The vast majority of plants on our planet – some 80% in the wild – live in symbiosis with beneficial fungi called mycorrhizal fungi or mycorrhizae. A mycorrhiza (from the Greek mycos for fungus and riza for roots) is a symbiotic relationship between a fungus and the roots of a vascular plant.
Essentially, mycorrhizae act like root extensions: their hyphae (the “roots” of the fungus) travel further out into the soil than the plant’s own roots, allowing the plant to obtain more minerals and water than the roots could have done on their own. In return for the extra food and water (after all, this is a symbiotic – that is, mutually beneficial – relationship), the fungi receive small quantities of carbohydrates, such as sugar and glucose.
Plants that do form mycorrhizal associations establish more quickly and grow faster and more densely than plants that don’t form such an association, get along with much less water and fertilizer, and are more productive as well, often considerably so. Moreover, several studies suggest that mycorrhized plants are often less susceptible to soil-borne diseases as well.
Mycorrhizal fungi are doubly attractive in a world where man disrupts the environment by releasing vast amounts of carbon dioxide into the air because they absorb carbon, often as much if not more than the plants they feed, and thus act as a living carbon sink.
In most natural soils, mycorrhizal fungi spores are abundant and quickly fix onto new plants as they germinate. However, in soils that have been disturbed by residential construction, frequent tillage (hoeing, rototilling, etc.) and applications of fertilizer, pesticides and other chemical products, the mycorrhizae content can be considerably diminished and may be insufficient to significantly enhance plant growth. Commercial potting soils (unless mycorrhizae have been added) are also usually devoid of beneficial fungi.
Since the early 2000s, formulations of mycorrhizal fungus spores have been available commercially. They can be sprinkled onto roots of plants as they are planted (for quantities, simply follow the instructions on the label) or added to the soil before seeding or taking cuttings. Considering that my yard was severely disturbed (it was originally covered in asphalt), I’ve gotten into the habit of simply adding mycorrhizae when I plant anything. I also use potting soil that already contains mycorrhizae, so my seedlings, cuttings and container plants can quickly start a symbiotic relationship.
Certain plants, for unknown reasons, do not form mycorrhizal associations. This is the case of entire families of plants, including crucifers (cabbage, cress, alyssum, etc.), chenopodes (beet, spinach, etc.), carnations and pinks (Dianthus) and most Crassulaceae (jade plant, sedum, kalanchoe, etc.). However, no harm comes from applying mycorhizae spores to plants that are non-mycorrhizal, so if it is more convent to simply apply mycorrhizae to all your plants without having to figure out their family relationship, go ahead. The roots of non-mycorrhizal plants so treated will simply not be colonized, that’s all.
Which Mycorhizae to Use?
Most herbaceous plants readily take up mycorrhizal associations with Glomus intraradices, by far the world’s most widely available mycorhizal fungus and the one most commonly offered in commercial mycorrhizal formulations. However, many woody plants form associations with other types of fungi. Some companies offer a blend of various mycorhizae, including those adapted to woody plants. If you have a choice, therefore, I suggest opting for “landscape” blends or “tree and shrub” blends and using them on all your plants: they contain the widest range of mycorhizae species and therefore are more likely to form mycorhizal associations with both herbaceous and woody plants of all kinds.
Currently, however, there is no commercial source for the type of mycorrhizae that colonize the roots of ericaceous plants (rhododendrons, blueberries, heather, etc.). When you plant rhodos and their friends, you’ll just have to hope that Mother Nature will supply the fungi they need or that the soil in the pots they are sold in was somehow inoculated.
Orchids, too, have their own special mycorrhizal fungi that are not included in commercial formulations. However, orchids are a special case: they generally only need their fungal friends to help their seeds germinate. Orchid-growers long ago learned how to germinate orchid seeds in test tubes using special nutrient formulations that get around the need for mycorrhizal fungi.
Avoid Phosphorous-rich Fertilizers
One final point: up to 80% of the phosphorus absorbed by plants is by mycorrhizae, but if there is a large quantity of phosphorous already present in the soil, plants will generally fail to form mycorrhizal associations. For that reason, it is best to avoid using high phosphorous fertilizers (those whose middle number is greater than 14) if you have applied or will be applying mycorrhizae.
If you’re a gardener, and especially if you garden in containers or disturbed soil, mycorrhizal fungi may be well worth trying. It’s worked for me!