(Editor's Note: This article was originally published on February 13, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)

They're rooting for us!

Mushrooms are not just for eating, and that white webby stuff you see under your organic mulch is not necessarily just the mulch slowly rotting away. Within your soil is a whole hidden world of activity that affects how well your plants grow. Teeming microorganisms in healthy soil, including fungi and bacteria, work alongside each other to break down organic matter and free up vital elements for use by your plants. But what about our gardens? Oftentimes, we are trying to grow our plants in severely disturbed soil, plots that were once farmland, or land bulldozed for housing developments. Such situations are poor in the kinds of soil biota that your plants need. To make things worse, the application of chemical fertilizer can throw things even further off balance. The out-of-balance situation fosters the growth of what we call pathogenic organisms. Our response to those is often the application of toxic pesticides. Is it no wonder why we sometimes find it difficult to grow a nice garden?

Does my plant need an innie or an outie?

Amidst the bustling action in healthy soil are fungi that have special relationships with plant roots. These fungi actually grow together with, or even within, roots and by so doing vastly expand the ability of the roots to access soil nutrients. Roots with such assistance are called "mycorrhizae", meaning "fungus roots". Pine (Pinus) and Barberry (Berberis) are two well-known genera of trees that have ectomycorrhizae, or mycorrhizae that grow externally to the root cells. An example of Rhizopogon ectomycorrhizae found on Barberry roots can be seen in the picture below, left. Another example, below right, is Amanita ectomycorrhizae on Pine roots. You can think of these fungi as the "outies". These are also the types that most often produce mushrooms in the soil near their hosts. For example, note that Amanita, shown on the pine roots, is the fly agaric mushroom, and Rhizopogon is the genus of the North American truffle.

Ectomycorrhizae on barberryAnother kind of mycorrhizal fungus is known as vesicular-arbuscular (VA), or endomycorrhizae. The thumbnail picture above shows an endomycorrhizal corn root, with spores also visible. These are the type found in association with your vegetable and flower plants. Hyphae of these fungi actually penetrate the root cells and tissue of the plants they are in association with. Endomycorrhizal fungi are the "innies". The innies are in the group of fungi known as "water molds". This group also includes such notorious pathogens as Pythium, the cause of damping-off, and Phytophthora, responsible for crown and root rot. The presence of the endomycorrhizae can help your garden plants fend off the nasties without the use of toxic chemicals.

Knowing whether your plant needs an innie or an outie is crucial to getting the kind of response you want from these special fungi. Inoculating an outie plant with an innie mycorrhizal fungus will give you little or no results. Since there are a large number of both types of fungi in nature, commercially available formulations of mycorrhizae are usually a blend of many species of the two types. Look for blends that include mycorrhizal spores, as some formulations may contain chopped-up fungal hyphae instead of spores.

Sometimes information on the mycorrhizal fungus specific to your plants is available. If it is, you can select the particular type that is best for your situation. Innies are generally non-specific as to their host, while outies are more particular to hosts.

Amanita ectomycorrhizae on pine

The difference is clear

I've grown Rainbow Eucalyptus tree seedlings with and without the mycorrhizae and the difference is dramatic. Eucalyptus happens to require the outie, or ectomycorrhizal, type of fungus. With the mycorrhizae, the plant is much greener and healthier even when fertilized less often. The fertilizer that is applied is utilized much more efficiently. Without the mycorrhizae, more fertilizer is required and the trees lack in overall health. One caution: too much fertilizer can defeat the purpose of mycorrhizal inoculation. These fungi are most helpful in making phosphorus more available to plants, so an abundance of available phosphorus in the soil will nullify their usefulness. Besides helping with nutrient uptake, mycorrhizae can aid your trees and plants in withstanding stresses such as drought and heat. They are also vitally important in restoring areas stripped of vegetation, such as some kinds of surface mining sites or waste areas.

Where can I get mycorrhizae?

Mycorrhizal blends are available from a number of online vendors. Several of these sources are Fungi Perfecti, Bio-Organics, and Mycorrhizal Products. If you are growing a variety of plants, including woody shrubs, trees, and vegetables or flowers, an endo-ectomycorrhizal blend is your best choice. For those whose primary interest is vegetable crops, an endomycorrhizal blend will be optimal. Some blends even include micronutrients with the mycorrhizae. Study the information presented on these and other websites and you will become knowledgeable about the types of mycorrhizae that will be most beneficial for your use.

Endomycorrhizal photo by Sara Wright, USDA-ARS and ectomycorrhizal pictures by Randy Molina, Forest Mycology Team, Pacific Northwest Research Station, Forestry Sciences Laboratory.