By Lauren Lewis
The weeks following our late winter rains, when the soil is as soaked as it’s ever going to be in SF, is when we’re most likely to find mushrooms in our gardens. If you do, it’s a reason to rejoice, because a mushroom is the above-ground evidence of fungi in the soil, and it’s hard to overstate just how important fungi are for healthy soil and healthy plants. We mostly don’t even notice its presence, but our plants depend on fungi for their growth.
Around 90% of all plants form a mycorrhiza with fungi, a symbiotic relationship whereby the hyphae of the fungus (a fungus’s massive underground network of microscopic filaments) connect to the plants’ roots and nutrients are passed between plant and fungus. Fungi receive carbon that was photosynthesized by the plant, and the plant receives…so much assistance. Nutrients: mycchorizae supply roots with necessary nutrients like phosphorus that are hard for the plant to absorb independently. The fungi exude acids that break down rock and solidified soil, which turns existing nutrients into a form that’s absorbable by plant roots. Water: the network of hyphae in effect expands a plant’s surface area in the soil, letting the plant reach more available water. Mycorrhizal soil also tends to have greater water retention capacity, so there’s more water for the roots and hyphae to reach. Communication: this underground network also creates a means of communication between plants– a plant enduring a pest attack sends out chemical signals to neighboring plants, and the mycchorizal network allows the warning signal to reach farther. The list of benefits received between fungus and plant goes on.
From a gardening perspective, we can celebrate fungi for their ability to support perennial plant growth over annual (i.e. weedy) growth. Research has shown that fungal soils can deter growth of weeds, while supporting other plants in the ways described above. Over time, myccorhizae create a soil environment that’s hostile to more short-lived plant species and welcoming to long-lived plants, mimicking mature “wild” plant communities.
Fungi support our gardening ambitions, and they can also help fix the messes we humans have made. In urban settings, where soils are often contaminated with oils and petrochemicals, fungi offer a grassroots solution via mycoremediation. Most fungi get their energy by breaking down large carbon-based molecules, like those in wood, which means they’re also good at breaking down petroleum and related carbon-based chemicals. Communities are starting to employ controlled use of fungi to clean up soils, and there is data to support the practice. It’s a method that can be done piecemeal, by leaving discovered garden mushrooms to do their thing and avoiding fungicides, or on a community level to solve more serious contamination. (To sate all your fungal curiosity, check out the indispensable book Mycelium Running: How Mushrooms Can Help Save the World, by Paul Stamets.)