Mycorrhizal Fungi: Does the Product in the Sachet Work?

Skip mycorrhizal sachets for an established bed or border. Mycorrhizal fungi are real and they do help plants, but most garden soil already has its own resident fungi working for free, and a 2025 meta-analysis found that over 80% of commercial inoculant treatments failed to establish even minimal root colonisation. The one case where a product is worth trying is a new-build garden on stripped, compacted subsoil, or a pot of fresh sterile compost, and even then it is plausible rather than proven. The two things gardeners most often do at planting time, feeding and digging, work against the partnership whether or not a product is added.

What mycorrhizal fungi are

Mycorrhizas are partnerships between plant roots and particular soil fungi. The most common type, arbuscular mycorrhizal fungi, grows inside root cells and sends fine threads (hyphae) into the soil well beyond what a root could reach, pulling in extra water and nutrients, phosphorus especially, in exchange for sugars from photosynthesis. Ectomycorrhizal fungi wrap around the fine root tips of particular trees such as oak, beech, birch, pine and hazel, and produce mushrooms. A third group, ericoid mycorrhizal fungi, partners with heathers, rhododendrons, azaleas and blueberries, which matters in Scotland given how much acid-loving planting its soils support. A sachet built around the arbuscular type isn't automatically doing anything for a rhododendron, blueberry or heather bed. Products for those plants are sold separately and say that they contain ericoid fungi, so check the label.

Almost all garden plants form one of these relationships. A few common vegetables don't: brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition → (cabbage, kale, broccoli, cauliflower, mustard, radish) and the beetroot family (beetroot, chard, spinach). A product applied to those has nothing to partner with.

The four claims, graded

Claim 1: mycorrhizal symbiosis is a real, beneficial phenomenon in nature

Verdict: True.

This is uncontroversial. The fungi extend a plant's effective root network, improve water and nutrient uptake, and can help plants cope with stress and some disease and grazing pressure. The marketing leap is from "this exists in nature" to "so adding a product from a sachet will improve your garden."

Claim 2: commercial products measurably help plants establish in an ordinary garden

Verdict: Disputed, leaning unproven to negative.

Two problems stack up. First, established beds, lawns and borders already carry their own resident fungi, which can resist colonisation by introduced strains, so adding a commercial fungus where a partnership already works doesn't necessarily improve on it. Second, a 2025 meta-analysis in New Phytologist (Koziol and colleagues) looked at 302 trials of commercial arbuscular mycorrhizal inoculants from around the world and compared them with fungi grown under laboratory conditions:

Colonisation is the measure that matters: it is the fungus doing its job inside the root, not just being present in the sachet. The authors found that where commercial products did improve growth, other ingredients, such as fertiliser or additives, were often responsible rather than the fungus. Formulations vary between manufacturers and not every product is inert, but the peer-reviewed evidence doesn't support a confident "it works" for an ordinary, established garden. Nor does the propagule count on the label reliably predict effectiveness.

Claim 3: products help in disturbed or sterile soil, new-build gardens and containers

Verdict: Unproven, but the most plausible use.

This is where the case is strongest in principle. Severely disturbed construction soils, degraded sites, and soilless or pasteurised growing media (containers, greenhouse compost) are where compatible fungi may be scarce or absent, so inoculation is at least biologically plausible. Products may help where soil is lacking in microbial diversity, and the RHS recommends choosing one containing native fungal species. It endorses one product, Empathy Rootgrow. That is a licence and a quality assurance standard, not a trial result.

No trial has tested a commercial product in sterile or heavily disturbed soil against an untreated control and measured a real growth outcome. A University of Zürich evaluation found that none of seven home-garden products it tested established the symbiosis under controlled conditions, and the meta-analysis raises the same doubt. A fungus that fails to colonise roots in a lab has little reason to do better in compacted clay. That is a reasonable inference, not the same as a study of disturbed planting holes. The product must also contain viable, compatible fungi, and no study names a brand as reliably effective, so a higher price or an award-style label isn't a useful signal.

Root dips sold for bare-rootA plant, usually a fruit tree or hedging, sold with its roots exposed rather than in a pot, and only available while dormant. Full definition → trees, hedging and roses work on the same logic. An established hedge line or old rose bed probably has its own fungi; a bare-root whip going into freshly stripped ground is the more plausible case. Those root dips and the ectomycorrhizal types are even less well studied than the arbuscular research above.

Claim 4: heavy feeding and digging suppress the symbiosis

Verdict: True.

Soil cultivation, fungicides and fertilisers all reduce mycorrhizal activity, because a well-fed plant needs its fungal partner less. High phosphorus in particular reduces colonisation by arbuscular fungi, and many long-established vegetable gardens test high in phosphorus after years of routine feeding. A gardener who adds a product and also feeds with a general-purpose or high-phosphorus fertiliser may be working against their own inoculation.

What this means in Scotland

On a new housing development the ground has typically been stripped of topsoil and recompacted by heavy machinery, and may sit on imported fill, which is the "severely disturbed" condition where native fungi are most likely to be scarce. Nothing suggests new-build subsoil is worse in Scotland than elsewhere in the UK, but a new-build garden on compacted subsoil is a stronger candidate for a product than a bed with years of undisturbed planting behind it, and the same applies to bare-root trees and hedging going into that ground. Vegetable gardens and allotments generally get fed more heavily than an ornamental border, so a keen grower's own feeding may do more to suppress mycorrhizal activity than any product could restore. Whether Scotland's cooler, wetter soil helps or hinders fungal establishment is an open question.

What to do instead

• For an established garden, protect the fungi you have: dig as little as you can, keep living roots in the ground with perennials or cover crops, and test soil before adding phosphorus.
• Skip the routine high-phosphorus feed at planting time if preserving the partnership matters to you.
• If you do buy a product for a new-build plot or a container, check the type: ericoid for heather, rhododendron and blueberry, arbuscular for most other plants, and not at all for brassicas and beetroot-family crops.
• Treat price, packaging and awards as no guide to performance.

Common questions

Weighing up other soil additives too? See does biochar work? and does rock dust work?

Sources