Mycorrhiza
Almost every plant on Earth shares its roots with a fungus. The fungus opens up soil the plant cannot reach on its own, delivers phosphorus and water, keeps root diseases at bay, and gets paid in sugar. This partnership is more than 400 million years old, older than the root itself.1,2 The catch: not every fungus fits every plant. Once you know which system a species has, you plan trees and shrubs differently, and you also understand why porcini grow under an oak and never under an apple tree. This page starts at the very beginning (what a fungus is, how it lives, what mycorrhiza means) and works its way into the detail from there.
From the big picture to the detail
Before we get to types and partners, it helps to keep three levels apart, otherwise everything later on sounds contradictory. Open whatever interests you; if you already know the basics, skip ahead.
Ask a child to draw a mushroom and you get an umbrella. But that is only one shape of fruiting body, and many fungi never form one at all.
An adult would try to be more precise: a living thing that feeds through threads instead of roots, does no photosynthesis and reproduces by spores instead of seeds. Sounds reasonable. Except that none of the three points holds up:
What remains is the cell wall.5
Every fungal cell is wrapped in a firm wall, and that wall contains chitin (C8H13NO5)n. Plants build their walls from cellulose (C6H10O5)n. Chemically they are almost the same molecule; chitin simply carries an extra bit of nitrogen on every building block. Animals have no cell walls at all; a beetle does carry chitin too, but as armor on the outside of its body rather than as a wall around every single cell. And bacteria use something else again, called murein.
In short: fungi are chitin beings, plants are cellulose beings.
The counter-test: downy mildew.
It forms threads, digests outside its body and in every respect lives like a fungus. It just isn't one. It belongs to the oomycetes (water molds), is more closely related to brown algae, and its cell wall is made of cellulose: a cellulose being playing at fungus.6 This is not an academic detail. Products against true fungi often have no effect on it.
You cannot tell by looking at it. People used to sort organisms by appearance and reproduction, and that is exactly why things that are not fungi sat among the fungi for decades. Only comparing their DNA set the record straight.
Not everything is what it looks like. Dolphins are not fish either, they are mammals.
And now to the point this page is about.
A fungus cannot make sugar and has to get it from outside. As we have just seen, that is not unique either: animals cannot make it, and neither can yellow bird's-nest. For this page only one question counts: where does a fungus get its sugar from?
Because that is exactly where the stories about the Wood Wide Web come from. One step at a time.
Everything hinges on where the sugar comes from. That gives four ways of life, sorted by what a fungus colonizes and whether it pays:
| Group | Lives on or in | Role in the garden |
|---|---|---|
| Mycorrhizal fungi | living roots, and pays for it | Supply. You want these. Porcini, chanterelle, truffle, fly agaric. |
| Endophytes | living tissue, without a trade | Stress tolerance. A bonus: they are there anyway. You never see them. |
| Pathogens | living tissue, at its expense | Disease. You don't want these. Honey fungus, Phytophthora, Monilinia (brown rot), scab. |
| Saprotrophs | dead material | Decomposition and humus. You need these. Oyster mushroom, shiitake, button mushroom. |
Saprotrophs are not pests. That is the most common mix-up. They break down leaves, mulch, prunings and deadwood, and so are the basis of all humus building. Without them, mulch just sits there and never turns into anything. White rot and brown rot are simply their two ways of working on wood, and they normally happen on dead wood: in the deadwood pile, in the stump, in the hügelkultur bed. Because they need no living partner, these are exactly the fungi that can be cultivated.
Where the boundary really is fluid: between endophyte and pathogen. The same fungus can sit harmlessly in the tissue for years and turn aggressive as soon as the plant comes under stress. That is a real mechanism, covered in the very next section. And the fact that honey fungus goes on decomposing the wood after it has killed the tree does not make it a decomposer: it can do both, but it started out as a pathogen.
They are everywhere: in leaves, needles, bark, wood, and yes, in roots too. Practically every plant carries endophytes, often dozens of species (including the apple tree you pick from).7 You do not see them because they form no fruiting bodies as long as the tissue is alive; they lie as thin hyphae between the plant cells.
