Mycorrhizal Fungi: Does the Product in the Sachet Actually Work?

There's a till-point display at most garden centres in late spring: a rack of small foil sachets promising "bigger blooms," "stronger roots," and "up to 10 times more roots" if you dust the powder into the planting hole before you back-fill. Rose specialists sell root-dip versions with the same message. It's cheap, it's easy to add at the point you're already digging a hole, and it plays on a fact every gardener has half-heard: mycorrhizal fungi are real, and they do help plants.

That last part is true. What's much less settled is whether buying a product and sprinkling it into an ordinary flower bed or vegetable patch makes any measurable difference, because most garden soils already have their own mycorrhizal fungi working away for free, and the two things gardeners are most likely to do at the same time (feed with phosphorus, dig the ground over) are the two things known to work against the symbiosis, inoculated or not.

In short: skip it for an established bed or border, where it's unlikely to do anything a resident fungus population isn't already doing. The one situation where it's worth trying is a genuinely new-build garden on stripped, compacted subsoil, or a pot filled with fresh sterile compost, even then, it's the plausible case rather than a proven one.

What mycorrhizal fungi actually are

Mycorrhizas are partnerships between plant roots and particular soil fungi. The most common type in garden soil, arbuscular mycorrhizal fungi, sends out fine threads (hyphae) into the soil well beyond what a root could reach on its own, pulling in extra water and nutrients, phosphorus especially, since it's often scarce and immobile in natural soils, in exchange for sugars the plant makes through photosynthesis. A second type, ectomycorrhizal fungi, wraps around rather than penetrating root cells and partners mainly with trees; a third, ericoid mycorrhizal fungi, is specific to ericaceous plants, heathers, rhododendrons, azaleas, blueberries, which matters in Scotland given how much acid-loving planting the country's soils support. This distinction matters when buying a product: a sachet formulated for the arbuscular type isn't automatically doing anything for a rhododendron, blueberry or heather bed, since those need the ericoid fungus specifically, not the type most general-purpose products are built around.

Almost all garden plants form one of these relationships with some fungus; a smaller number of common vegetables, including 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) and beetroot family crops (beetroot, chard, spinach), don't form the association at all.

This is old, well-studied biology, not a marketing invention. The marketing invention is the leap from "this exists in nature" to "therefore adding a product from a sachet will improve your garden."

The four claims, graded separately

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

Verdict: True

This is uncontroversial in soil science. The fungi extend a plant's effective root network, improve water and nutrient (particularly phosphorus) uptake, and in some cases offer a degree of protection against root disease, describing the sugar-for-nutrients exchange and noting phosphorus uptake as a key benefit, particularly because phosphorus is often in short supply in natural soils.

Claim 2: Commercial root-dip/granule products measurably help transplants establish in an ordinary garden with existing soil biology

Verdict: Disputed, leaning unproven-to-negative

Two separate problems stack up against this claim. First, established garden beds, lawns and borders generally already carry their own resident mycorrhizal fungi, built up through the existing vegetation and soil history, adding a commercial fungus to soil where a partnership is already functioning does not necessarily improve on it.

Second, and more damagingly for the product category specifically, a 2025 meta-analysis in the peer-reviewed journal New Phytologist (Koziol et al.) examined 302 observations from 94 globally sourced commercial arbuscular mycorrhizal inoculants, and compared them against fungi grown under laboratory conditions:

Colonisation is the thing to focus on in that table: it's the fungus actually doing its job inside the root, not just being present in the sachet. Of the commercial products that did show a growth benefit, nearly half did so with essentially no fungal colonisation detected at all, the study's authors put this down to other ingredients in the product (labelled or unlabelled fertiliser and additives) doing the work, not the fungus.

Not every product on a shelf is necessarily inert, and formulations vary between manufacturers, but the peer-reviewed evidence available does not support a confident "it works" for an ordinary, already-established garden.

