By Dr. Avishesh Neupane, UConn Soil Nutrient Analysis Lab 

Walk into a garden center, and you may find bags, tubs, and packets of “mycorrhizal inoculants” shelved beside fertilizers and transplants. Their labels promise stronger roots, better yields, improved drought tolerance, and healthier plants. 

The science behind mycorrhizae is real. The more practical question is whether the product on the shelf can deliver those benefits and whether your garden needs it at all. 

What mycorrhizae actually are 

Mycorrhizae are partnerships between plant roots and certain soil fungi. The fungi extend fine threads called hyphae into the surrounding soil, increasing the area from which plants can absorb water and nutrients. In return, the plants provide sugars produced through photosynthesis. These ancient partnerships play an important role in the growth of many plant species. 

Arbuscular mycorrhizal fungi, or AMF, are the most common type of mycorrhizal fungi associated with lawns, vegetables, and many ornamentals and may be labeled “endomycorrhizae.” Tomatoes, peppers, corn, squash, beans, grasses, and many flowers form these partnerships. Other plants rely on different mycorrhizal fungi. Oaks, birches, hickories, and many conifers commonly associate with ectomycorrhizae, while blueberries and rhododendrons form ericoid mycorrhizae. The inoculant must therefore be compatible with the plant. 

What may already be in your soil 

In many established garden beds, lawns, meadows, and landscapes, resident mycorrhizal fungi are already present. Their abundance and diversity depend on vegetation, disturbance history, fertilizer use and soil conditions. Adding a commercial fungus to an established soil community does not necessarily improve a partnership that is already functioning. The introduced fungi must still be alive, compatible with the plant, and able to compete with the organisms already there. 

Recent research gives gardeners good reason to be cautious. A meta-analysis published in the journal New Phytologist found that roughly 80% of commercial inoculant treatments resulted in minimal or no mycorrhizal root colonization. A separate 2025 evaluation from the University of Zürich of sixteen commercial products found that none of the seven products marketed for home gardening successfully established mycorrhizal symbiosis under controlled conditions. 

There are several possible reasons. Mycorrhizal fungi are living organisms and may lose viability during storage. Fungal species also differ in their ability to associate with particular plants and perform under different soil conditions. Independent quality standards and verification remain limited, making it difficult for consumers to know how many viable fungal propagules remain in a product by the time it reaches the garden. 

When inoculation may help 

Inoculation is most plausible where suitable fungi are scarce or absent, such as in some severely disturbed construction soils, degraded sites, or soilless and pasteurized growing media used for containers and greenhouse transplants. Even then, the plant must be capable of forming the partnership, and the product must contain viable fungi that are compatible with that plant. 

Tree and shrub plantings require extra caution. Many woody species rely mainly on ectomycorrhizal or ericoid fungi rather than AMF, so a generic product labeled “mycorrhizae” may contain the wrong organisms for the plant. Read the label carefully and look for the fungal type and species included, not just the word “mycorrhizae.” 

Why established gardens often see little benefit 

For an established Connecticut vegetable garden or perennial bed, several factors work against a bagged inoculant paying off. 

First, the soil may already contain a functioning fungal community. Introduced strains often struggle to establish in soils with diverse resident organisms. When they do establish, they may persist, decline, or alter the existing community, so their effects are not automatically beneficial. 

Second, high phosphorus commonly reduces AMF colonization and the plant’s reliance on the partnership. Many long-established vegetable gardens test high in phosphorus after years of compost, manure, or fertilizer applications. A soil test is especially useful before buying an inoculant or adding more phosphorus. 

Third, some common crops generally do not form functional mycorrhizal associations. These include broccoli, cabbage, kale, cauliflower, mustard, and radish. Beets, chard, and spinach are also generally considered non-hosts. Applying a mycorrhizal inoculant to a bed planted mainly with these crops is unlikely to provide a meaningful benefit. 

Supporting beneficial fungi 

For most established Connecticut gardens, lawns, and landscapes, mycorrhizal inoculants may not be necessary because beneficial fungi are often already present. They may be more useful in highly disturbed soils or growing media where compatible fungi are scarce. 

Good soil management can help support the fungi already in the soil. Avoid unnecessary tillage, which can break fungal hyphae, and keep living roots in the ground when practical through perennial plantings or compatible cover crops such as annual ryegrass and crimson clover. Test the soil before adding phosphorus, since high phosphorus can reduce the plant’s reliance on mycorrhizae. 

Before buying an inoculant, consider whether improving soil conditions and supporting the existing fungal community may be the better first step. 

The UConn Home Garden Education Office at the George Leigh Minor Plant and Soil Health Center supports UConn Extension’s mission by providing answers you can trust with research-based information and resources. For gardening questions, contact us toll-free at (877) 486-6271, visit our website at homegarden.cahnr.uconn.edu, or reach out to your local UConn Extension center at extension.uconn.edu/locations.   

This article was published in the Hartford Courant August 9, 2026