You plant a tomato next to a basil and another two rows down. Three weeks later the basil is flowering, the close tomato is thriving, and the distant tomato is also healthier than the year you planted it alone. Most companion planting books explain this as "good neighbors." The newer answer, backed by 30 years of forest ecology research, is that the plants are connected underground through a fungal network and they are sharing more than just soil space.
Mycorrhizal networks (sometimes called the "wood wide web") are the threadlike fungal connections that link plant roots beneath your garden. They move water, nutrients, and chemical signals between plants of the same and different species. They have been documented in peer-reviewed studies since the 1990s and they are doing real work in your backyard whether you notice them or not. This guide covers what they actually are, what the science says (and does not say), and how to garden in a way that keeps them functioning.
Key Takeaway
Mycorrhizal networks are real, documented fungal connections between plant roots. They transfer phosphorus, nitrogen, water, and chemical defense signals between linked plants. About 90 percent of garden plants participate, with brassicas (cabbage family) and chenopods (beets, spinach, chard) as the main exceptions. The practical gardener takeaway: minimize tillage, go easy on synthetic phosphorus fertilizer, and consider mycorrhizal inoculant at planting time. Recent research has tempered some early claims about "mother trees" deliberately feeding offspring, so treat the network as a functioning ecosystem rather than a conscious organism.
What mycorrhizal networks actually are
The word "mycorrhiza" comes from Greek roots meaning "fungus" plus "root." It describes the symbiotic relationship between specific soil fungi and the roots of about 90 percent of land plants. The fungi grow inside or around the plant roots, then extend long threadlike structures called hyphae far out into the surrounding soil. When the hyphae of one plant's fungal partner meet and fuse with the hyphae attached to another plant, you get a network.
Two main types matter for backyard gardeners:
- Arbuscular mycorrhizae (AM), formed by fungi in the phylum Glomeromycota. These partner with most vegetable garden plants: tomatoes, peppers, corn, beans, squash, strawberries, fruit trees, herbs, grasses, and ornamentals. The fungi penetrate into plant root cells and form tree-shaped structures called arbuscules where nutrients are exchanged.
- Ectomycorrhizae (EM), formed by fungi that wrap around root cells without penetrating them. EM partners are mostly trees: oak, pine, beech, birch, maple. If you have fruit trees or shade trees in your yard, they likely host EM fungi.
Source: Arbuscular mycorrhizal structure documented by SPUN (Society for the Protection of Underground Networks).
What plants share through the network
Suzanne Simard's 1997 Nature paper was the first to use radioactive carbon tracers to prove that carbon moved between trees through mycorrhizal networks. Since then, peer-reviewed studies have documented transfer of several substances:
- Carbon (sugars). Trees and plants in shade can receive sugars from neighbors with more sun access. Documented in PMC studies of belowground carbon transfer.
- Nitrogen and phosphorus. Mycorrhizal networks move these nutrients between connected plants, particularly to plants that need them more, per Frontiers in Plant Science research.
- Water. During drought, mycorrhizae act as extensions of plant root systems, pulling moisture from soil zones too far for roots to reach alone.
- Defense signals. Babikova et al. 2013 (Ecology Letters) documented that when one bean plant was attacked by aphids, neighboring plants connected by mycorrhizal networks received chemical signals and began producing defensive volatile compounds before the aphids reached them.
Why This Works (the permaculture lens)
Companion planting works partly because companions feed and protect each other through the mycorrhizal network. A diverse polyculture with no tillage builds a thick belowground web that turns the entire bed into a single connected organism. This is the same principle as a forest, scaled to your backyard garden's underground network. The plants that share well do better together than apart.
Plants that participate (and the ones that do not)
| Plants that form mycorrhizae | Plants that do NOT |
| Tomatoes, peppers, eggplant (Solanaceae) | Broccoli, cabbage, kale, Brussels sprouts (Brassicaceae) |
| Corn, beans, peas, squash, melons | Mustard, radish, turnip, arugula (Brassicaceae) |
| Strawberries, raspberries, blueberries | Spinach, beets, chard (Chenopodiaceae/Amaranthaceae) |
| Fruit trees, nut trees, most herbs | Buckwheat (Polygonaceae) |
| Roses, lavender, native flowers | Most knotweeds |
Source: Non-mycorrhizal plant families documented at mycorrhizas.info and Britannica's Brassicaceae list.
