller, or choose a product that prints one.
How long does Dr Forest mycorrhizal powder last?
Use it within 9 to 12 months of purchase, and by the date printed on the pouch. Keep it sealed, cool and dry once opened.
Can I tell if it is still alive by looking at it?
No. Testing it means growing a host plant with it and checking the roots for colonisation weeks later. In practice, go by the date and how the bag has been stored.
Is it worth using out-of-date powder?
It will not harm the plant, but it may colonise fewer roots, so use it on low-stakes jobs and buy fresh for anything that matters.
Can I keep leftover slurry?
No. Mix slurry fresh for each planting job and use it the same day. Store the powder dry.
Sources
- Habte M, Osorio NW (2001). Arbuscular Mycorrhizas: Producing and Applying Arbuscular Mycorrhizal Inoculum. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. ctahr.hawaii.edu
- Püschel D, Kolaříková Z, Šmilauer P, Rydlová J (2019). Survival and long-term infectivity of arbuscular mycorrhizal fungi in peat-based substrates stored under different temperature regimes. Applied Soil Ecology 140: 98–107. doi:10.1016/j.apsoil.2019.04.020
- Ruiz-Lozano JM, Azcón R (1996). Viability and infectivity of mycorrhizal spores after long term storage in soils with different water potentials. Applied Soil Ecology 3(2): 183–186. sciencedirect.com
- Boussageon R, Eigenmann M, Frommenwiler J, van der Heijden MGA (2025). Poor quality of commercial arbuscular mycorrhizal inoculants used for agriculture and home gardening. Journal of Sustainable Agriculture and Environment 4(4): e70107. doi:10.1002/sae2.70107
- Logi C, Sbrana C, Giovannetti M (1998). Cellular events involved in survival of individual arbuscular mycorrhizal symbionts growing in the absence of the host. Applied and Environmental Microbiology 64(9): 3473–3479. doi:10.1128/AEM.64.9.3473-3479.1998
Related: Dr Forest Mycorrhizal Fungi Powder · Which mycorrhizal fungi should you buy in the UK? · How and when to use mycorrhizal fungi · What are mycorrhizal fungi?
Founder of Dr Forest. Joe started the business in 2020 and formulates the blended fertilisers and mixes and packs them in small batches in Stockport.
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