Plants and fungi have co-existed for millions of years and have developed interdependent or symbiotic partnerships. When planting trees, roses and other shrubs, we are encouraged to give our plants the best chance by putting a couple of pinches of rootgrow in the planting hole This contains a mix of mycorrhizal fungi that will work with the plant roots.

Perhaps, inevitably, this is only part of the story. Untold thousands of fungi have formed partnerships with 90% of land plants. The two main types of mycorrhizae that associate with the trees and shrubs we grow in our gardens are ectomycorrhizae (ECMs) and arbuscular mycorrhizae (AMs). The AMs, the most common on trees, penetrate cells within roots and develop a huge network of tiny filaments, or hyphae, around their hosts. ECMs function outside the plants, forming a sheath round the roots and growing a network of hyphae perhaps covering hundreds of metres. Both types collect water, nutrients and many elements for plants in exchange for vital sugars from their hosts. Phosphorus, which fosters root development, is specially important. The mycorrhizae also help fight off pathogens that would otherwise damage their hosts.

Some of our plants rely on one type of mycorrhiza, with others using both, simultaneously or at different times. This often depends on surrounding soil conditions.

Local soils also contain mycorrhizae that suit and may have adapted to these conditions and, in his recently published book, Trees, Peter A Thomas writes that the introduction of different and possibly incompatible fungi could be counterproductive. Manufacturers of these inoculants don’t say what’s in the powder we buy or their origins, so even experts like Thomas can’t assess their value.

But some gardening methods make rootgrow pointless. For example, fungicides in spring lawn-care and some rose blackspot treatments inevitably leach into the soil. These fungicides kill all fungi they contact, not just your problematic ones.

Every garden, especially an established one, contains its own suite of mycorrhizal fungi, so why not save your money, dig up a spadeful from somewhere else and put in the planting hole?

Be careful not to over-fertilise the ground before and after planting. The fertiliser contains as much phosphorus as the tree or rose needs, so it won’t use mycorrhizae to get more. Thomas notes that as a result, nearby mycorrhizae may turn pathogenic and damage their potential host. The American Rose Society quotes Dr Linda Chalker-Scott, who writes: “To encourage these hard-working and beneficial fungi in your plant community, you’ve got to cut down the junk (plant) food – stop using phosphate fertiliser.”

Plant of the week

Kale Cavolo nero, Black Tuscan or a host of other names is looking luscious as a result of the autumn rains, having put on a great spurt of growth after the dry summer. The long leaves are at their most succulent just now so enjoy. Its botanical name is Brassica oleracea palmifolia and it does look a little like a palm tree especially when you have harvested the lower leaves and there is just a tuft of younger ones still to develop.