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Mycorrhizal products for landscape trees: what extension and trials report

Soil products carrying mycorrhizal fungi are sold for newly planted trees. This page reports what a University of Arizona sheet, an Iowa State article and a Clemson University greenhouse study say about them.

Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.

This page reports what three sources say. It does not recommend or discourage any product, and it names no brands. The guide on fertilizing trees and deficiencies covers what other sources say about feeding trees, and the guide on planting and soil standards covers the planting side.

What mycorrhizae are

The University of Arizona Cooperative Extension sheet Backyard Gardener 105, adapted by Jeff Schalau and dated October 2, 2023, describes mycorrhizae as fungi that form a two-way relationship with living plants. It says the fungus helps the plant take up water and minerals, with fine strands that reach a much larger volume of soil than roots, and the plant gives the fungus sugars. It says about 80% of higher plants found in nature and all conifers can be colonized. It separates two groups: ectomycorrhizae, which grow outside root cells, and endomycorrhizae, which enter them and are also called vesicular-arbuscular.

What the sources say about added products

The Arizona sheet says manufacturers suggest the inoculum can be added to young plants in the nursery or to the soil before planting, and that there are biological benefits, such as protecting roots from root rot fungi and improving phosphorus uptake where phosphorus is low. It then says many studies have been done and there is little evidence the products help growth and survival after planting in native soils. It says relatively undisturbed sites with native plants nearby would likely already have working mycorrhizal populations. It reports that Linda Chalker-Scott of Washington State University Extension, after literature reviews, drew two themes: healthy undisturbed soils already hold native mycorrhizae, and where soils are impaired too far for native fungi to live, added mycorrhizae alone will not improve things.

Iowa State University Extension's Yard and Garden article by Jeff Iles, published April 23, 2004, is titled "No Answers Yet". It lists published trials of spores added to backfill or root zones of trees. Its summaries include these.

  • Silver linden inoculated at transplanting grew more and kept its fall leaf color longer, with the response first noted two years after installation.
  • Established willow oak, northern red oak and pecan showed more fine root growth and mycorrhizal development four to seven months after treatment with fertilizer, inoculant or both.
  • Live oak transplants given spores in the backfill showed no significant effect on stress, survival or growth.
  • Willow oak inoculated at installation showed no survival or growth benefit after one year.
  • Established pin oak, willow oak and red maple showed no measurable growth benefit from a commercial product unless it was combined with fertilizer.

The article calls its list the sum total of the published research and says it seemed premature to make adding spores standard practice when installing woody plants. It says the unproven treatment may be embraced as more is learned.

The Clemson greenhouse study

Wiseman, Colvin and Wells, in the Journal of Environmental Horticulture, March 2009, tested commercial arbuscular mycorrhizal products bought anonymously through mail-order channels. Their abstract says the infectivity of such products had rarely been independently tested. In corn and sorghum, colonization rarely exceeded 5% with the commercial products, against 38 to 61% with a viable lab-cultured inoculant. The products generally improved shoot growth and soil nutrient levels. On trident maple and sweetbay magnolia liners, the commercial products had no effect on colonization or shoot growth. Magnolias grown from seed developed little or no colonization with the products, against 74% with the lab-cultured inoculant.

The authors write that the growth effect in corn and sorghum suggests non-mycorrhizal ingredients acted as fertilizers or growth regulators, and that high soil fertility commonly inhibits this kind of fungus. They say manufacturers must show that their products promote colonization under their intended conditions of use.

Where the three agree and where there is a gap

  • Arizona and Iowa State say the evidence for benefit after planting in ordinary landscape soil is thin or mixed, and the Clemson study found the tested products rarely colonized roots.
  • Arizona and Iowa State both describe a role for the fungi in plants, and neither denies it.
  • Only the Clemson study set out to test how infective the commercial products themselves were, buying them anonymously and measuring root colonization.
  • No source opened for this page covers compost teas or biochar as mycorrhizal treatments.

Sources