Your plants pay for the partnership in sugar, and in rich soil they get very little back for it. Three mycorrhizae disadvantages show up in real gardens. Plants hand over carbon they made themselves, whole crop families ignore the fungus, and most bagged products never colonize enough root to matter. In a fed, watered vegetable bed you can end up paying the full price for almost no return.
"So the fungus works for free?" My neighbor asked me that across the fence, holding a fresh bag of inoculant for the snap pea rows in her raised bed. It is not free. Penn State puts the transfer at up to 20% of the total carbon a plant assimilates, handed straight to the fungal partner. Her peas were already climbing hard in soil she fed and watered all spring, so that sugar would buy them almost nothing.
Treat the carbon cost of mycorrhizae as a purchase your plant makes. A 2020 review put the loss at 10% to 20% of the photosynthates a mycorrhizal plant produces while it takes in extra nutrients through the fungus. That sugar could have gone into leaves, roots or fruit instead. On thin, dry ground the trade is worth every bit of it, because the fungal threads pull water and phosphorus out of soil that root hairs never touch. In a bed you feed and water each week, your roots already reach that, and you are buying something you own.
Some crops give you nothing back at all. Brassica-family vegetables like cabbage, broccoli and kale are largely non-host plants, so the fungus finds no partner in their roots. Soil chemistry shuts the door too. Above about 100 parts per million of soil phosphorus, roots do not need fungal help, and that covers plenty of older beds that have taken compost and fertilizer for years. A basic soil test tells you this for a few dollars, and it is the one number worth checking before you spend anything.
Mycorrhizae disadvantages shrink fast on poor ground. In sandy or freshly stripped soil, the fungal network reaches water and phosphorus that your roots cannot, and up to a fifth of the plant's sugar is a fair price for that. The same bag in a rich raised bed moves carbon out of your crop and into the soil. So the real question is never whether the fungi work. It is whether your soil is poor enough for the trade to pay off.
The bag on the shelf is its own problem. A 2025 review of commercial inoculants found that 84% produced under 5% root colonization. Fewer than 12% gave both a growth benefit and living fungus on the roots. Those are the mycorrhizal inoculant limits that never make it onto the label. Shelf life typically runs up to two years. That is longer than most other soil biologicals last, but a jug that sat through two warehouse summers may hold little that still works.
Keep your money in your pocket in fed vegetable beds and brassica rotations. Skip it as well when a soil test comes back high in phosphorus. In my own garden I save inoculant for poor subsoil and for container mixes that start out with no fungal life in them. Check the production date before you buy and pick the freshest jug on the shelf. Your compost pile will not top the fungi up either, since they generally do not survive composting or compost tea.
Read the full article: Mycorrhizae: What Works and What Is Hype
