A couple of weeks ago I was talking with a local landscaper. At one point I asked about mixing organic amendment into the soil. “Oh no,” she replied, “I use (gobbledygook).” I had no idea what she said. It sounded like “Mygo Rygo.” A few days later, I asked my gardener about it. She said “Oh, you mean mycorrhizal fungi.” What? The next day I asked my mom, who said “Oh, yes, I read a whole book about that.” So maybe I’ve been living under a log (with the fungi?), but just in case you have been too, this post is a brief introduction to mycorrhizal fungi and some suggestions for applying this knowledge to help the plants in your yard.

Mycorrhizal fungi are organisms that live in the soil, embed in the roots of plants, and support the plants by effectively extending their root system and helping to extract and transport minerals and water from the surrounding soil. The fungi also benefit from this symbiotic relationship because the plant sends down sugars to feed the fungi, which do not have access to the sun and photosynthesis.


This diagram enumerates many of the positive effects of a common type of mycorrhizal fungi (AMF), from a paper by Catherine N. Jacott, Jeremy D. Murray and Christopher
J. Ridout, licensed under Creative Commons.

In many cases this relationship is not just a nice-to-have, it’s a necessity. Tom Bruns, Professor Emeritus of Fungal Ecology and Systematics at UC Berkeley, was interviewed for The Mushroom Hour Podcast a few years ago. He described an experiment at the edge of a Point Reyes forest, in which pine seedlings farther from the edge were less likely to be colonized by mycorrhizal spores. “In a setting like Marin County, where you have the seasonal dry, if they’re not colonized that first year, they’re not there that second year. It’s find a fungus or die.”

Land plants have always been this way, having evolved with this relationship 450 million years ago. At the time, the Earth was largely an oceanic planet, and plants like algae could easily find the nutrients they needed dissolved in the water around them. As the oceans receded, plants had to figure out how to survive on land. As Bruns put it: “The colonization of the land was not an easy thing for marine or freshwater plants to do. It was a stressful environment. Instead of being bathed in a nutrient broth, they had to find those nutrients in the soil, probably very little soil. From the get-go, fungi and plants have had this partnership.”

Amazingly, scientists have found direct fossil evidence of plant-fungal interactions from that time. A genetic analysis of modern-day plants shows that nearly all of them share the same genes involved in this symbiosis. Only about 5% of plants today are not mycorrhizal, and that is only because they have given up this ability along the way. (1)

There are several different classes of mycorrhizal fungi, with the two main ones being “endo” and “ecto”. About 80-85% of plants, including most green leafy plants and shrubs, most farm crops, and many deciduous trees, are endomycorrhizal. You can find a partial list here. These plants have fungi developing inside of their root systems (hence “endo”). The fungi’s small filaments (“hyphae”), extend out from the roots and help to acquire and transfer nutrients to the plant, provide some resistance to disease, and store and transfer water.

A different type of fungus works with woodier plants. Most conifers, plus some northern hardwoods like oaks, lindens, and beeches, are ectomycorrhizal. They work with a type of fungi that binds to the outside of their roots, penetrating only into the outer layer (hence “ecto”). Many of these fungi have mushrooms above ground. Mushrooms in a forest or in your yard are often a sign of an active ectomycorrhizal environment. (2)


I wanted to share with you some pictures of mycorrhizal fungi, but because of copyright restrictions that usually means I have to take them myself. I happened to be near a pine forest when writing this post, and it was easy to find these examples of fungi. But I am told they are more likely to be wood decay fungi than mycorrhizal fungi.

One of the powerful properties of mycorrhizal fungi is that they form networks through the soil, effectively connecting plants and trees with one another, allowing them to share resources and better tolerate environmental stresses. For example, researchers have traced sugars flowing from an evergreen spruce to a nearby deciduous beech in early spring, when the bare branches of the beech limited its photosynthesis. Then in summer, after the beech leafed out, the flow reversed and the conifer received the sugars. The flow reversed yet again in fall. These two distinct species were helping each other to survive.

Michael Phillips, the late farmer and author of The Mycorrhizal Planet, gives an example in a separate episode of The Mushroom Hour Podcast of how a shared mycorrhizal network can improve resistance to pests and disease. “Not only are nutrients passed on these fungal networks, but also chemical signals. Here’s an example. We’re in the garden, and on one end of the garden we’re growing some cucumbers, and cucumber beetles come along. The very first cucumber beetle bites into the very first cucumber plant, and that plant has a phytochemical reaction to try to resist the cucumber beetle. But it also does two things. It sends off volatiles in the air to alert the rest of the plants nearby, including its other plant parts, that cucumber beetles are here and you might want to up the production of a certain phytochemical compound that they don’t like so much. But that same sort of signal goes through the roots and reaches much further. And those further-out plants take that signal, and they might start producing some of those compounds. But even more so, they emit volatiles that call beneficial insects that eat cucumber beetles … ‘Come over, we’ve got a feast over here….’”