What they live on: whatever is lying around between the cells anyway, sugars and amino acids that seep through the cell walls. Very little, lost in the noise. That is why the plant does not notice. And that is precisely the difference from mycorrhiza, which receives large amounts of sugar, but through a real organ and in return for something. The endophyte only skims off.
What they do, three things that matter in the garden:
This is even put to use: pasture grasses with endophytes are tougher, and become toxic to grazing animals.9 It is the best-known case of this double nature.
The point about stress. It is not the endophyte that wears the tree down. Rather, many of these fungi are opportunistic pathogens. As long as the tree is vigorous, its defenses keep them in check and they stay harmless. When drought knocks out those defenses, they switch strategy and start to grow. That is why branches suddenly die back in drought years without any disease having "arrived": the pathogen had been sitting inside for years. Diplodia on pines and Botryosphaeria on fruit trees work exactly like this.10
And one more thing that closes the circle: when the leaf falls, many of these fungi are the first to start decomposing it. They are already inside and can get going right away. That does not make them decomposers; endophyte is a way of life of its own. It just means that nobody has to arrive from outside before a freshly fallen leaf starts to break down.11
Bacteria are just one group among several, and for trees and shrubs the smallest:
And the distinction that is worth most in the garden: a fungus on dead wood is not a pathogen. Whatever grows in the deadwood pile, in the stump or in the hügelkultur bed is decomposing; it is not attacking anything. Only when fruiting bodies appear on a living trunk is it worth a second look.
For a tree, then: whoever talks about disease means a fungus in the vast majority of cases. And whoever hears about a giant fungus in the forest is not necessarily hearing about a partner: the record holder among fungi is a tree killer. More on that further down.
Literally "fungus root", from Greek: mykes fungus, rhiza root. And that already says the most important thing:
Mycorrhiza is not a species of fungus or a group of fungi, it is a connection. It means the shared organ of fungus and living root, not the fungus on its own. You cannot buy it or hold it in your hand; it only comes into being where both sides meet.
Three features define it: close, regulated contact between fungus and living root; an exchange from which both sides benefit; and a distinct structure at the contact point that can be recognized under the microscope.
A fungus that can form such a structure is called a mycorrhizal fungus. As a rule it does not maintain just one such connection but many at once, to several roots and often to several plants.
This is where things almost always get muddled, so here it is before the types come up:
The key point: which variant forms depends on the plant, not on the fungus. One and the same ectomycorrhizal fungus forms ordinary ectomycorrhiza with a pine, an arbutoid one with a strawberry tree and a monotropoid one with yellow bird's-nest: one individual fungus, three variants.12,13
An analogy: charging cables.
Where the analogy breaks down: a plug is a question of fit, a mycorrhiza is a trade. And the socket is not there beforehand: fungus and root build it together, from scratch, every time.
When this page talks about "the two systems", it means the two big families: arbuscular and ecto. That is the distinction that matters when you plant.
No, quite the opposite: they are a minority. The vast majority of fungi are decomposers. Only certain groups can form mycorrhiza, and those that can usually depend on it.
You can see one consequence of this in the supermarket: button mushrooms, oyster mushrooms and shiitake are for sale, porcini and chanterelles are not (or only wild-picked). The first are decomposers and grow on straw, manure or wood. The second need a living tree and therefore cannot be grown in a shed; you can only gather them or plant a tree whose roots have been inoculated beforehand. That is exactly what is done with truffles.
The flip side: a fungus growing in your compost is not a partner for your tree. And a mycorrhizal fungus cannot live on compost.
The basics
A fine root is about half a millimeter thick. A fungal hypha is twenty to fifty times thinner, which lets it into soil pores that stay closed to the root. A single gram of soil often holds tens of meters of hyphae.14,15 This multiplies the surface through which the plant takes things up, without it growing a single extra millimeter of root.
Four services lie behind this:
The most important point. Phosphorus is usually plentiful in soil, but tightly bound and barely mobile. Around a root it is used up after a short time, and the plant cannot reach the rest. The fungus fetches it from areas well beyond that zone.15
The same reach applies to water. Colonized plants often come through dry spells better16, and on free-draining soils that is often the difference between establishing and struggling.
