The vast majority of fungi in our gardens are beneficial saprotrophic fungi. They

Decompose means to break down or decay into simpler substances. In gardening, it often refers to the process of organic matter breaking down when added to compost or soil. This process is important for creating healthy soil and providing nutrients for plants.

Without the decomposing activities of saprotrophic fungi we would disappear under a mountain of unrotted dead leaves and logs! As leaves and wood decompose, animal biodiversity such as springtails, beetles and worms, can gain access to the cavities and resources that emerge and these organisms continue the breakdown process.

All fungi usually exist in a microscopic form as a network of threadlike cells called the mycelium, but occasionally they produce their reproductive structures, or fruiting bodies (e.g. mushrooms, brackets, etc.) and it is at this point they become noticeable to the home gardener. Fruiting bodies themselves are used as food and habitats for animal biodiversity, including beetles, flies and small mammals. These organisms in turn feed higher trophic level creatures such as spiders, bats and birds, which can help keep other herbivorous animals in check.

Saprotrophic fungi produce enzymes that allow them to decompose the tough plant cell wall compounds: cellulose, hemicellulose, and pectin. Some wood decaying fungi can also decompose lignin. These processes enable the recycling and recovery of large amounts of nutrient resources (particularly carbon and nitrogen) from dead organic matter.

Saprotrophic species prefer different types of dead plant material: leaves of different trees, specific types of wood in various sizes, grass stubble, buried roots, herbaceous stems, nut cases and cones. Most of the time, the mycelium of saprotrophic fungi is largely concealed in the substrates they inhabit. However, there are signs we can observe that give us insight into this largely unseen world:

• Fruiting bodies – Often the most recognisable sign of a fungi’s presence. Fruiting bodies of saprotrophs come in an astonishing array of forms and colours: from large mushrooms to curious puffballs to tiny jelly dots or paint-like crusts. You may see these on the lawn, soil, fallen wood and declining plants. The production of fruiting bodies can be infrequent and they may only appear long after the fungus has become established.  
• Mycelium – This is the main body of the fungus. It is made up of tiny filaments called hyphae which en masse can be seen as white fungal growth comprising of threads. You might find it on the soil surface, in bark mulches and compost. Collectively, hyphae can form cords to transport nutrient resources, or hard survival structures called sclerotia.  
• Breakdown of wood – To access the energy in dead wood, saprotrophic fungi release potent enzymes capable of breaking down tough materials like cellulose and lignin. You may notice that dead wood in your garden decays faster than expected, affecting not only log piles but wooden furniture and fence posts directly embedded in the soil. Wood can become stained during this breakdown, some species produce bright colours such as the turquoise elfcup Chlorociboria aeruginosa. On cross sections of wood you may see lines and staining demarking the interactions between different fungal communities.  
• Breakdown of leaf litter – Fallen leaves, dead herbaceous stems and other contents of compost piles are good sources of food to saprotrophic fungi, which turn this dead organic matter into good quality soil.  
• Heartwood rot – Saprotrophic species contribute to the natural decay of older trees. Their decay activity generates hollows containing partially decayed materials that are excellent environments for invertebrates to thrive. (see Heartwood fungi)  
• Increased diversity of wildlife – The breakdown of organic materials makes new habitats and accessible food sources to support a wide range of invertebrates, birds and mammals. Invertebrates also graze on the mycelium and can benefit the fungi by dispersing spores and providing sources of nutrition when they die.  
• Improved soil, improved plants – The release of nutrient resources from saprotrophic fungal action encourages a dynamic community of microorganisms in the soil, which contributes to the retention and distribution of nutrient resources across a garden and retention of moisture. These healthier soils should increase the vigour of your plants and even make them less susceptible to disease.

The cells of saprotrophic fungi are extremely thin, elongated tubes known as hyphae which grow throughout the dead material they decompose. Collectively, the hyphae make up the mycelium – the main body of a fungus. Hyphae are often invisible to the naked eye, but sometimes, aggregated mycelium becomes visible if growing prolifically or forming cords.

