Should I add mycorrhizae to the soil when I plant native plants?

| Colorado Native Plants

This question and answer are part of our Ask CSU Extension Anything (About Native Plants) series. We appreciate CSU Extension for answering these questions to expand our community’s knowledge of native plant landscaping. Send us your questions to the Ask CSU email account.

ANSWER: While at least one study has found that in extremely degraded sites the naturally occurring mycorrhizal inoculum is not sufficient to quickly recolonize restoration plants, many others have shown that adding mycorrhizae can have no effect on plant performance in the field or in container culture. While buying and adding mycorrhizae is unlikely to hurt anything, working on building your soil’s tilth by adding compost (if the organic matter % is low) and fighting compaction will likely bring at least as many benefits as adding purchased fungi. Mycorrhizal fungi and their spores are all over the place, so building a habitat for them will likely result in their rapid colonization of plants in that habitat without the need (or potential risks) of adding non-local species or genetic lines. Focusing on biological diversity and the soil microbiome overall is more effective than a reductionist approach focused on mycorrhizae.

Inoculation is more beneficial when:

  • Fungal species are selected to be compatible with both the target plant and the soil it is growing in.
  • Soil has been physically or chemically disturbed – killing both fungi and host plants.
  • Site has lost much of its native identity (i.e., native plant complexes have been functionally displaced by introduced species).
  • Inoculant is dispersed as close to the root zone as possible to ensure rapid root colonization.
  • Physical soil disturbance, such as tillage, can be eliminated or reduced to keep fungal networks intact.
  • There is always a living root in the soil. This is easier with perennials than seasonal annuals
  • Fertilizer and fungicide use are limited to keep fungus alive and also to incent plants to invest carbohydrates for fungal symbiosis – instead of relying on soluble nutrient sources.

Answer developed by:

John Murgel (Extension County Specialist, Horticulture and Natural Resources, Douglas County) and Derek Lowstuter (Extension Mountain Regional Specialist, Expanded Rural Engagement, Food, and Agriculture) Colorado State University Extension.

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