Claim 3: Products help specifically in disturbed/sterile soil, new-build gardens with compacted subsoil, or containers/potting compost

Verdict: Unproven, the most credible use case in principle, but not confirmed by any trial

This is where the case for a product is strongest in principle, and it's backed by the same institutional sources rather than contradicted by them. Severely disturbed construction soils, degraded sites, and soilless or pasteurised growing media (containers, greenhouse compost) are the situations where compatible mycorrhizal fungi may genuinely be scarce or absent, which is the condition under which inoculation is at least biologically plausible. Mycorrhizal fungi may be less effective on frequently cultivated soils, implying the reverse is also true: soil that hasn't been repeatedly disturbed retains its fungal population better.

No trial has actually tested a commercial product's performance in genuinely sterile or heavily disturbed soil against an untreated control and measured a real growth outcome. The Koziol meta-analysis and an earlier University of Zürich evaluation (which found none of seven home-garden-marketed products it tested established symbiosis under controlled conditions) both raise doubts about whether the products colonise roots reliably at all, regardless of how depleted the starting soil is. A fungus that fails to colonise roots reliably in a lab setting has little reason to do better watered into compacted clay, that's a reasonable inference from the colonisation data, but it isn't the same as a study that tested disturbed-soil planting holes directly. Treat this claim as the most plausible use case for a product, not a proven one.

The root-dip version 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 partners mainly with ectomycorrhizal fungi rather than the arbuscular type most of the research above covers, and the same caution applies by extension rather than by a separate trial. An established hedge line or an old rose bed is likely to have its own resident fungi already; a bare-root tree or hedging whip going into freshly stripped, disturbed ground, a common scenario on Scottish new-build plots and windswept shelter-belt planting alike, is the more plausible case for a product to matter, on the same logic as the new-build/container case above.

No study names a specific brand as reliably effective, and the peer-reviewed meta-analysis found no consistent link between a product being on the shelf and it actually colonising roots, a name on the packet, or a higher price, isn't itself a useful signal of whether a product works.

Claim 4: High-phosphorus fertiliser use suppresses the symbiosis

Verdict: True

Both institutional sources agree on this independently. Phosphorus-rich fertilisers are thought to suppress mycorrhizas, and high phosphorus levels reduce colonisation by arbuscular mycorrhizal fungi, and many vegetable gardens test high in phosphorus after years of routine feeding, meaning a gardener who both applies a mycorrhizal product and feeds with a general-purpose or high-phosphorus fertiliser at the same time may be working against their own inoculation, inoculated or not.

Where this leaves Scotland

Scotland's own housing boom makes the new-build case a practical one here, not just a theoretical exception. Ground on a new housing development has typically been stripped of topsoil and recompacted by heavy machinery during construction, and may sit on imported fill rather than the site's original soil, the same "severely disturbed" condition named as the scenario where native mycorrhizal fungi are most likely to be scarce. Nothing here suggests new-build subsoil is worse in Scotland than anywhere else in the UK, but a new-build garden on compacted subsoil is a stronger candidate for a mycorrhizal product than a bed in an established garden with years of undisturbed planting behind it. The same logic extends to bare-root trees and hedging going into that same disturbed ground, since the underlying soil condition is what makes the difference, not the plant type.

A quieter connection runs through Claim 4. Vegetable gardens and allotments generally get fed more heavily and more often than an ornamental border, and it's common for a well-worked veg plot to run high in phosphorus after years of routine feeding, exactly the pattern flagged as working against the symbiosis. A keen veg grower's own feeding habits may be doing more to suppress mycorrhizal activity than any product could restore.

Whether Scotland's cooler, wetter soil conditions themselves help or hinder fungal establishment, independent of the disturbance and phosphorus factors above, remains an open question, one this article can't answer with confidence either way.

Practical takeaway

• Check what type of fungus a product actually contains before buying it for an ericaceous plant (heather, rhododendron, blueberry), a general-purpose arbuscular product isn't built for that partnership, and paying for the wrong type is money wasted twice over.
• Skip the routine high-phosphorus feed at planting time if preserving the fungal partnership matters to you; it works against the symbiosis whether or not a product has been added.
• Minimising digging and soil disturbance protects existing mycorrhizal networks more reliably than any product can rebuild them.
• A higher price or an award-style label isn't itself a signal of effectiveness, none of the institutional evidence links either to whether a product actually colonises roots. An RHS-branded product, Rootgrow, is sold commercially, but its branding shouldn't be read as an independent performance claim, since it isn't backed by a trial named above.

Common questions

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

Sources