The fungal sheath occupies the root surface and makes it inaccessible to harmful fungi; on top of that come defensive compounds and a primed immune response in the plant.17,18 Citrus is considered especially dependent on its fungal partner.19
Everything attached to the same fungus is connected. It is proven that carbon, nitrogen and phosphorus move from one plant to another along the way; how much and how far is what the argument is about. More on that further down.
The forms
Now the second level: within the "mycorrhiza" way of life there are four types. They differ in how closely fungus and root grow together: does the fungus grow into the root cell or only wrap around it? In practice it is enough to tell the first two apart; the other two concern blueberries and orchids. If the levels are not clear yet, look them up above.
| Form | Who | What defines it |
|---|---|---|
| Arbuscular mycorrhiza | About 72% of all vascular plant species: almost all fruit, vegetables, herbs, grasses, palms, legumes and nearly all tropical crop trees | The fungus grows into the root cell and forms little tree-shaped exchange organs there, the arbuscules. It forms no visible fruiting bodies, so you never get to see it. Only a few hundred fungal species serve this huge group of plants (244 had been described by 2015, DNA analyses point to 300 to 1,600)20, and accordingly they are not very picky. |
| Ectomycorrhiza | About 2% of species, but they include the dominant forest trees of the temperate and boreal zones plus some tropical families | The fungus wraps the root tip in a sheath and does not enter the cells. Around 20,000 fungal species20, many of them tied to just a few tree species, and almost all well-known edible wild mushrooms belong here: porcini, chanterelle, truffle, bay bolete. |
| Ericoid mycorrhiza | The heath family: blueberry, lingonberry, cranberry, rhododendron, heather | Specialized in acidic, wet, humus-rich soils. The fungus unlocks nitrogen directly from undecomposed raw humus, which is why blueberries grow where there is nothing to be had for other plants.21 |
| Orchid mycorrhiza | All orchids, about 10% of all vascular plant species | An orchid seed is fine as dust and has no reserves. Without the right fungus to feed it during germination, it does not come up. That is why you cannot simply sow orchids.22 |
Two variants, and why they are not types of their own
Books often list two more types. But they are not types: both are formed by perfectly ordinary ectomycorrhizal fungi and therefore belong in the same drawer as row two.
Arbutoid mycorrhiza: strawberry tree and bearberry. A sheath as in ectomycorrhiza, but the fungus also enters the cell.
Monotropoid mycorrhiza: yellow bird's-nest and its relatives, plants without chlorophyll. One and the same individual fungus is attached to a spruce on one side and to yellow bird's-nest on the other, which lives entirely on the tree's sugar.13,23 The best evidence that carbon flows from plant to plant through a fungus.
In short: which variant forms depends on the plant, not on the fungus. In the analogy above they are USB-A, USB-C and Micro-USB, but all of them USB. One individual fungus can serve a pine, a strawberry tree and yellow bird's-nest at the same time. When this page talks about "the two systems", it means the two big connector families, arbuscular and ecto, and these two variants count as ecto.
Two things do not belong on this list even though they often turn up here. First, the dark septate endophytes: fungi that live in roots without forming any of the classic structures. They are widespread on extreme sites and their effect is unclear, but they are endophytes, not mycorrhiza. Second, the plants with no fungal partner at all, about eight percent of species (they get a section of their own in a moment).
Worldwide guide
The rule of thumb holds on every continent: arbuscular is the norm, ectomycorrhiza the exception.3,24 But that exception is concentrated in a few families, and those very families make up the great forests of the northern hemisphere, Australia and Southeast Asia.
