Making an Off-grid DIY Mushroom Fruiting House

| Melliodora, Mushrooms | comments | Author :

Here’s how we made a DIY off-grid mushroom fruiting house – for creating a humid forest simulation chamber, to grow LOTS of mushrooms right on our back porch.

You can grow mushrooms in lots of different ways. We grow them in reusable containers – buckets and jars, and also in gardens, on logs and stumps – all with a focus on using waste materials (sawdust, straw, forest thinnings, spent coffee grounds) and avoiding single-use plastic growbags.

For the outdoor cultivated mushrooms that we grow on logs, stumps, and gardens, harvests are more seasonally dependent – we harvest mushrooms when conditions are right, and the mycelium is ready to fruit.

The alternative, which we use for the mushrooms we cultivate on pasteurised substrate in buckets and jars, is an ‘improved environment’ for our mushrooms to fruit in – helping to create the right conditions that they need to fruit.

Low tech DIY climate modification, if you will.

Most mushrooms love to fruit in high-humidity conditions – ie think a shady wet forest, at certain times of year, with great airflow and a bit of light. So how can you re-create this same climate-controlled environment down the side of your house, without large energy inputs or fandangled gear?

The way that most commercial growers do this is usually with expensive and energy intensive insulated + air-regulated grow rooms (or underground tunnels, if they’re lucky)… but we wanted to see if we could make a grow room from junk, and the absolute minimum amount of technology, using the temperate environment that we have.

Enter the offgrid, DIY mushroom fruiting house. It’s not perfect, but it does a darn fine job, and all made from scrap materials (with a few exceptions for the tricky bits).

Making the mushroom house

First, we headed to our local landfill’s tip shop and got materials – 3-4 different sets of glass shower screens, scrap aluminium framing, supermarket shelving, wire rack shelves, dishwasher trays, shadecloth.

Then, working with the factors that the mushrooms need, and what our home could provide, we got cracking on a design. And built it.

And now, we feast on mushrooms regularly. Hooray!

So what do mushrooms need to fruit properly? Here’s a summary:

  • Temperature stability – mushrooms need things stable – some like it hot, some like it cold, but they all like it consistent, once they’re fruiting. Which can be tricky to maintain in a home growing context.
  • High humidity – mushrooms need about 90% humidity to fruit really well.
  • Airflow – mushrooms need a good amount of fresh air exchange (called FAE in mushroom lingo) to fruit properly. Which is extra tricky once you’re trying to also keep them humid.
  • Good light – mushrooms need light to fruit properly, to the tune of normal daylight hours. Yes I know this runs contra to the ‘keep em in the dark and feed em shit’ mushroom joke but there you go. Light is important once mushrooms are ready to fruit.

Before we go any further, if you’re not that familiar with mushroom growing you might like to check out our other mushroom growing posts – how to make buckets of mushrooms out of a packet of spawn and so on – here’s all our mushroom articles.

So… how to make all that happen out of junk, with minimal power… took some thinking. Here’s the design:

And here’s how we dealt with the factors that mushrooms need to fruit in dependable abundance…

Creating a humid environment

the aim is to create a 90+ % humidity environment in the chamber. We achieved this with the glass panel walls, which are great for keeping the humidity in, while allowing for good airflow.

The humidity comes from a centrifugal humidifier (for reference, this type of thing) – which was the only slightly expensive bit of kit in the build. It’s mounted in a solid plastic tub in the top of the chamber, with a water inlet coming from our garden hose (which runs from our water tanks). We added a plastic toilet float valve to control the water level in the tub. The tub also has an overflow outlet, so if the float valve fails, the water doesn’t flood the electrics for the humidifier.

This humidifier is mounted at the top of the system so that the humid air falls downwards. The humid air blows through a short bit of aircon ducting, to deliver the humid air directly over the wire racks of fruiting mushrooms.