The non-mycorrhizal families produce glucosinolates (brassicas) or other compounds that suppress fungal colonization. This is one reason why eggplant and brassicas are not great neighbors. The eggplant wants its mycorrhizal partners and the brassicas suppress them.
How tilling destroys the network
Mycorrhizal hyphae are physically delicate. They extend through pore spaces between soil particles, often reaching 10 to 50 times further than plant roots alone. A peer-reviewed PubMed study on tillage impact documented 30 to 50 percent reductions in mycorrhizal colonization after conventional tillage. The hyphae get cut into pieces, and recolonization takes 2 to 3 growing seasons under typical garden conditions.
This is one of the strongest arguments for no-dig gardening. Every time you rototill or double-dig a bed, you break the network that was helping your plants. The plants that grow back next season have to rebuild everything from spores in the soil.
How synthetic phosphorus fertilizer suppresses mycorrhizae
This is the counterintuitive one. Plants form mycorrhizal partnerships partly because the fungi help them access phosphorus that is otherwise bound in soil minerals. If you flood the soil with synthetic phosphorus fertilizer, plants no longer need the fungal partnership and stop investing carbon in feeding the fungi.
PMC research on phosphorus availability and mycorrhizal resistance documents this trade-off: high-P soils show 40 to 70 percent lower mycorrhizal colonization. Over multiple seasons of heavy fertilizer use, the network can effectively disappear from a bed even if the spores are still present.
The practical implication is to use organic phosphorus sources (compost, bone meal, rock phosphate) at modest rates and let the mycorrhizal network do its job, rather than dumping cheap synthetic phosphate that short-circuits the relationship.
Using mycorrhizal inoculant in your garden
Commercial mycorrhizal inoculants are spore-based powders or granular products containing dormant Glomeromycota spores. Rodale Institute's inoculum guide documents when they are worth using and when they are not.
Worth using: new garden beds, soil that has been heavily tilled or sprayed, container plantings, transplants going into compacted soil, after building disturbance.
Not worth using: existing no-dig beds with active soil life, beds that already grow healthy mycorrhizal-hosting plants, soil that has been resting under mulch.
Inoculant Reality Check
Mycorrhizal inoculants need to touch the root system to work. Sprinkling powder on the soil surface and watering it in is mostly wasted. Apply directly to the root ball at planting, or dust into the planting hole where the roots will sit. Match the inoculant type to the plant: AM products for vegetables and most plants, EM products for trees in the pine, oak, beech, birch families.
What the science still does not say
The "wood wide web" story became famous in part because of Suzanne Simard's mother tree research and popular books that described forests as conscious cooperative ecosystems. Recent peer-reviewed work has pushed back on some of the strongest claims.
Undark Magazine's 2023 coverage of the Karst, Jones, and Hoeksema critique documents that some "mother tree" claims (mothers deliberately favoring kin, networks as forest-wide information systems) are not as well-supported as the popular narrative suggests. The basic phenomenon (carbon, nutrients, signals moving through networks) is solid science. The interpretation (forests as conscious cooperative organisms) is more speculative.
For a backyard gardener, this distinction does not change much. The network exists and it benefits plants whether or not trees are deliberately nurturing offspring. The practical gardening choices (avoid tilling, avoid heavy synthetic phosphorus, plant diverse polycultures, consider inoculant at planting) work regardless of the metaphysics.
Five things you can do this season
Stop tilling existing beds
Switch to no-dig methods. Top with compost and mulch instead of rotating the soil. Existing networks recover in 1 to 2 growing seasons.
Use mycorrhizal inoculant on transplants
Dust the root ball or planting hole of tomato, pepper, fruit tree, or other mycorrhizal transplants. Skip brassicas and beets where it does not work.