| Species or group | System | Note |
|---|---|---|
| Apple, pear, quince, cherry, plum, peach, almond | arbuscular | All pome and stone fruit, plus strawberry, raspberry, blackberry, currants and gooseberry. |
| Walnut | arbuscular | Often mixed up: despite its size, the walnut is not an ectomycorrhizal tree. Its close relative the pecan, on the other hand, is.25,26 |
| Hazel, oak, beech, birch, sweet chestnut, hornbeam, pecan | ecto | The edible-mushroom trees. Truffles grow under hazel and oak, birch boletes under birch, porcini and bay boletes under spruce and pine. |
| Pine, spruce, larch, fir, Douglas fir, cedar | ecto | Conifers are almost without exception ectomycorrhizal partners. Without colonization, pine seedlings practically do not grow on raw soil. |
| Alder | ecto | Doubly equipped: ectomycorrhiza for phosphorus, plus Frankia nodules for nitrogen from the air. That is why it copes with raw soils where nothing else will establish. |
| Oleaster (Russian olive), sea buckthorn | arbuscular | Also with Frankia nodules, but with the other fungal system than alder27, a difference that is easy to overlook when putting a group together. |
| Willow, poplar | both | The most important exception of all. They can form arbuscular and ectomycorrhiza side by side and so bridge two groups. Ectomycorrhiza usually dominates; the arbuscular form joins in mainly in young plants and on wet sites.3,28 |
| Eucalyptus, many acacias | ecto (acacias arbuscular depending on the species, some also ecto) | Australia is the great ectomycorrhiza continent outside the north. |
| Mango, avocado, papaya, banana, citrus, coffee, cocoa, olive, fig, date and coconut palm | arbuscular | Tropical and subtropical crops practically without exception. Citrus is considered especially dependent, above all on rootstocks with coarse roots.19,29 |
| Corn (maize), wheat, millet, rice, potato, tomato, onion, leek, carrot, squash, bean, pea, clover, alfalfa | arbuscular | Almost all field crops and vegetables. Leek and onion are considered especially heavily colonized and are often used as indicator plants in trials. |
| Blueberry, lingonberry, cranberry, rhododendron | ericoid | A system of its own, with fungi of its own. A blueberry bed hardly benefits from an ectomycorrhizal neighbor and not at all from an arbuscular one. The well-known exception: a few fungi from the group around Rhizoscyphus ericae colonize heather and pine at the same time, but only over a few centimeters. No large shared network comes out of it.30,31 |
| Cabbage, rapeseed, mustard, radish, arugula, cress, horseradish | none | All brassicas. What that means in practice. |
| Beetroot, chard, spinach, quinoa, amaranth | none | Goosefoot and amaranth family. |
| Rhubarb, sorrel, buckwheat, knotweed | none | Knotweed family, mostly not colonized or only weakly. |
| Lupin | none | Instead it forms dense root clusters that dissolve phosphorus out of the soil with acids on their own32, its own solution to the same problem. |
The exceptions
About eight percent of vascular plants manage entirely without mycorrhiza, and another seven percent are colonized sometimes and sometimes not.3,33,34 They gave up the partnership again over the course of evolution and unlock phosphorus themselves: with acids that release it from its bonds, and with strikingly dense root clusters. It works, but it costs the plant more energy, which is why these species are typically at home on disturbed, nutrient-rich sites.
And the kitchen garden, of all places, is full of them: cabbage in all its forms, radishes, mustard, arugula, cress, horseradish. Plus beetroot, chard, spinach, rhubarb and most lupins.
This leads to a practical point that you rarely find written down anywhere: mustard and oilseed radish as a cover crop do not feed the fungi. Where only brassicas grow for a year, the amount of arbuscular fungi in the soil drops noticeably because they lack a host; in one trial it fell by a good half after brown mustard.35,36,37 Brassicas probably also release compounds that inhibit colonization.38
In the vegetable garden this usually has no consequences. Before planting trees and shrubs, before corn or before strongly dependent crops such as citrus, though, it is a real drawback. If you choose green manure with the fungi in mind, go for phacelia, clover, vetch, rye or simply a grass mix, and leave out mustard and oilseed radish, which form no partnerships, as well as buckwheat, which is only weakly colonized.39
The honest answer
The obvious line of thought goes like this: every tree has mycorrhiza, fungal threads are everywhere in the soil, so the whole forest is one network, stretching for kilometers. That is the story that has been told for years under the name Wood Wide Web. The truth is more interesting than the story, but it takes three steps.
Almost everyone knows the figure: in the Malheur National Forest in Oregon there lives a honey fungus that spreads over about 965 hectares (some 2,400 acres), a single clone, estimated at 1,900 to 8,650 years old.40 That is no rumor, it is true.
