June Garden Update – Underground Fungi Keep the Garden Alive

 Photo 1

The 20th Street Garden is over two and a half years old, growing under nearly thirty-year-old trees. Over this time, the garden has been managed lightly without chemical fertilizers. No new soil amendment or mulch has been added since planting. There is still occasional handwatering but not much rain has fallen. How do the plants remain green and alive? How have the street trees managed to grow during decades of neglect between their planting in 1987 and the installation of the garden in 2012?

Photo 2
(Courtesy of Dr. Jim Downer, University of California Cooperative Extension, Ventura County)

The secret is underground, living on the roots. Mycorrhizae (my-ko-RYE-zee -“fungus roots”) are fungi that naturally colonize the roots of plants, allowing them to take up water and nutrients more efficiently. It is estimated that over 90% of the world’s plants have a relationship with mycorrhizal fungi. Without these organisms most plants would fail to thrive.

This is an old arrangement – these fungi have co-evolved with plants for the last four hundred million years. Although there are some generalist mycorrhizae, most have adapted to live with a specific plant species – meaning there are at least as many kinds of mycorrhizae as there are kinds of plants in the world.

Photo 3
(Courtesy of Dr. Jim Downer, University of California Cooperative Extension, Ventura County)

The relationship is beneficial to both parties. The fungi live on sugars that the plant produces via photosynthesis. As the fungi grow, they create a fibrous network spreading throughout the soil. These fibers increase the surface area of the roots 100 to 1000 times, making them more able to take up water and nutrients. Mycorrhizae also play a role in protecting the roots from disease-causing fungi in the soil.

There is still much to learn about what these amazing fungi are capable of. Some studies show that mycorrhizae can hoard nitrogen, a vital plant nutrient, when surrounding soil nitrogen is low. One species lures and kills a specific soil-dwelling insect in order to absorb nitrogen from its body, then share this nitrogen with the plant.

Photo 4
(Courtesy of Dr. Jim Downer, University of California Cooperative Extension, Ventura County)

If mycorrhizae are so great, can we package them, sell them, and use them to improve plant health? Many companies do sell “mycorrhizal inoculants” as an additive to your garden soil. It sounds like a good idea, but not so fast! Dr. Jim Downer from the University of California shares the following cautions in an online presentation available at http://www.sfp.ucdavis.edu:

• In independent tests, about 50% of the inoculants did not have viable spores – no living mycorrhizae.
• Most plants evolve with their own species of mycorrhizae, and these products cannot possibly contain all the species that every plant would need.
• The inoculants contain other ingredients such as wetting agents and fertilizer which are beneficial for plants on their own. It is impossible to test the effectiveness of the commercial mycorrhizae separately from these additives. Until the commercial mycorrhizae can be independently tested for effectiveness, no science-based horticulturalist can confidently recommend them.

Photo 5

The good news is, it is possible to encourage the natural mycorrhizal populations in your soil with the following methods (from the Colorado State Extension website http://www.ext.colostate.edu/):

• Add organic matter. Apply compost and/or wood-chip mulch on top of the soil, and sweep fallen leaves and other organic debris into the garden bed rather than take it away. Don’t use plastic sheets under the mulch as it will limit water and air movement.
• Water but don’t over-water – soggy soil inhibits many beneficial soil organisms.
• Avoid unnecessary roto-tilling as it will destroy mycorrhizae.
• Avoid unnecessary use of pesticides especially fungicides. Determine that you actually need a fungicide before using, and use only in the amounts and locations necessary.

As beautiful as the garden is above ground, what’s going on below is far more interesting! Let’s appreciate fungi both on our plates and for their work in the soil keeping all of us alive.

Ellyn Shea is a gardener and garden consultant in San Francisco. Visit her at http://www.garden-guidance.com

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