How to Reuse Mushroom Substrate

What is Flushing Mushroom Substrate?

Mushrooms after flushing substrate

After your first crop of mushrooms has sprouted, the substrate is known as "spent". Much of its nutrients have been consumed by the mycelium, the vegetative part of the fungus, but it’s not entirely depleted. Paul Stamets, author of Mycelium Running: How Mushrooms Can Help Save the World, explains that mycelium continues to provide valuable nutrients even after an initial harvest _[1]. Many mushroom growers find that their substrate can yield a second or third flush of mushrooms before it’s entirely spent.

Here are two effective techniques to encourage that next flush, along with key elements for maintaining humidity, temperature, and substrate health.

The Dunk

The dunk technique is well-known for boosting yield and is a reliable method. After the first flush, carefully harvest the mushrooms, ensuring you don’t damage the substrate. Twist and pull gently or cut them at the base with sterilised scissors. Check the substrate for any decaying or mouldy material, removing any remnants of broken mushrooms or contaminated areas to prevent spreading.

Submerging used substrate during the dunking process

To rehydrate the substrate, fill a large vessel with room-temperature water. Submerge your cake or substrate block completely for up to 24 hours, weighing it down gently with a clean object to keep it submerged. I tend to dunk for a couple of hours to prevent over saturation. This rehydration process ensures the mycelium receives enough moisture to trigger a second flush. Once rehydrated slowly and carefully pour off any excess water.

If you’ve got small cakes of substrate, after dunking, you can roll the substrate in a layer of sterilised vermiculite, which helps retain moisture during the next flush. Return the cake or substrate to the grow chamber, increasing humidity back to 90-95% and maintaining proper ventilation. Keep the temperature steady at around 21-25°C, providing indirect light for about 12 hours a day.

Misting and Fanning

Another effective technique is to mist and fan the cake. For this method, instead of fully dunking, mist the surface of the substrate with clean, sterilised water. I recommend using a heat-resistant jar in a pressure cooker for 30 minutes to sterilise the water adequately. If you don’t have a pressure cooker, boiling the water for 10-15 minutes will kill most bacteria and harmful organisms. I advise against using the fridge to cool your jars, as this poses a risk of further contamination. You can leave the jar for an hour and then place it in a cold water bath to cool down.

Misting is an effective technique to maximise substrate

Mist the substrate thoroughly until the surface glistens, but avoid over-saturating it. Continue misting 2-3 times a day to maintain moisture, focusing particularly on areas that appear dry. Additionally, fan your growing chamber 3-4 times a day to increase oxygen flow, stimulate the mycelium, and initiate a second flush.

Once your substrate is sufficiently hydrated, carefully pour off any excess water. Now it’s time to focus on the environment. Maintain high humidity levels (90-95%) through regular misting, keeping a stable temperature of around 21-25°C and providing 12 hours of indirect light daily. Proper distribution of fresh air and humidity will help reduce contamination.

Within a week or so, you should see new pins forming. Continue with the misting and fanning regimen to encourage healthy mushroom development. For a third flush, repeat the same misting and fanning process after harvesting the second flush. I’ve found that this technique produces more even pin sets albeit with generally smaller mushrooms.

Even with optimal conditions, reusing substrate has limitations. Each subsequent flush of mushrooms is likely to be smaller, and the risk of contamination increases as the substrate age. As Tradd Cotter points out in Organic Mushroom Farming and Mycoremediation:

each flush depletes the substrate further, and contamination becomes a greater challenge with each cycle _[2].

However, achieving a second or third flush is still a significant win!

Compost your Spent Substrate

After a few flushes, even the most magical substrate will run out of steam. But don't toss it yet! Here are some excellent ways to continue its lifecycle. Once the substrate is fully spent, it makes an excellent addition to compost. According to Michael Phillips, author of The Holistic Orchard says that spent substrate:

is a brilliant source of organic matter for your garden, improving soil structure, moisture retention, and nutrient availability _[3].

Mushroom waste is healthy for composts

Adding mushroom substrate to your compost mix provides nitrogen, speeding up the decomposition process.

As your compost breaks down, it creates nutrient-rich humus that can be returned to your garden beds, whether for growing vegetables, flowers, or even more mushrooms!

You can even introduce worms into your compost. Amy Stewart, in The Earth Moved: On the Remarkable Achievements of Earthworms, notes that worms thrive on decomposing organic matter, including spent mushroom substrate _[4]. The result? Worm castings, which are nutrient-rich organic fertiliser that can enhance soil fertility in garden beds or potted plants.

Simply mix the spent substrate with other organic material like kitchen scraps or shredded paper, and your worms will do the rest.

So can I reuse Mushroom Substrate?

Yes, you can reuse mushroom substrate, but it’s important to manage your expectations. A second or third flush of mushrooms is possible, and with a little ingenuity, you can refresh and revive spent substrate to extend its life. When it’s finally time to retire it from mushroom growing, composting or vermiculture will give that substrate new life in your garden. As Paul Stamets would say, fungi are nature’s recyclers, so why not take a cue from them?

Get Mushroom Substrate from the experts

So where can I get sterilised Mushroom Substrate?

Right here. We provide Mushroom Substrate sterilised in ISO 5 rated environment, expertly mixed and ready to go! Okay, so you got me - this might be a shameless product plug squeezed in here, but I genuinely believe it! And as master cultivators (or soon to be) - when you try it you will too!

★★★★★ Never let me down yet will keep purchasing.

- Lee edwards on 18th October 2024