Catriona McLeodCheap potting mix brings uninvited visitors

A fascinating and harmless visitor leaves its mark in pots containing low-grade potting mix. I made a critical error in gardening. I admit it, and the proof is evident. I bought poor quality potting mix, erroneously thinking all potting mixes were alike (ignoring both my common sense and the Australian Standards potting mix rating system – more on this later). There have been three notable repercussions; two of which are undesirable, and the third is the topic of this article.

Cyathus olla Photo Fred Stevens

Cyathus olla Photo Fred Stevens

First, the main constituent in the inferior potting mix has dried to a dense, shrunken clump of organic matter that is hydrophobic. Second, the plant roots have been trapped and dried in this unyielding, compacted mass. Third, a fungal visitor has appeared in the pots that contain this cheap mix: Cyathus olla, commonly known as Birds Nest Fungi (family Nidulariaceae), have grown happily in the potting medium. Cyathus olla is distinguished by its relatively large ‘eggs’ or peridioles and is common and widespread throughout Britain and Ireland and occurs in many other parts of the world including South America, Asia, mainland Europe, North Africa and Australia.

Cyathus olla Mycobank

Cyathus olla Mycobank

This gasteromycete fungus (of the obsolete class Gasteromycete, meaning ‘stomach fungus’) was first described in 1783 by German mycologist August Johann Georg Karl Batsch who named it Peziza olla; however, the botanist generally regarded as the founding father of systematic mycology Christiaan Hendrik Persoon (1761 – 1836) transferred this species to the genus Cyathus in 1801, thus establishing its now accepted scientific name Cyathus olla.

Cyathus comes from the Greek prefix kyath– meaning cup shaped (like a chalice). Less obvious is the meaning of the specific epithet olla, which means a squat, rounded pot or jar used to cook or store food, in this case in reference to the silvery ‘eggs’. In certain villages in Ladakh (northern India) Bird’s Nest Fungi are known as Nasi-baangah, a combination of two words: Nasi (taken from Nass meaning wheat or barley grain) and baanga, which refers to the traditional storage bins made up of mud bricks. Local folklore also holds that sighting a group of seven or more in the crop fields foretells good fortune and a bumper crop of grain.

Cyathus olla Photo Fred Stevens

Cyathus olla Photo Fred Stevens

The life cycle of this fungus allows it to reproduce both sexually, with meiosis, and asexually via spores. The funnel- or cup-shaped fruiting bodies form initially as small, fluffy, brown ‘knobs’ on woody debris such as twigs, woodchips, and humus in woodland and on compost or (as in my case) pine bark mulch. They become darker with age, and the lids fall away to reveal the peridioles. Each peridium or ‘nest’ typically contains four or five silvery flattened ‘eggs’. The outside of each peridium is covered with fine grey-brown hairs; the inner surface, in contrast, is smooth and hairless. The peridia are 6mm – 10mm across and up to 15mm tall, with flared rims and the individual peridioles are typically 3mm – 3.5mm across. The silvery-white peridioles soon darken, and their attachment to the base of the nest-like cup is easily broken when raindrops hit them, and they are thrown out, where they eventually split to release their spores.

Eggs splashed up on stems

Eggs splashed up on stems

The force of splashing rain can propel the peridioles several metres. Other species of Bird’s Nest Fungi have ‘spore packages’ that stick to an object or surface by a coiled spring with a sticky end. When the package is splashed out of the cup, the coil (or funicular cord) snaps and extends a microscopic tail that catches on any blade of grass, stem or twig, and then wraps around that object.

Another member of the Nidulariaceae family, the aptly named Artillery Fungus (genus Sphaerobolus – Greek for ‘sphere thrower’) is even more forceful:

“The fruiting body of this fungus orients itself toward bright surfaces, such as lightcolored houses or parked automobiles [and] ‘shoots’ its black, sticky spore mass, which can be windblown as high as the second story of a house. The spore mass sticks to the side of a building or automobile, resembling a small speck of tar.” (Dr. Donald D. Davis, Penn State)

Currently, and in the tradition of research into the benefits of ‘good fungi’ Cyathus olla is the subject of agricultural research to determine its potential as a means to accelerate the breakdown of crop residue, and reduce the population of plant pathogens. In particular, Cyathus olla is being studied in Canada for its potential as an inoculant to accelerate decomposition of canola stubble, and hence reduce the incidence of stubble-borne diseases of this crop, such as blackleg caused by Leptosphaeria maculans and blackspot caused by Alternaria brassicae. Cyathus olla is commonly found colonising and fruiting on the stubble of canola; in the absence of such an accelerant, the basal stem and root of canola, which are woody and generally resistant to decomposition, could provide an overwintering site and food source for pathogens for many years.

Empty nests

Empty nests

Bird’s Nest Fungi are saprophytes that decay wood, bark and mulch, and do not harm plants, so control of these fungi in the garden is not normally necessary. Should Bird’s Nest Fungi become a nuisance, removal of the mulch, coupled with decreased watering can minimise the production of ‘nests’. Penn State researchers are also recommending blending at least 40% spent mushroom compost with landscape mulch as the compost contains microbes that compete with, or actually destroy, ‘nuisance’ fungi such as the Artillery Fungus and Bird’s Nest Fungi.

I am now in the process of replacing all of the affected potting mix, taking care to remove the spongy hyphae, empty ‘nests’ and flung spore capsules on stems and under leaves. Although I am intrigued by this phenomenon, I would prefer my pots only support vegetable seedlings and ornamentals. Interestingly, the fungus doesn’t seem to be growing in garden soil, only on the surface of the potting medium in pots. My soil, particularly in the vegetable garden, has been enriched with animal manures and seaweed, and it could be the lack of wood cellulose that dissuades the spores from taking hold.

Australian-standards_red_webAs an afterword: Always take care when using potting mix as health risks include Legionnaires’ disease, infection of open wounds, Tetanus, respiratory ailments and dermatitis. The Australian Standard for potting mixes was written in the late 1980s in response to perceived disparity in the quality of potting mixes. The Standard measures physical properties such as percentage of air and water and weight; chemical properties such as pH and salt levels and; levels of nutrition and biological properties that might cause damage to plant roots. The black tick Australian Standard indicates the mix meets all criteria but does not include any nutrients (fertiliser). The red tick Australian Standard for premium potting mix indicates the mix meets all criteria and does include enough nutrients (fertiliser) to last for three months from the date of manufacture.

Like this post? Why not share it with a friend?


Catriona McLeod

About Catriona McLeod

Dr. Catriona McLeod is a writer, environmental & cultural consultant, horticulturalist and designer based in Tasmania. Catriona worked as a horticulturalist in plant nurseries, while studying architecture and landscape architecture in Brisbane. After travelling, and working as a designer and horticultural consultant, she completed a PhD in green design. Until 2012 she taught design principles, landscape architecture, ecology, design theory, horticulture and sustainable practices at the University of Tasmania. She is a passionate advocate of wild places and the preservation of threatened environments and species and is, of course, an avid and always-learning gardener.

Leave a Reply (no need to register)