Winter mushroom project

Over a year and a half ago, we received a Next Stage Grant from the Mill City Farmers Market Charitable Fund.  Our project was to build an indoor fruiting chamber so we could grow mushrooms inside during the winter – for farmers market and grocery sales. In order to grow, mushrooms need optimal temperature, humidity, and fresh air (CO2).  Here’s how we hope to maintain control of these conditions:

    • temperature – the chamber is insulated because it’s located in a re-purposed walk-in cooler box, and using electric heat to heat it.  We are also preheating fresh air as it comes in (see below).
    • humidity – we are installing water misters to maintain 90% humidity, under pressure and after reverse osmosis treatment. We’re still working on how to provide this much humidity!
    • fresh air (CO2) levels – we are using a fan to force preheat fresh air through a pre-conditioning plenum.  Fresh air is necessary to maintain the quality of mushrooms.

Someday we’ll write up a detailed list of the shenanigans involved, but this project has had it’s shore of challenges right from the start. We re-purposed a walk-in cooler for the chamber and on day 1 we could not get the walls to go back together! Adding insulation, additional flooring, piping, electrical, lights and so forth for the first iteration has gone slowly, and not without some adventures!

We finally got the water hooked up with pumps, misters, reverse-osmosis, etc and we’ve spent the last couple months tracking down all the various leaks and replacing various joints, elbows, and other random pieces. It’s like that old game Water Works – we’re almost done, almost to our goal, and someone puts a leaky pipe somewhere in the works!

The worst of it was last Sunday when too much pressure built up (we’re still not sure how or why) and the pipes in our basement (that lead out to the fruiting chamber) burst, filling the basement with water! (Don’t worry, we got the water all pumped out by the end of the day and our basement is back to normal.)

Our current challenge is getting humidity up to 90%, which we are hoping to do with misters piped in along the ceiling.  For this we need filtered water at 40psi (pounds/square inch).  Our approach has been to channel water into a reverse osmosis unit, then re-pressurize the cleaned water – which is stored in a 55 gallon tank.  For this system to work however you need just the right pump (not the one we currently have!) that pressurizes to 40psi but with a very low water flow about the water flow we need – only 5 gallons per hour! It sounds like a pump with too much water flow will burn out quickly if the water flow that you allow is much lower than the maximum water flow allowed by the pump.

We are currently re-evaluating our humidification system.  We may use an in-line filter instead of RO, which will allow us to maintain the pressure from our municipal water.  Or we may use a system that re-pressurizes the water just right with a compressor.  We’ll keep you updated!

However, despite all this, we’ve been able to maintain a moderately humid environment for the mushroom blocks – by doing things by hand.  For example we’re out there two or three times a day misting the blocks with a hand plant mister!  We’ve had a small number of mushroom blocks fruiting away, growing shiitake, oyster, lions mane, and chestnut mushrooms, and we’re happy to say we’ve been able to meet much of the demand for fresh mushrooms at the last two Mill City Farmers Market winter markets!

A big thank you to the Mill City Farmers Market Charitable Fund and the folks that donate to that. This project took way longer than we thought it would, but we wouldn’t have been able to get started without their help!


fruiting chamber feb 2019
large pipe is fresh air intake small pipes will provide water to misters
chamber interior drainage
drainage channel – interior of fruiting chamber
chamber drainage and sink
exterior of fruiting chamber – including drain pipe for chamber and hand washing sink
preconditioning plenum1
preconditioning plenum – insulated chamber to pre-heat fresh air
preconditioning plenum2
interior of pre-conditioning plenum. Duct fan blows air into plenum ducts, and into pipe leading to fruiting chamber