A mycorrhiza is a symbiotic association between a fungus and a plant root, benefiting both organisms. The term combines the Greek words for “fungus” (myco) and “root” (rhiza). This relationship is formed by fungal threads called hyphae, which colonize the plant’s root system.
The fungi greatly extend the root’s reach into the soil, creating a vast underground network. In return for carbohydrates from the plant, the fungi efficiently gather and transport water and mineral nutrients, such as phosphorus and nitrogen, back to the host. This enhanced absorption capacity is beneficial in nutrient-poor or stressed soils, helping plants achieve better overall health and tolerance to adverse conditions.
Selecting the Right Inoculant
Not all mycorrhizae are compatible with all plant species, making the selection of the correct inoculant necessary. The two most common types are arbuscular mycorrhizae (AM), also known as Endomycorrhizae, and Ectomycorrhizae. AM fungi associate with approximately 80 to 90 percent of all terrestrial plants, including most vegetables, grasses, shrubs, and fruit trees.
Ectomycorrhizae are primarily associated with certain woody plants, such as pine, oak, fir, and birch trees. Checking the product label to ensure the fungal species matches the plant type is important for successful colonization. Inoculants are commercially available in several forms, including powders, granular products, and soluble liquids or gels. The physical form chosen depends on the application method, but the goal is always to achieve direct contact between the fungal spores and the plant root.
Application Methods for Transplants and Seeds
The most effective application of mycorrhizae occurs during the initial planting phase, when direct root-to-fungus contact is most easily achieved. For seeds, coat the surface before planting. This can be done by creating a slurry of powdered inoculant mixed with water or by lightly dusting the seeds with the powder.
For seedlings and bare-root transplants, the “dusting” method is highly effective. Sprinkle the granular or powdered product directly onto the moist root ball just before placing it into the planting hole. Alternatively, roots can be dipped into a suspension of soluble inoculant mixed with non-chlorinated water, allowing the roots to absorb the solution for about ten to fifteen seconds.
When preparing a planting hole or potting mix, granular inoculants can be incorporated directly into the soil. This involves mixing the product into the backfill soil or adding a small amount to the bottom of the hole so the plant’s roots sit directly on the inoculant. After planting, watering the area with a liquid inoculant solution helps settle the soil and encourages the fungi to establish contact.
Applying Mycorrhizae to Established Plants
Applying inoculants to established plants requires different methods, as the roots are not exposed. Spreading the product on the soil surface is ineffective because the fungal spores cannot survive when exposed to sunlight. The inoculant must be placed directly into the active root zone to initiate colonization.
One method is soil drenching, which involves mixing a soluble inoculant powder into non-chlorinated water and pouring the solution around the plant’s drip line. This liquid application carries the spores down through the soil profile toward the roots.
For trees and shrubs, a more direct technique is drilling or injection. Use an auger or rod to create several small holes around the plant’s canopy edge. The granular or liquid product is then poured directly into these holes to a depth that reaches the active root system, depositing the fungi near the root tips. This localized application bypasses the surface soil and provides the necessary contact for the fungi to begin their symbiotic relationship.
Supporting Fungal Colonization
Once mycorrhizae have been applied, certain gardening practices ensure the fungi survive and successfully form their network. Consistent moisture is necessary during the initial establishment period to support the growth and spread of the fungal hyphae. Avoid heavy tillage near established plants, as this action can break the delicate hyphal networks, forcing the fungi to restart colonization.
High concentrations of certain nutrients can inhibit the fungal relationship. High phosphorus fertilizers specifically suppress mycorrhizal growth, since the plant will not need to trade its sugars for an abundant nutrient. If fertilization is necessary, using low-phosphorus or balanced fertilizers is recommended to encourage the plant to partner with the fungi for nutrient uptake. Promoting general soil health with organic matter also helps, as this improves soil structure and creates a more favorable environment for the fungal network.