Mycorrhizae: What they are and how gardeners can protect them.

Fungi growing in wood mulch – photo by Jane Zoutman

By Jane Zoutman, Master Gardener

Mycorrhizae means fungus roots. A mycorrhiza is a symbiotic relationship between fungi and plant roots. Fungi are very common in the soil and many types require this symbiotic relationship with plants for long-term survival. The relationship is also necessary for most plants, from trees to annuals, to thrive.

This relationship is beneficial to both the plant and the fungus. The fungus attaches to the plant roots and sends out hyphae (very small root like structures) to places in the soil the plants roots are too large to go. The fungus provides the plant with water and nutrients like phosphorus and other minerals it cannot otherwise access. The fungus absorbs carbohydrates and other nutrients it cannot produce from the plant. The fungi form an interconnected network between many plants and trees. The fungus allows the plants to thrive and because the plants are healthier they are better able to withstand environmental stress, pests and disease.

It is estimated that there are over 20,000 species of mycorrhizal fungi. They have always existed in our soils and their spores can be found in the soil and near the surface in the organic matter. These fungi form a symbiotic relationship with about 80% of all plant species and 100% of all trees. How do we know if mycorrhizal fungi are in our soils? Look for mushrooms.

There are two main types of mycorrhizal fungi: ectomycorrhizae and endomycorrhizae. Ectomycorrhizae are a large number of fungi that associate with a limited number of tree species: most conifers, larch, beech, birch, willow, oak, eucalyptus, etc. Ectomycorrhizae do not penetrate the roots, instead they cover the ends of young roots. Ectomycorrhizae are generally very specific to some plant species. This is also the type of fungus that produces mushrooms.

Endomycorrhizae penetrate the root cells to maximize nutrient and carbon exchange. There are a limited number of fungi species in this group, which associate with about 80% to 90% of the world’s plant species that include grasses, herbs, and tropical trees. Endomycorrhizae are commercially available and are found in containers at our local nursery.

All the mycorrhizal fungi our plants need are already in the soil but we can damage the mycorrhizal fungi in our soils by: removing the top soil, unnecessary fertilizer use (especially phosphate), excessive tilling (double digging and rototilling), and through soil compaction and flooding. Overuse of phosphate fertilizers inhibit mycorrhizal activity. Excessive use of tilling destroys the mycorrhizal hyphae and the soil structure. Compacting of soils and flooding reduces the amount oxygen in the soil, since mycorrhizae require a certain amount of oxygen in the soil, they will be unable to establish. Mycorrhizae can recover and re-establish within a system if given enough time and damaging activities are avoided.

We can purchase mycorrhizal fungi in plastic containers at most plant nurseries. There is some evidence that the use of these products by nurseries and greenhouses to grow plants in sterile growing medium has some benefits.

Studies have shown that when the commercial mycorrhizae are added to gardens when new plants are planted, the added commercial mycorrhizae disappear and are replaced by native mycorrhizae, which are better adapted to the local ecosystem. There is also the question of what is in those plastic containers? Are the mycorrhizae even suitable for our soil and climate; are they the appropriate type for the plants we are planting; are they even alive after sitting for months/years on a shelf?

What can we do to benefit the naturally occurring mycorrhizae already in our soils?
1. Have our soil tested and add only the fertilizers that are needed.
2. Do not dig, double dig or rototill our gardens.
3. Avoid compacting the soil. Don’t walk in your garden. Create paths with a thick layer of mulch or straw.
4. Use compost but use it as a top dressing – do not dig it in. The organic matter will find its way naturally into the soil.
5. Use arborist’s wood mulch. This is a good source of fungal spores.
6. If you want to introduce mycorrhizal fungi into garden containers use some of the soil from your garden.
7. Plant a diversity of plants in your landscape, especially trees and shrubs, as this will encourage mycorrhizae diversity.

Further information on the topic:
• “A Gardener’s Primer to Mycorrhizae: Understanding How They Work and Learning How to Protect Them” Linda Chalker-Scott Washington State University
• Allies in the Soil: Mycorrhizal Fungi | Dr. Adam Cobb, Soil Food Web School – online presentation
• FINDING THE MOTHER TREE, Discovering the Wisdom of the Forest, Suzanne Simard, ALLEN LANE/PENGUIN RANDOM

Published:  September 2022