But honey fungus is not a mycorrhizal fungus. It is the opposite of a partner: a root parasite that attacks and kills trees and then decomposes their wood. And it can grow so large because it can do something mycorrhizal fungi cannot: it forms black, cord-like strands that let it bridge open stretches of soil without needing a root along the way. That is why it is also called the bootlace fungus.
That this particular record is regularly cited as evidence for the Wood Wide Web is one of the most persistent fallacies in the whole debate.
It pays to separate these carefully here, otherwise you get a contradiction where there is none.
How far does the fungus grow from a root into open soil? In the arbuscular form, a few centimeters: just enough to bridge the depleted zone around the root and reach fresh phosphorus.41 In ectomycorrhiza considerably further, depending on the species; species of the so-called long-distance exploration type also form strands and use them to tap much larger volumes of soil.42
How large does the individual fungus grow overall? Considerably larger than that reach. Many species form small, short-lived individuals; some grow to dozens of meters. The largest described so far, a bolete of the species Suillus pungens under pines, measures 40 × 14 meters.43,44
The reason for this difference is exactly the point: the fungus does not bridge the distance freely through the soil, it works its way along from root to root. It grows to where the next fine root already is, colonizes it and grows on from there. Without roots in between, it gets nowhere.
A tree does not keep its fine roots at the trunk; they are spread across the whole crown area and usually well beyond it.45 Two trees standing ten meters apart therefore have a zone in the middle where their fine roots intermingle. If an individual fungus sits there, it is attached to both: not because it has grown ten meters, but because it met both of them in the middle.
And for everything beyond that: the tree itself is the next link. A fungus on the north side and a fungus on the south side of the same tree are not directly connected, but both are attached to the same tree, and the tree carries things from one side to the other in its own tissue. So the chain runs fungus → tree → fungus → shrub → fungus, alternating, across the whole area.
Large connections can certainly arise this way. Just not as one giant fungus, but as a chain of many short sections, and every gap in that chain breaks it.
Besides the continuous fungal connection, there is a far less spectacular route by which substances get from one plant to another. Hyphae and fine roots constantly die and are renewed, and roots release substances all the time. Whatever is set free this way ends up in the soil, and is taken up again by the neighbor and its own fungus.
This is not a directed transfer, just plain recycling: not a baton being handed over, but one that is dropped and picked up again. It also works between plants that have no fungus in common.
And this is exactly the methodological difficulty of every transfer experiment in the field: when a labeled substance turns up in the neighboring plant, it is usually impossible to say which of the two routes it took.46,47 That is one of the main reasons why the numbers on the Wood Wide Web vary so widely.
The experiments with labeled carbon really do exist: Simard and colleagues, 1997, in Nature.48 Paper birch and Douglas fir were bagged individually in chambers and exposed to carbon dioxide whose carbon could be recognized later; nine days on, the trees were harvested and analyzed.
The result is real: carbon moved in both directions, with a net flow toward the Douglas fir, and the more shaded it was, the more it received. This is the basis for everything that has been written about helpful trees since.
Two details are missing from almost every retelling. First, the net amount was small: in the second year of the trial an average of 6% of the labeled carbon taken up by both trees, just under 10% in deep shade. In the first year there was no net transfer at all. Second, and this is the more interesting point, a western red cedar stood alongside as a control. It belongs to the other fungal system and so was not attached to the network in question at all. It still received an amount equal to 18% of the exchange between birch and Douglas fir.47,48
In other words: a considerable share simply takes the route through the soil, not through the fungal network. That is exactly where the criticism comes in. Simard and colleagues counter that the criticism rests on too narrow a selection of literature, and other researchers also regard transfer through fungal networks as established.49,50 The debate continues to this day.
The fairest summary is probably: small, but measurable. Saying "nothing at all" is just as wrong as telling tales of the talking forest.
