Dry spores in a sealed bag stay usable for up to two years. That window comes from the 3 Biotech 2020 review, and it holds only while the bag sits in the conditions the label asks for. Once the powder hits the soil, mycorrhizal inoculant shelf life stops being the question you should ask. No solid number covers how long the fungus keeps working down there.
The bag on the shed shelf above my potting bench had gone stiff, and the powder inside rattled like dry sand when I shook it. I scraped the dust off the back seam and checked it. I found a date stamp three summers old. Two years is the usual limit these products get, and that beats what most other biological additives manage, so a bag that far past it was promising me nothing.
Storing mycorrhizal inoculant and keeping a network alive in the ground are two separate jobs. The product is dry spores held in a carrier, with a date printed on the bag. The network is a live trade between fungus and root, and it runs only while the plant keeps sending sugar down to feed it. Pull the plant and the trade stops, no matter how fresh the powder was.
Inside the root, the swap happens in arbuscules, small branched structures the fungus builds into the cells. Each one lasts under 15 days. The fungus tears them down and builds new ones for as long as the roots keep feeding it, season after season. So the powder you buy is a starter, and the plant does the rest of the work.
A printed date is a ceiling, not a promise. The two-year figure assumes the bag was held the way the maker says to hold it, and that line on the back is the only storage guidance you get. Lab-grown inoculants are held to peer-reviewed reporting on how they were grown, fed, stored and handled. Commercial products mostly report none of it.
The results split along the same line. In 2025 testing, 84% of the commercial products came in under 5% root colonization. Only 12% of them reached a level counted as enough. Among the lab-grown ones, 63% cleared that same bar. A fresh bag from a supplier that tells you nothing about its handling is still a gamble.
- Date it first: Find the production stamp and count two years forward from it, not from the day you bought the bag or opened it.
- Follow the label: Store the bag the way the maker prints on the back, since that instruction is the only storage rule you have to work with.
- Skip the home test: No retrieved source gives a way to check at home whether the spores in an open bag are still alive.
- Buy from a supplier willing to tell you the production date and how the stock was held before it shipped.
Judge mycorrhizae product viability by the date and the storage, because those are the only two things you can check. A bag inside its two-year window, kept the way the label says, is the best you can confirm before it goes in the ground. Past that, you are guessing, and so is anyone selling you the bag.
Out in the soil, you can only improve the odds. Fallow ground, frequent or deep tillage and heavy phosphorus all push mycorrhizae down. So keep something growing in the bed, cut the digging back and go easy on the phosphorus. Those choices make the ground friendlier, but none of them come with a timeline you can hold anyone to.
No source I can point to supports a figure for how long a fungal network takes to establish or how long it lasts in the soil. Anyone quoting you a number for that has gone past the evidence.
Read the full article: Mycorrhizae: What Works and What Is Hype
