How exactly do mycorrhizal inoculants work?

Q:

I keep seeing articles about how beneficial mycorrhizae are for plants, and that it’s possible to add them to my growing medium. My question is: How often do I need to add mycorrhizae to my soil to ensure my plants constantly have enough?

A:

Symbiosis begins when fungal spores germinate and emerging, threadlike structures called hyphae enter the epidermis of plant roots. After colonization of the roots, the fungus sends out a vast network of hyphae throughout the soil to form a greatly enhanced absorptive surface area. This results in improved nutrient uptake by plant roots.

When using those beneficial mycorrhizal inoculants, the timing of application is critical: they must be applied at the right place and the right time.

Every time you transplant is the time to add mycorrhizal inoculants. The mycorrhizal inoculant must be applied all around the plant’s root ball. This way, the new roots that emerge from the root ball will be in contact with the inoculant. The key is to use the mycorrhizal inoculants when the plant is actively growing.

When transplanting a plant to a larger pot, make sure there is mycorrhizal inoculant all around the planting hole, because that’s where the feeding roots of plants will grow out. Once it’s been applied, it normally remains present for the plant for the entire duration of the crop cycle.

If you transplant another time, just add more mycorrhizal inoculant all around the root ball again. The bigger the plant, the more mycorrhizal inoculant is required. For example, for a small plug, a table spoon would be enough all around the planting hole. When the plant is bigger, you need to use more so that the planting hole is well covered and that all the roots come in contact with the inoculant.

In short, mycorrhizae are beneficial to plants because they help them acquire more nutrients and water and resist stressful conditions such as extreme heat, drought and diseases. However, know that overfertilizing plants will reduce the efficacy of mycorrhizal inoculants.

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Written by Susan Parent
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Susan Parent is a horticultural specialist at Premier Tech Horticulture and has a master's degree in Ecology of Mycorrhizae. She has worked in R&D on the production of arbuscular mycorrhizal fungi and has tested mycorrhizae in the fields of horticulture and agriculture for many years.

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