Mycorrhizae 101
A plain-English guide to how mycorrhizal fungi work - written by Dr. Mike Amaranthus, one of the world's leading mycorrhizal researchers.
Myco-What?
We live on top of a hidden world. Beneath the soil surface is a fascinating combination of life forms interacting in an abundance of ways. The vast majority of plant species form a mutually beneficial living relationship - a "symbiosis" - with beneficial fungi. It's a 1+1=3 relationship.
The roots of an estimated 85% of the world's plant species are colonized by symbiotic fungi. We call this root/fungus combination a mycorrhiza (plural: mycorrhizae). Mycor-rhiza literally translates into "fungus-root." The mycorrhizal fungi are the network of fungal filaments that permeate into the soil from their home on the plant root.
The body of the mycorrhizal fungus consists of microscopic filaments called hyphae. An individual hypha is approximately 1/25th the diameter of a human hair and can grow 15 to 25 inches in length. These hyphal strands spread out from the root cells of the host plant into the surrounding soil, greatly increasing the effective surface area of the root system.
The Two Main Types
Mycorrhizal fungi fall into two major groups. Knowing which type your plants need is the key to choosing the right inoculant.
What They Do for Your Plants
The mycorrhizal fungi act as the extended stomach of the plant - producing enzymes that digest and unlock nutrients in the soil, then transporting them back to the plant. Research confirms they are particularly important for accessing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese, and sulfur.
Nutrient Uptake
Hyphae penetrate tiny soil pores roots can't reach, breaking "tightly held" chemical bonds and transporting nutrients back to the plant - often dramatically reducing fertilizer requirements.
Drought Tolerance
Mycorrhizal fungi find and retain water where plant roots cannot, slowly releasing it during dry periods. Plants in natural areas tolerate drought far better than cultivated ones largely due to this fungal network.
Disease Resistance
Mycorrhizae release suppressive compounds that inhibit fungal root pathogens including Phytophthora, Fusarium, Pythium, and Rhizoctonia. They also form a tough physical barrier made of chitin around root cells.
Carbon Sequestration
Mycorrhizal fungi produce glomalin - a sticky, carbon-rich compound that binds soil particles together. Glomalin may account for up to one-third of the world's soil carbon, making these fungi key players in climate stability.
The Underground Network
Much like the internet, mycorrhizae can link all plant species together into an underground network known as the "common mycelial network." New scientific evidence shows this network does more than transport water and nutrients - it acts as a communication system, sharing chemical signals between plants.
Studies have documented plants using this network to warn neighbors of aphid attacks, share defensive molecules against root pathogens, and even inhibit the growth of competing plants. Interplant communication via the common mycelial network is now well established in the scientific community.
Why Modern Soils Are Often Deficient
In natural, undisturbed soils, most plant roots become readily colonized by indigenous mycorrhizal fungi. But modern agricultural practices - tillage, heavy fertilizer use, fumigation, and soil disturbance - dramatically reduce or eliminate native mycorrhizal populations.
Plants that germinate or are planted in soil without a mycorrhizal network become colonized much more slowly, and sometimes not at all. That's where commercial inoculants come in: products containing mycorrhizal propagules ("fungal seeds") reintroduce the relationship and give plants an immediate advantage from day one.
Inoculation is most effective when the product is placed in direct contact with the root system at planting - in the hole, on the root ball, or worked into the growing medium.
Frequently Asked Questions
How do I apply mycorrhizal inoculants?
Apply directly at the root zone at planting time — direct root contact is essential. For transplants, add granular or powder inoculant to the planting hole or coat the root ball. For seeds, dust with powder inoculant before sowing. Soluble products like MycoApply Soluble Maxx can be applied as a root drench at transplant. Broadcasting without root contact produces poor results.
How long does mycorrhizae take to work?
Colonization begins within days to a few weeks of inoculation. Visible growth benefits typically appear within 4–8 weeks. A full mycelial network can take one complete growing season to develop, with improvements in drought tolerance and nutrient uptake compounding as the network expands.
Can mycorrhizae be used with fertilizers?
Yes, but use caution with high-phosphorus fertilizers. Plants naturally reduce their mycorrhizal relationship when phosphorus is already abundant — the symbiosis is less valuable to them. Applying high-phosphorus starter fertilizers at the same time as inoculant can suppress colonization. Use moderate fertilizer rates at planting, especially directly at the root zone.
Are mycorrhizal inoculants compatible with fungicides?
Some fungicides are compatible; others will kill mycorrhizal spores. Triazole-class fungicides (propiconazole, triadimefon, fenarimol) should be avoided at inoculation time. Fungicides containing fosetyl-AL, mefenoxam, or azoxystrobin are generally safe. See our full Fungicide Compatibility Chart for the complete list.
What plants do NOT respond to mycorrhizal inoculants?
About 5% of plants do not form mycorrhizal relationships. This includes the Brassica family (broccoli, cabbage, kale, cauliflower, mustard), the Ericaceae family (blueberry, rhododendron, azalea, cranberry), and a few others including beet, spinach, and orchids. Applying inoculant to these plants is harmless but will not produce results. See the full Plant Compatibility Guide.