Mycorrhizal fungi are specialized soil organisms that form a mutually beneficial, symbiotic relationship with approximately 80% of all land plants, significantly expanding the plant’s root system. The term “mycorrhiza” literally means “fungus-root,” describing an extensive network of microscopic fungal threads, called hyphae. These threads reach far beyond the plant’s own roots to forage for nutrients and water, allowing the plant to efficiently access resources like phosphorus and nitrogen. In return, the plant supplies the fungi with carbohydrates produced through photosynthesis. Introducing these beneficial fungi can improve plant health, growth, and resilience to environmental stressors like drought. Effective application requires understanding the different fungal types, selecting the right product, and ensuring proper post-application care.
Understanding Mycorrhizal Types
The fungi that form these beneficial associations are broadly categorized into two major groups based on how the fungal hyphae interact with the plant’s root cells. The most common type is Endomycorrhizae, also known as Arbuscular Mycorrhizae (AM), which colonizes approximately 80 to 85% of all plant species. Endomycorrhizal fungi penetrate the cell wall of the plant’s root cortex cells, forming highly branched structures called arbuscules that facilitate the nutrient exchange. This group is generally used for common garden vegetables, grasses, flowers, and most agricultural crops.
Ectomycorrhizae (ECM) is associated with a smaller percentage of plants, typically 5 to 10% of plant species, primarily specific trees. Ectomycorrhizal fungi do not penetrate the root cells but instead form a dense sheath, called a mantle, around the root tips. They also grow between the root’s cortical cells, creating a network known as the Hartig net. This type of fungi is important for certain hardwood and coniferous trees like pines, oaks, beeches, and birches. Applying the wrong fungal group will not result in colonization and offers no benefit.
Selecting the Appropriate Inoculant Product
Mycorrhizal inoculants are commercially available in several forms, each suited to different application methods. Dry powders are often the most concentrated form, consisting of fungal spores and hyphae mixed with a carrier like clay or talc. Powders are typically used for direct root dusting or for making a root-dip slurry, providing immediate, high-concentration contact for seedlings or bare-root plants.
Granular products incorporate the fungi into water-insoluble pellets, often mixed with a carrier like compost or volcanic rock. These are best suited for mixing directly into the planting hole or blending into bulk potting mixes for localized release. Liquid concentrates suspend spores in water, allowing application as a soil drench or through irrigation systems. This method is convenient for treating established plants but requires monitoring to ensure adequate spore delivery. When selecting a product, check the label for the propagule count, which indicates the concentration of viable spores or fungal fragments per unit. A higher count generally suggests a greater chance of successful colonization.
Step-by-Step Application Techniques
Successful inoculation requires fungal propagules to make direct contact with actively growing roots. The most effective application method is typically during the planting process, when the root system is fully exposed. For small plants, seedlings, or bare-root transplants, the root dipping or dusting technique is effective. Before placing the plant in the soil, the root ball should be lightly dusted with the powder or dipped into a slurry mixed with non-chlorinated water.
When planting a larger shrub or tree, or when using granular inoculants, the product should be mixed directly into the planting hole. Apply the recommended dosage to the sides and bottom, ensuring the granules are positioned where the new roots will grow. This localized application maximizes the chance of the fungi encountering the root tips as the plant establishes itself. Always follow the product’s dosage instructions, which often specify the amount per plant size or container volume.
Established plants can still be inoculated, though the process is more challenging because the fungi must reach the existing root zone. A liquid drench is effective for this purpose; the soluble inoculant is mixed with non-chlorinated water and poured directly around the plant’s base, allowing the solution to soak down to the roots.
For trees and deep-rooted plants, soil injection tools can deliver the liquid inoculant directly into the root zone several inches below the surface. Top dressing or lightly tilling a granular product into the soil surface is a less direct method, but it can be used if the product is watered in thoroughly to help move the spores toward the roots.
Optimizing Conditions for Fungal Establishment
The environment immediately surrounding the roots plays a large part in successful fungal symbiosis establishment. Mycorrhizal fungi are suppressed by high levels of readily available phosphorus (P) in the soil. Plants signal the fungi to colonize when they detect low P levels, as the fungi’s primary benefit is P uptake. Therefore, use fertilizers low in phosphorus at the time of inoculation to encourage the plant to establish the partnership.
The fungi are living organisms, and their viability is sensitive to certain chemicals. Systemic fungicides, which are absorbed by the plant, can inhibit the growth and colonization of the beneficial fungi. If fungicides must be used, select compatible products or wait approximately three weeks after inoculation to allow the symbiosis to establish. Maintaining moderate, consistent soil moisture and ensuring the soil pH is within a general range of 6.0 to 7.5 will also support the fungi’s initial growth.