Reports about how plants communicate are sometimes laced with references to “magic” or “spirit” or “intelligence” or “superpowers”, which can seem pretty woo-woo. But the wisdom of observant forest ecologists and experienced indigenous peoples is increasingly backed by rigorous science, tracing chemical isotopes through networks, analyzing DNA, and running controlled experiments. The science of studying fungi has come a long way. Bruns describes how, when he first started working in this area, it took a long time just to identify which fungi were active. They would get a soil core, wash off the soil, extract root tips, pick them apart, and then use a dissecting microscope and then a compound microscope to try to identify what it might be. When molecular analysis came along, with more precise identification, they still had to extract out individual root tips and then amplify a particular molecule before analyzing it in a lab. “Between when we’d pull the root out and when we’d have an answer would be months. We’d have two days of field work and then six months of lab work to figure out what we saw…. Our resolution of which species were out there and which ones were dominant was embarrassingly bad.” But the tools now are much better; you can get DNA from the soil, sequence everything in it, and have a computer identify all of the fungi in it in days. “The whole game has changed.”

Work in this field has taken off over the last decade, and that is a good thing. The more we know about this critical underground support of our plant world, the more we can help to fortify our forests and crops and even our yards from the changes in temperature, moisture, pests, and disease that are coming with climate change. We can also better understand how to use these fungal networks to store more carbon underground.

You may be wondering how this applies to your yard, where you are unlikely to extract a soil sample and send it off for DNA analysis. One thing you can do is to start paying attention to what your plants are growing in. Phillips, a farmer with a practical streak, put it this way: “The soil is not dirt. The soil is a medium where all kinds of life lives -- the bacteria, the fungi, all kinds of organisms -- working together to bring nutrients to plants…. When you come to really honor this, you become a better grower. By honoring it, it’s mostly about not screwing it up, not disturbing it, not breaking apart the mycelium, not pouring chemical insecticides and herbicides onto that ground.”

That might mean, for example, using a fork instead of a rototiller to aerate the soil, and choosing to do surface mulching instead of stirring in amendment. (3) Putting in a diversity of plants, not too widely spaced, can provide extra support for the plants via a shared mycorrhizal network, and more photosynthesis will bring more nutrition to the fungi, which in turn will strengthen the underground network and further fortify the plants. (4)

Consider taking some soil from a place with a good diversity of healthy plants, one where there are no fertilizers or herbicides in use. That soil is likely to have a healthy population of many types of mycorrhizal fungi. If you take some soil near roots, or even with some root tips in it, those fungi can take up residence in your own garden. You can put the soil in a new planting hole, or just scrape off some mulch and put the soil near the root tips of some plants that would benefit.

Commercial products with mycorrhizal spores are also available. The better ones will have many different types of spores in them, since some work better with some plants than others, and more generally we want to maintain a diverse underground ecosystem. Phillips recommends sprinkling some in planting holes to help reduce transplant stress. There are a few different ways to apply it, and once should be enough as the fungi will propagate within the garden once they attach to a root system. The mycorrhizal fungi, unlike fertilizers, will help to improve soil structure, prevent root disease, facilitate inter-plant signaling, and absorb and store water, while also nourishing the plants. They complement compost or other amendments by helping to break down organic material and extract the nutrients. And they will not pollute the water or lead to excess emissions, as fertilizers can do. You can find some pictures showing growth with and without mycorrhizae here.

I am really looking forward to trying some of these ideas. I am also hoping that some of you are already on top of this and have some experience that you can share. Please let us know in the comments.

Notes and References
0. For those that want to learn more, I really enjoyed The Mushroom Hour Podcast episode 29 with UC Berkeley Professor Emeritus Tom Burns, and episode 53 with the late farmer and author Michael Phillips. There are numerous books and even TED talks on the topic as well. Some of you will be familiar with Suzanne Simard’s work and/or her doppelganger in Richard Powers’ The Overstory. You can find some information related to mycorrhizae and home gardening at Pacific Horticulture, including pointers to commercial products.

1. The members of the mustard family are probably the most common plants that are not mycorrhizal.

2. For those of you who have seen snow plants in conifer forests in the spring, these are also signs of a robust ectomycorrhizal network. The snow plants, which have no chlorophyll and so are unable to photosynthesize, are parasites, taking the sugars from the mycorrhizal network, essentially robbing the fungi of its food and indirectly harming the conifers.


Credit to Max Listgarten for the photo on the left.

3. Phillips mentioned chipped wood as one of the mulches that he likes, and specifically endorsed small branches (2 inches or less), because they have relatively more cambium, where the nutrients are stored.

4. Not everything is kumbaya in the plant world. Some plants only get along with certain kinds of fungi and vice versa, particularly on the ecto side of things. (Endomycorrhizal plants tend to be more generalists.) So not all groupings of plants, or plant-fungal groupings, work well. As far as I can tell, the study of this is still a work in progress, as with many mycorrhizal-related questions.

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