Are mushrooms sustainable?

As mainstream mushroom production still relies on peat, Hugh Thomas hears from farms at two ends of the spectrum and looks into some of the sustainable alternatives for growing.

Autumn is the season for mushrooms, as the lower temperatures and wetter weather provide the perfect conditions for fungi to flourish.

For most mushroom producers though, every season is mushroom season. When cultivated indoors under the right temperatures and humidity levels, growers can mimic the environments mushrooms need to produce, or ‘fruit’.

As this increases the frequency at which they can be harvested, it could be argued this is the kind of production we need to support if we are to incorporate more vegetables into our diets. More, however, doesn’t necessarily mean better – indoor cultivation, for example, means less exposure to sunlight. Though mushrooms don’t require natural light to grow fruit, their nutritional value declines without it. One study showed that outdoor-grown mushrooms can contain ten times the amount of Vitamin D as they’re exposed to more UV light.

log grown shitake
Indoor versus log-grown Shiitake mushrooms. 

“All the familiar mushrooms you see in the supermarket are grown indoors,” says Noel Hegarty, chief commercial officer at supermarket supplier Monaghan Mushrooms and spokesperson for UK and Ireland Mushroom Producers association. Monaghan produces 6,000 tonnes of mushrooms a month with the help of substrates – typically straw, sawdust, woodchips, or manure – providing the beds for mushrooms to grow and take up their nutrients. Anything that was formerly a plant, even cardboard, can be used as a substrate.

Once they’re sterilised to kill off any competing moulds or bacteria, substrates are inoculated with the spawn of the chosen mushroom, which, within a few weeks, will fruit. “The sterilising of substrates is heavy on cheap fossil fuels,” says Tasha Stevens Vallecillo, forager and production manager at Somerset retreat and regenerative farm 42 Acres. “And the discarding of growing mediums to sterilise again. Because with indoor setups, you have just a six-week turnaround.”

Some growers have switched to more sustainable substrates, such as used coffee grounds from cafes, or spent malt from breweries. These often skirt around the problems with sterilisation, as the heat involved in brewing coffee and beer typically kills off any unwanted organisms.

Mushrooms are primarily grown in substrates, like straw or woodchips, but can also be grown on logs. 

One particular growing medium, however, is not doing the mushroom industry any favours environmentally. Although it’s fractional compared to other uses in horticulture, commercial mushroom producers commonly employ peat as a ‘casing’ – essentially another layer of soil that helps keep moisture in, therefore dramatically improving yield. Since peat comes from bogs and moorland, it’s very efficient at retaining moisture. But it’s also very efficient at retaining carbon. Around a quarter of global soil carbon is stored within peatland, despite it taking up only three per cent of the Earth’s surface. Should peatland harvesting persist, more of that carbon will continue to be released.

Northern Ireland’s Department of Agriculture, Environment, and Rural Affairs is encouraging the government to ban the use and import of peat by 2022 (the UK government set a similar target for 2020, which failed). While alternative casings exist, a suitable one is lacking – coffee can’t be cultivated locally, the mineral vermiculite is too expensive, and wood fibre doesn’t retain nearly as much water. Indoor exotic mushroom producers are also avoiding the peat problem by growing in smaller containers meaning they don’t need as much help to retain moisture.

Tasha Stevens Vallecillo runs a regenerative farm where she produces mushrooms outdoors.

Eliminating the need for casings entirely, Stevens Vallecillo grows Shiitake mushrooms outdoors on cut beech, birch, and oak logs. “Mushrooms are a good food source regardless of indoor or outdoor,” she explains, “but outdoor log-grown mushrooms are infinitely higher grade and more nutritious. The Japanese value log-grown extremely highly and have been doing it at a huge scale for hundreds of years.”

While 42 Acres’ Shiitakes are at the whims of the season, and therefore fruit only two or three times a year, log-grown mushrooms can be integral in helping woodland ecosystems function. Sporing mushrooms attract insects, which attract birds, which carry and drop seeds. Some varieties also help restore the land – oyster mushrooms have been documented to ‘clean up’ petrol-contaminated soils, while a species indigenous to the Amazon is able to turn plastic into compost in a matter of months.

Especially among exotic mushrooms (things like Shiitake, Maitake, King Oyster, Lion’s Mane), supermarkets have witnessed a boom in recent years (a 240 per cent year-on-year increase, in some cases) due to health benefits and a growing interest in meat alternatives.

Growing on logs is irregular but can add biodiversity into local ecosystems. 

But the question remains: can sustainably-grown mushrooms reach the scale of output asked of them in the UK?

The general consensus is that exotic mushrooms need to be grown indoors to achieve a consistent crop all year round, with small-scale operations like Forest Fungi and Dart Valley Fungi able to produce enough to supply national organic veg box company Riverford.

It’s certainly less predictable and much lower volumes growing outdoors – at 42 Acres, Stevens Vallecillo will produce around 700kgs for every 1,000 logs, and aims for her harvest to be ready in time for key events like Glastonbury Festival. It’s also a labour intensive process, which makes it expensive to run, a reason cited by the owner of the largest shiitake log farm in Britain (20,000 logs) for why he is unable to make a profit.

But Stevens Vallecillo says growers are just starting to realise alternative approaches are possible. “What I’m seeing is people are now fruiting strains of shiitake, for example, that will come out in colder environments. So I don’t think it’s limited to the summer, spring, or autumn. It could also be viable in the winter.”


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