Certain: one individual fungus can serve several plants at the same time, and carbon flows through it. The best evidence is yellow bird's-nest: a plant with no chlorophyll at all that makes not a single gram of sugar itself and still flowers and sets seed. It lives entirely off a nearby tree, through an individual fungus attached on one side to the spruce root and on the other to the plant. Without a real connection this plant could not exist.13,23
Not certain: everything beyond that. How much a mature tree actually gives or receives this way, whether it matters for its growth, how far it reaches, and above all whether a plant steers it deliberately. The story of mother trees that recognize which seedling is their own and supply it preferentially is the most popular part of the narrative and the most weakly supported.51 Several reviews from recent years conclude that the evidence for it is much thinner than its popularity would suggest.46,47
What this means for planning
Neighborhood is what counts, not kilometers. Because the fungus works its way from root to root, a new tree gets colonized quickly if a woody plant of the same system stands close enough for their fine-root zones to overlap. If it stands alone in the corner, it has to start from scratch, waiting for a spore to find it by chance.
That is exactly why Permaplans has a dedicated view for this: the "Mycorrhiza root zones" layer colors the root zone of every tree and shrub by its system and so shows where isolated islands have formed on a site instead of connected areas (calculated from mature width and mycorrhiza type). The screenshot below shows what that looks like.
In practice
The questions that actually come up when you plant, from "do I need to do anything?" to "how do I connect two groups?".
It depends on the system, and the difference is big.
Arbuscular mycorrhiza: already there. Every meadow and every field is full of it, because grasses and herbs are partners of these fungi themselves. An apple, a walnut or an elder that you plant in a meadow is colonized within a few weeks to months. You do not need to do anything.
Ectomycorrhiza: this is the problem case. In a meadow without host trees there are hardly any spores of these fungi. An oak, beech, hazel or sweet chestnut on open ground finds little at first. Spores arrive by wind and animals, and colonization takes months to years. In a trial with oak seedlings on fallow land, all seedlings up to eight meters from the forest edge were colonized; from about 15 meters on, colonization dropped off markedly.52
And there is a sequence: the pioneer fungi come first (deceivers, earthfans, poisonpies). The late colonizers, meaning brittlegills, webcaps and amanitas, need decades and an established stand.53,54 So you can easily have half a forest for ten years that is still thinly equipped on the fungal side.
It saves you exactly that waiting time, and that is the real benefit of planning a group in the first place.
If you plant an ectomycorrhizal shrub next to an established tree of the same system, the existing mycelium grows into the new roots. That happens within one season, instead of waiting years for spores to drift in by chance.52,55 A hazel planted next to an established oak is therefore something completely different from the same hazel alone in the corner of a meadow.
That is exactly why it pays to group trees and shrubs of the same system instead of scattering them across the site.
No, they do not get in each other's way. Hyphae of both systems live side by side in the same soil, each on its own roots. A mixed planting is no disadvantage.
What you have to let go of is the idea of one big network. Many individual fungi are only a few meters across, the largest known ones a few dozen; any piece of land is a patchwork of many such individuals, not one continuous cable. The "road" mentioned on the plant guilds page is therefore more of a chain of handover points than a cable. Covered in detail further up.
There is an elegant solution for this: willow and poplar can form both types, and so can eucalyptus and alder. Ectomycorrhiza usually dominates; the arbuscular form joins in mainly in young plants and on wet or extreme sites. Worldwide, just under 90 plant genera with this kind of dual mycorrhiza are known28; in the garden these four are the obvious choices.
A row of basket willows between an oak and hazel group and a fruit group is therefore the connecting element, and on the side it gives you prunings, early-spring forage for bees and weaving material. If you want to put the idea into practice on your own land, this is where to start.
For fruit trees and bushes it is almost always money down the drain: they need arbuscular fungi, and those are already present in established garden or meadow soil.
It makes sense for oak, beech, hazel and sweet chestnut on open ground, exactly where the right fungi are missing and colonization would otherwise take years. And on land where soil life is fundamentally lacking: long-term arable fields, raw subsoil after construction, stripped topsoil, land reclamation.
Even then, though: a wheelbarrow of soil from the root zone of an old, healthy tree of the same species is usually more effective and cheaper than any product. You get exactly the fungi that occur locally and cope with the site. Many products you can buy contain only arbuscular fungi anyway and are therefore useless for an oak or a birch from the start.
