Mycorrhizae are created by fungi naturally found in the soil that form beneficial relationships with the roots of plants. In exchange for sugars provided by the plant, the slender branches of mycorrhizae offer an extended reach for nutrients and other resources and impart greater plant resilience in periods of stress.

They are of enormous importance to plants in nature, but studies have shown that the use of mycorrhizal-boosted potting soils or the addition of commercial mycorrhizal inoculants* to garden and planter soil are generally ineffective. One of the reasons is that the type of mycorrhizal network that provides benefits varies from one type of plant to another, so the inoculant needs to be very specifically chosen.  There are also a small percentage of plants that do not appear to benefit from mycorrhizal associations at all. And, cultivated plants are less reliant on these structures as their growing environment and medium are more carefully tended and improved. 

Soil that is high in organic matter will naturally invite fungi into the soil. Gardeners will usually do more for their plants by building healthy soil than by adding mycorrhizal products which may not be a match for their plants or remain in the soil. Organic compost refreshments to gardened areas, limiting soil disturbance and compaction, avoiding the use of fungicides and high phosphate products will all assist with the natural colonization of the soil by beneficial mycorrhizae. 

If you decide to try enhancing your plant's resilience by using inoculants, select a high quality product that is specific to the plant species you are growing and apply it as instructed. You could also try introducing a small amount of soil from beneath a thriving plant to the soil of a struggling plant of the same type.

Courtesy of NYBG Plant Information Service

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* An inoculant is a horticultural amendment that can be added to soil to introduce beneficial microbes.