Autumn and winter are the key tree and shrub planting seasons in the South East of England and gardeners are hearing a lot of references to ‘Mycorrhiza’ so this seems a good time to explore what it/they do. Mycorrhizal fungi are present naturally in the soil but can be depleted in soil or growing compost that has been deeply dug over, sterilised or treated with fungicides as with much nursery produced stock. These tiny fungi attach to the roots of plants and form a symbiotic (mutually beneficial) relationship: the fungi has a huge network of fine roots and increases the uptake of water and nutrients, especially phosphorus, making it available to the host plant; in return the plant supplies some food energy stored by photosynthesis to the fungus. It is thought that up to 85% of plant species benefit from some species of mycorrhizal fungus and that these organisms have been around for 400 million years. What is new though is the development of off-the-shelf products for gardeners to add to their soil, likely to be most use if you have bought in sterilised top soil or have a recently re-landscaped garden (they hate being disturbed) and least use if you have a long established organically improved soil which would have a healthy population anyway. The fungi do not travel far through the soil so need to be added in granule form close to the roots at planting time. Trials have shown significant results for establishment and growth of trees and shrubs – RHS and Kent County Council schemes included – and it is starting to look as if planting with mycorrhizal fungi can significantly reduce the incidence of ‘Rose sickness’ where new roses fail if planted directly into the same soil when an old rose is removed.
I now plant all trees and shrubs with a combined organic granular fertiliser and mycorrhizal fungus supplement both at home and for clients; it’s a small outlay compared to the disappointment of a plant that fails to establish.