What helps without inoculating: keep the soil covered, turn it as little as possible, leave roots in the ground over winter and avoid long periods without plants. Fungi live on living roots; bare soil starves them.
Because the two groups differ fundamentally in one respect: how picky they are.
Arbuscular fungi are generalists. There are only a few hundred species for around 200,000 partner plants, so they simply cannot afford to be picky. One and the same fungus colonizes apple, tomato, corn and lavender. That is why a single universal product really is enough here, and most contain only a handful of species.
Ectomycorrhizal fungi are often specialists. Over twenty thousand species, and many of them are tied to one tree genus or even a single species: the birch bolete grows only with birch, the larch bolete only with larch, the black truffle only with oak and hazel.56 An all-purpose product cannot exist here even in principle, hence the split into conifer, broadleaf, oak and so on. Then there is the site: conifer fungi are mostly adapted to acidic soil, others to chalky soil.
Still, the choice is smaller than the labels suggest. Only what can be propagated can be sold, and that means mainly pioneer species, the generalists among the specialists: dyeballs (Pisolithus), common earthballs, deceivers, false truffles (Rhizopogon), poisonpies. Exactly the fungi that would find a young tree first anyway. The late colonizers that characterize an old stand cannot be raised in a bag.
And what most people actually want does not exist. Porcini and chanterelles still cannot be grown commercially (for the chanterelle it worked once in the lab, in 1997)57,58, so there is no product, however high the demand. The one exception is the truffle, and that is not sold as powder but as a ready-inoculated, certified young tree from the nursery.
Part of the variety is simply product range. The market is barely regulated, and tests of commercial products regularly find discrepancies between label and contents, right down to packs with no detectable viable fungi. In a worldwide test of 28 products, 84% led to no colonization at all in sterile soil.59,60,61
What this means in practice is exactly what the previous answer says: where inoculation is needed at all, a wheelbarrow of soil from the root zone of an old, healthy tree of the same species is the more reliable route. The right species are guaranteed to be in there, alive and used to the site.
The finding is clear, the cause is not. A study in 48 Czech castle parks examined the soil under 73 non-native tree species: there, pathogens made up twice as large a share of fungal species and three times as large a share of fungal biomass as under native trees, while ectomycorrhizal fungi made up a smaller share. The longer a tree species had been growing locally, the lower the share of pathogens.62
What causes this, though, the study does not say. It could be poorer supply with suitable fungal partners; it could just as well be which pathogens were imported together with the plants. Both are plausible, neither is proven.
Put honestly, the finding is: fewer fungal partners, more pathogens, cause unknown. It still works as a rule of thumb for planting, as an observation rather than an explanation.
Yes, but it is a different route from the inoculation above, even though the same fungal groups are involved. If you want to harvest truffles, porcini or chanterelles, you buy specifically inoculated planting stock (a hazel or oak whose roots were colonized with a particular fungal species at the nursery) and you need the right soil to go with it, which for truffles above all means a high pH. This has nothing to do with an all-purpose mycorrhiza powder.
Mushrooms such as oyster mushroom or shiitake, on the other hand, are not mycorrhizal fungi at all but decomposers: they are grown on logs or substrate, independently of any living tree. That is why you can buy them and not porcini.
Next steps
Three sentences are enough for everyday use. First: a fungus only colonizes plants of its own system, so if you put a forest tree among nothing but fruit trees, you give it no partner. Second: neighborhood matters, not area; place trees and shrubs of the same system so that their root zones touch. Third: a plant guild should not be locked into a single system, otherwise it has no connection to whatever grows next to it.
How to turn this into a complete planting, with an anchor plant, roles and a timeline, is covered on the sister page: Plant guilds: what belongs together.
Evidence
The superscript numbers in the text point here, and ↩ takes you back to the text. Where textbook knowledge is concerned, the standard work is cited; where experts disagree (above all on the Wood Wide Web), both sides are listed. Planning conclusions such as grouping trees and shrubs are our own inferences from these findings, not study results.
Sources last checked: October 2026. Spotted a mistake? Let us know.