The humidifier turns on and off via a cheap digital timer (for reference, this type of thing), and is plugged into our power. When we’re fruiting lots of mushrooms, the time turns the humidifier on for 5 mins each hour, or less depending on the weather and conditions.

In addition, all our jars and buckets of substrate are soaked in water for 12-14 hours before going into this chamber, which ensures they start out nice and moist.

Technical bits: We’re offgrid on a small stand-alone solar system, so minimal power usage is critical for us. The humidifier uses a 85w motor and  it turns on for 5 minutes 12 times per day. Power usage is therefore about 85/1000kW x 5/60hrs x 12 = 0.085 kWh (kilowatt hour). To put that into perspective, a modern small energy efficient fridge will use at least 1kWh per day… so it is about 1/12th as much as that.

The water usage is very minimal – about 2-4L per day.

Creating good airflow

The chamber is not sealed – the  bottom of the chamber rests on open dishwasher racks (very sturdy plastic) topped by mesh to ensure good airflow without entry for insects – fruit fly netting or similar is good here, to protect against small fungus gnats. The top of the chamber is also screened.

This open top and bottom means that whenever the humidifier dumps moist air into the top of the chamber, it also has the effect of flushing the chamber with fresh air.

As the humid air falls downwards and slowly exits out the bottom, fresh air is drawn into the top of the chamber, replenishing the fresh air in the mushroom house.

This fresh air exchange (FAE) is critical to effective mushroom fruiting – in commercial setups, large fans are used for this purpose. This home system, with the humidity being provided by humid air entering at the top, performs this second function of FAE as a side effect, which is great for us (and the mushrooms).

Creating temperature stability

We’ve opted out of using expensive air conditioning and heating systems – partly because we’re off-grid on stand alone solar, partly because we don’t want to use a bunch of coal-fired energy to grow our food.

Instead, we’ve passively regulated the temperature of this chamber by placing it on the colder, shadier southern side of our house. The chamber is also up against a mudbrick wall, and on brick pavers.

All this thermal mass, combined with a lack of sun exposure, helps hugely with thermal stability – keeping the temperature of the chamber cool and stable.

We’re also adding some foil insulation under the verandah tin roof over this chamber for Summer, to further reduce heat.

The other thing we do is grow different species seasonally – some mushrooms prefer cooler temperatures, some prefer hotter. By growing different mushrooms at different times of year, we match the seasons with different varieties, and still get very regular harvests of home grown goodness.

Temperature regulation is just as important (if not more so) as any other factor for successful mushroom fruiting. Which is why it’s hard to get this aspect right without either a nice underground cave, or alternatively lots of artificial heating and cooling. Definitely a big factor to consider, when choosing where to fruit your shrooms.

Ensuring daylight

The chamber’s walls are glass panels, which means lots of light, at a normal daylight length for the mushrooms. Becuase the chamber is under a wide verandah on the shady side of the house (and with a concrete watertank facing) it gets zero direct sunlight, which is important for both temperature stability AND humidity.

Other factors we needed to consider were:

Fireproofing – we live at Melliodora, an iconic living permaculture system designed by David Holmgren with many specific design aspects dedicated to bushfire readiness. One of these is ensuring no plastic roofing or components anywhere near the mudbrick houses, as a fire reduction strategy – plastic burns! Mud and glass does not. And so, the greenhouses and skylights on the property are all toughened glass. This was why we went with glass panels for this chamber, which would be up against the wall of our home – turns out it was a good idea for other reasons too.

The results

The result of all this tinkering and thinkering has been a dependable supply of home grown mushrooms! What more could anyone want. We eat them fried, dried, powdered, souped and pie-ed. We trade them with friends for things we need. Deliciousness – made with scraps from the tip and a bit of experimentation. It’s a satisfying meal for sure.

Further reading + resources:

Big thanks to our new resident illustrator Brenna Quinlan for the diagram, and to the general mycophile network for the advice, sharing and learning that takes us all forward.

There’s a whole chapter on Growing Mushrooms in our book MILKWOOD if you’d like to learn more.

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