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Adding mycorrhizal fungi

Complete guide to mycorrhizal fungi

Sue Fisher

Published: Thursday, 5 January 2023 at 2:52 pm

What is mycorrhizal fungi and how does it work? Our complete guide introduces these incredible fungi and explains how to use mycorrhizal products in the garden.

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Out of sight, below the ground, the soil teems with life. Much of the living mass of soils is made up of fungi, which form extensive underground networks, working in partnership with plants and "supercharging" their root systems.

More than 90 per cent of all plant species form associations with mycorrhizal fungi. Scientific research continues to make discoveries about the world beneath our feet, revealing how important it is to take care of our soil health by adopting sustainable practices such as composting, using leaf mould and not using synthetic fertilisers.

What are mycorrhizal fungi?

Applying mycorrhizal fungi powder in the base of a planting hole

The word mycorrhizal relates to "myco" (fungi) and "rhiza" (root). It refers to the symbiotic relationships that fungi have with plant roots, aiding absorption of nutrients and helping them grow. Mycorrhizal fungi pass water and nutrients to the plant, and the plant in turn supplies the fungi with some of the food that it generates by photosynthesis – the process by which plants use sunlight to transform water and carbon dioxide into oxygen and energy.

Because the fungi are far more extensive than the plants’ roots alone, they efficiently ‘mine’ the soil for nutrients and water, and the food from the plant boosts the fungi growth.

Mycorrhizal fungi have been around for hundreds of millions of years and research suggests that they are the root of all life on land. Underground, mycorrhizal fungi form an extensive network, known as mycelium, of microscopic thread-like strands or hyphae, which becomes far more extensive than the actual roots of a plant. Incredibly, the fungal network can cover up to 700 times more soil than the plant roots alone. Fungal networks also connect individual plants together to share resources in a natural ecosystem, often described as the ‘wood wide web’ for its similarities with the internet or ‘world wide web’.

The benefits to plants of mycorrhizal fungi

Mycorrhizal fungi gel on tree roots

Gardeners can boost natural levels of mycorrhizal fungi by composting, mulching the soil with organic matter such as well-rotted horse manure, letting leaf litter remain on the soil and top-dressing with leaf mould. You can also buy mycorrhizal fungi and add it to the planting hole when planting new plants. This can help plants establish more quickly and boost healthy growth by supplying additional water and nutrients. Mycorrhizal fungi therefore help plants resist drought, produce bigger crops and flowers, resist diseases, and thrive on poor soils or adverse conditions. The primary beneficiaries of mycorrhizal fungi are woody plants, but other types of plant also benefit. Mycorrhizal fungi also help newly planted roses overcome rose replant disorder.

Plants that do not benefit from mycorrhizal fungi

Plants in the Brassica family do not use mycorrhiza. The Brassica family includes vegetable crops such as broccoli, Brussels sprouts, cabbage, cauliflower, kale, rocket, swede , and turnip. Ornamental flowering plants in the Brassica family include candytuft (Iberis), sea kale (Crambe) and wallflower (Erysimum).

How to use mycorrhizal fungi

Mycorrhizal fungi powder around rootball of tree

Buy mycorrhizal fungi in powder, granule, or gel form
The most important use of mycorrhizal fungi is as a one-off aid when planting, especially of bare rooted trees, shrubs, roses, hedging, fruit, and perennials. Mycorrhizal fungi is also beneficial when growing from seed, including lawns, and bulbs
Mycorrhizal fungi is also an effective aid to establishing container grown plants
Use powder or granules when planting container grown plants. Mix the mycorrhizal fungi into the planting hole and sprinkle onto the plant roots to ensure immediate contact
When planting bulbs or seeds, sprinkle a little of the mycorrhizal fungi into the soil or planting hole first
Mycorrhizal fungi is also beneficial for plants growing in containers
Ericaceous (lime hating) plants like rhododendrons, azaleas, and blueberries need a particular type of mycorrhizal fungi
Use gel when planting bare rooted plants (these are only available during the winter months, whilst dormant). In a bucket, mix up the gel with water as per the manufacturer's instructions and dip the plants’ roots in before planting
Do not apply chemical fertilizers, or phosphorus-rich organic ones such as bonemeal, as these would suppress mycorrhizal fungi activity
Avoid using chemical weedkillers, pesticides, or fungicides
When not to apply mycorrhizal fungi
Do not use if fungicidal chemicals have been applied recently as the fungicide will kill the beneficial fungi
Chemical fertilizers inhibit the growth of mycorrhizal fungi
Avoid using mycorrhizal fungi alone on poor or over-cultivated soil. Improve the overall soil condition first by applying plenty of organic matter, such as garden compost, well-rotted manure, composted chipped bark or garden shreddings, or leaf mould
Do not apply on soil where brassicas have been grown

Advice on buying mycorrhizal fungi

A wide range of products containing mycorrhizal fungi are available. Most contain Rootgrow ™, the first and only product so far to be licenced by the Royal Horticultural Society. Ensure you buy the correct one for your plants by checking the list above

Where to buy mycorrhizal fungi

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