Cut synthetic phosphorus to zero
Replace with compost, bone meal, or rock phosphate. Organic phosphorus releases slowly enough that plants keep investing in mycorrhizal partners.
Plant diverse polycultures
Mix mycorrhizal-hosting plants throughout the bed. A diverse network with multiple plant partners is more resilient than a monoculture.
Separate brassicas from mycorrhizal-dependent crops
Plant cabbage family crops in their own rotation block. The glucosinolates they produce suppress mycorrhizae in nearby beds.
For broader context see our companion planting chart, our soil microbes guide, and the wider soil health pillar.
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Read the Free GuideFrequently Asked Questions
What is a mycorrhizal network?
A mycorrhizal network is the underground web of fungal threads (hyphae) connecting the roots of plants in a shared soil ecosystem. The fungi (mostly in the Glomeromycota phylum for vegetables) trade water, nutrients, and chemical signals with plant roots while also linking different plants to each other.
Is the "wood wide web" real?
Yes, the underlying science is real. Peer-reviewed studies since Suzanne Simard's 1997 Nature paper have documented carbon, nutrient, and chemical signal transfer between plants through fungal networks. Recent 2023 critiques have tempered some of the strongest interpretations (forests as conscious cooperative organisms) but the basic phenomenon is solid.
Which garden plants benefit from mycorrhizal fungi?
About 90 percent of plant species, including tomatoes, peppers, corn, beans, squash, fruit trees, berries, roses, and most herbs. The main exceptions are the brassica family (cabbage, broccoli, kale, mustard, radish), chenopods (spinach, beets, chard), and buckwheat.
How do I encourage mycorrhizae in my garden?
Stop tilling. Reduce or eliminate synthetic phosphorus fertilizer. Plant diverse polycultures with mycorrhizal-hosting species. Mulch heavily. Use mycorrhizal inoculant on transplants going into new or disturbed beds. Avoid heavy chemical pesticides that suppress soil fungi.
Are mycorrhizal inoculants worth buying?
Worth it for new garden beds, transplants into disturbed or compacted soil, container plantings, and after building construction. Not worth it for established no-dig beds with healthy soil life, where the spores are likely already present and active.
Does fertilizer kill mycorrhizal fungi?
Synthetic phosphorus fertilizer suppresses mycorrhizal colonization because plants stop investing carbon in fungal partners when phosphorus is abundant in soluble form. Studies show 40 to 70 percent reductions in colonization under heavy P fertilization. Organic phosphorus sources (compost, bone meal) do not suppress mycorrhizae because they release nutrients slowly.
Who coined "the wood wide web"?
The term was coined by Helen Hooper in a 1997 Nature commentary accompanying Suzanne Simard's research paper on belowground carbon transfer between trees. The phrase has become widely used in popular science writing about mycorrhizal networks.
Build a Living Garden System
Our free permaculture starter guide walks you through soil health, companion planting, mycorrhizal networks, and food forest design. Practical steps designed for weekend gardeners with limited time and real budgets.
Start with the Free GuideResources
- Net Transfer of Carbon Between Ectomycorrhizal Tree Species (Simard 1997, Cornell PDF)
- Underground Signals via Common Mycelial Networks (Babikova 2013 Ecology Letters)
- Belowground Carbon Transfer Across Mycorrhizal Networks (PMC)
- Phosphorus Availability and Mycorrhiza-Induced Resistance (PMC)
- Impact of Tillage on Arbuscular Mycorrhizal Fungal Community (PubMed)
- Where the Wood Wide Web Narrative Went Wrong (Undark Magazine)
- Mycorrhizal Fungi Explainer (SPUN)
- Mother Tree Project (Suzanne Simard)
- Mycorrhizal Fungi: Beneficial Microbes (Cornell Cooperative Extension PDF)
- Mycorrhizae Underground Support Network (UC ANR)
- On-Farm Mycorrhizal Inoculum Production (Rodale Institute)
- Non-Mycorrhizal Plants Reference List
