Mycorrhizae are beneficial fungi that form a symbiotic association with plant roots. The fungi create a vast network of microscopic threads, known as hyphae, which extend the plant’s root system. This extensive network allows the plant to scavenge for and absorb water and nutrients, particularly phosphorus, that would otherwise be inaccessible. Applying these fungi to established plants can improve overall health, enhance resilience to environmental stresses like drought, and increase nutrient uptake efficiency. The process requires ensuring direct contact between the fungal spores and the plant’s active root tips to initiate this long-term biological partnership.
Selecting the Appropriate Mycorrhizal Type
Selecting the correct fungal type is necessary, as plants form relationships with specific groups of fungi. The two primary groups are Endomycorrhizae, also known as Arbuscular Mycorrhizae (AM), and Ectomycorrhizae. Endomycorrhizae are the most common type, associating with roughly 80 to 85% of all plant species, including most vegetables, grasses, shrubs, and many common trees. Their hyphae penetrate the cortical cells of the plant root to facilitate nutrient exchange within specialized structures called arbuscules.
Ectomycorrhizae form a dense sheath or mantle around the root tip and grow between the root cells without penetrating them. This type is primarily found in about 5 to 10% of plant species, most notably conifers and specific hardwood trees such as oaks, pines, and birches. Before purchasing an inoculant, confirm which type of fungi your established plant requires to ensure successful colonization.
Commercial mycorrhizal products are typically available as dry powders, granules, or wettable liquid concentrates. The product form often dictates the best application method: powders and granules are used for top dressing or mixing into a slurry, while liquid concentrates are mixed with water for root drenching or deep root injection.
Direct Application Methods for Established Plants
The goal of application is to introduce fungal spores directly to the actively growing root tips beneath the soil surface. For smaller plants in garden beds or containers, the soil drench method using a liquid or soluble product is the simplest approach. After mixing the inoculant with non-chlorinated water, pour the solution over the root zone, ensuring a deep soak that carries the spores down to the roots.
For larger, established trees and shrubs, accessing the active root zone requires more targeted methods. Deep root injection or augering is often the most effective technique, especially for compacted soils. This involves creating a series of holes, six to twelve inches deep, within the area beneath the canopy using a soil auger or probe.
These holes are then filled with either a slurry of the product and water or a measured amount of dry granular inoculant. Creating these access points ensures the inoculum bypasses the surface soil and reaches the fine, fibrous roots where colonization can occur. An alternative for established beds or shallow-rooted plants is top dressing, which involves gently scraping back surface mulch. The granular inoculant is then lightly worked into the top few inches of the root zone, followed by a thorough watering.
Optimizing Soil Conditions and Timing
Timing and soil management are necessary to ensure the applied mycorrhizae successfully colonize the plant’s roots. The fungi work best when applied during periods of active root growth, such as the start of the growing season or during cooler, moist periods. Avoid times of extreme heat or drought. Consistent moisture after application is important for spore germination and hyphal growth, but the soil should not be allowed to become waterlogged.
Certain common garden practices can inhibit the establishment of the fungal relationship. High concentrations of water-soluble phosphorus fertilizer suppress the chemical signal required to initiate colonization. Avoid applying high-phosphorus fertilizers for at least three weeks following inoculation.
Many systemic fungicides, especially those applied as a soil drench, can be detrimental to the fungi, so a buffer period should be observed after fungicide use. Highly compacted soil can also impede fungal establishment; in such cases, aeration or minor soil amendment prior to application may improve the colonization rate. The plant itself provides the fungi with the necessary sugars, meaning they do not require additional feeding.