A single fallen log is likely to contain many different fungal mycelia. The species present will change over time. When fungal mycelium encounters the mycelium of different species, it responds antagonistically, either by inhibiting the opponent’s growth or by parasitizing it. You might see dark lines inside decaying wood demarcating the territories of separate mycelia (spalting lines), which are formed by the fungi releasing defensive compounds.

When the favoured environmental conditions of a species are met, the mycelium will develop complex structures for reproduction: the fruiting bodies. The most recognisable versions are mushrooms, but they can take many shapes including brackets, oysters, fans, crusts, cups, corals etc. These structures carry out sexual reproduction by producing and releasing spores which can travel long distances by the wind, rain or animals. After landing on a surface, spores germinate into hyphae, which form a juvenile infertile mycelium. Fungi produce spores in vast numbers: most spores will not successfully germinate, and most spore-borne mycelium will not create a new fertile fungus as to do so it must fuse with another compatible mycelium.

Fruiting bodies usually enlarge very rapidly, becoming engorged with water in just a few hours. This hydraulic pressure enables fruiting bodies to push through soil or wood so that they can reach the air to release their spores. In some fungi, such as puffballs, the hydraulic pressure can be so great that it can damage tarmac and lift paving slabs.

Effects on Wildlife
Saprotrophic fungi can be hugely valuable for wildlife. These fungi produce aromas, known as volatile organic compounds (VOCs), which helps draw wildlife to the area. Breaking down dead wood and leaf litter creates soft, damp habitats and breeding sites, including hollows in tree stumps, which become populated with animal biodiversity. These species, including springtails, beetles, and worms, then continue the breakdown process.

Fungal fruiting bodies themselves provide food and shelter for many animals too. Fruiting bodies can be eaten by a variety of creatures, including invertebrates and mammals. Grey oysters (Pleurotus ostreatus), for example, can be a nutritious source of vitamin D, which can be a valuable resource for animals during the winter months.

The invertebrates that live in the decay sites or fruiting bodies are attractive to predators such as spiders, birds and bats, further increasing the biodiversity in the garden. These predatory species could help control populations of unhelpful herbivorous wildlife and reduce feeding damage on garden plants. The products of fungal decay can also recruit soil bacteria, and a healthy soil microbiome supports better plant growth.

The variety of wildlife that can be found associated with saprotrophic fungi will depend on how advanced the decay is, along with other factors such as the plant species, their age, aspect and the climate. Decaying plant material is not a single habitat but consists of a complex series of changing microhabitats, supporting a wide range of organisms.

In the majority of cases saprotrophic fungi growing in gardens do not need to be controlled. These fungi play an important role in the long-term health of a garden and we encourage viewing them as an important part of the ecosystem while enjoying their diversity of forms.

A small number of species have poisonous fruiting bodies that would harm humans or pets if consumed. No fungi will harm you to touch, but if you are worried about fruiting bodies being eaten you can lift them and relocate them to somewhere they won’t be disturbed but can still spread their spores in the wind and serve wildlife, such as under a bush or on top of an open compost heap.

To maintain and support a complex ecosystem of saprotrophic fungi in gardens, aim to minimise soil disturbance and avoid the use of chemical fungicides. Leave disease-free dead wood, woodchip mulches and leaf litter on top of the soil, and as this breaks down it will create a healthier soil that plants will thrive in.

Honey fungus is a poor competitor for woody resources that are already dead. By encouraging or introducing saprotrophic fungi in your garden, we expect this will create greater competition against harmful pathogenic fungi. With fewer resources available, we expect that pathogens would be less capable of damaging your plants.

In some cases, especially if the ground is poorly drained, mycelium can grow in excess and create a water-repellent environment. This effect is similar to a conditions found in lawns called ‘dry patch’. To remedy this, fork over the soil to break up the mycelium.