You’ve earned this badge! You now understand how to compost your spent substrate and return nutrients back to nature!
Congratulations on your first harvest! But here's the good news: your bucket isn't done yet.
After harvesting, the mycelium is still alive and ready to produce more mushrooms. You can do this in two different ways: either wait for more flushes, or restart your growkit. On this page, we explain how to do these steps.
A flush is a complete cycle of mushroom growth from pins to harvest. Think of it like a fruit tree: after picking the first apples, the tree can produce more fruit if conditions remain favorable. Your mycelium works the same way.
After you've harvested your first cluster and cleaned away the leftover mushroom pulp from the hole, you can help the fungi produce a next flush.
The mycelium has lost moisture during fruiting, so you need to add a small amount of water to rehydrate the substrate.
Use a plant spray to mist the exposed mycelium through the holes from the outside and spray directly into the bucket.
If you have some space left in your bucket, you can also add a thin layer (about 1 cm) of fresh, moist coffee grounds. This provides additional nutrients for the fungi. Then close the lid and wait. Within 1 to 2 weeks, you can see new pins emerging, either from the same holes or from different ones.
If no pins are emerging, you can give the mushrooms another cold shock, just like in the ‘Growing & Harvesting’ step.
The growkit is made out of 100% recycled plastic. Unfortunately, we cannot make the bucket out of organic materials, because then the material would be eaten by the fungi, and eventually break down. The pro of using plastic is that you can use it over and over again, and it holds water and moisture best.
One of the beautiful things about the rotterzwam grow kit is that the plastic bucket is designed for multiple uses. Reusing your grow kit multiple times dramatically reduces waste and maximizes resource efficiency. A single bucket can theoretically produce mushrooms for years if properly maintained, turning hundreds of cups of coffee waste into kilograms of fresh food.
This is circularity at the household level: regenerative, local, and delicious. Growing mushrooms doesn't have to be a one-time experience: you can restart your bucket and keep the circular cycle going indefinitely. However, restarting requires some special attention to give your mycelium the best chance of success.
After your second or third flush, when mushroom production slows down or stops completely, it’s time to refresh the system. Here’s the step-by-step process:
Scoop out most of the spent substrate (old coffee grounds and exhausted mycelium), leaving behind about 20% of the healthiest, whitest mycelium in the bucket. This remaining mycelium will serve as your “starter culture” for the next round.
Bright white colour with a dense, rope-like texture.
Fresh smell: earthy and mushroom-like, not sour or rotten.
No discolouration: avoid keeping any sections with green, orange, black or pink spots. The brown or orange liquid (metabolites) you might see is normal, but you don't need to keep it for the restart. Focus on the cleanest, whitest patches of mycelium.
Active growth: mycelium that still looks vibrant, not thin or yellowed.
Follow the same process from Step 1 and Step 2. Start with about 100g of moist, cooled coffee grounds mixed with the remaining mycelium, then layer new grounds every time the mycelium fully colonizes the previous layer. You're essentially "rebooting" the bucket with a head start from existing mycelium.
As you reuse the mycelium over and over, it will gradually lose vigor. This is natural. Like any living organism, mycelium ages. You'll notice:
Slower colonization: it takes longer for white mycelium to spread through new coffee
grounds
Increased contamination risk: weaker mycelium struggles to outcompete green molds and bacteria
Smaller yields: mushroom clusters may be less abundant or slower to develop
If your mycelium is exhausted or heavily contaminated, you can start over with a new spawn bag and new tape. These supplies are available on the rotterzwam webshop.
Restarting with fresh spawn and new tape gives your bucket another new life. Plus, you’ll already know exactly what to do: you’re a mushroom-growing pro now!
As a special bonus for experienced growers like you, use the discount code “GrowMore!” at checkout to support your continuous growing journey.
So, what do you do with the 80% of substrate you removed from the bucket? Don't throw it away! The mixture from your grow kit (used coffee grounds mixed with exhausted mycelium) is what we call "spent substrate" and it's an excellent soil amendment for your garden.
If you have a vegetable garden, flower beds or houseplants. Mix the spent substrate directly into the soil.
No garden? No problem.
If you don’t have outdoor space, you can also add it to your compost bin. The mycelium will continue breaking down organic matter in your compost, speeding up decomposition.
Many municipalities compost this material and use it in parks and public gardens, so your coffee grounds still complete the circular journey, just on a larger scale
You can also read our blog about returning to the soil:
Coffee grounds are high in nitrogen, phosphorus, and potassium: the building blocks of healthy plants
The decomposed mycelium adds organic matter that helps soil retain moisture and improves aeration
The mycelial network encourages beneficial bacteria, fungi, and earthworms in your garden
Coffee grounds become less acidic as they decompose, making them safe for most plants
If you have a garden and want to supercharge your soil, rotterzwam regularly has bags of spent substrate available for free pickup at our Rotterdam facility.
Sign up on our website to get notified when fresh batches are ready. Urban gardeners and community gardens love this stuff. It's free, local, and packed with nutrients.
Take a moment to celebrate: you’ve just completed a full circular cycle! You’ve turned waste into food, learned a new skill, and contributed to a more local, sustainable food system. Let’s put some numbers to your achievement.
Based on your harvest and the coffee grounds you used, here’s an estimate of the positive impact you made:
You diverted waste, saved water, reduced carbon emissions, and produced fresh, local, nutritious food, all from your kitchen counter. That’s the power of circular thinking!
rotterzwam is more than a mushroom farm. We’re a social enterprise building circular food systems in Rotterdam.
By purchasing grow kits, attending our programs, or spreading the word, you’re supporting:
Local, sustainable food production in urban environments
Social impact: employing people with distance to the labor market
Education and awareness about circular economy
principles
You’ve mastered the basics of mushroom cultivation, but there’s so much more to explore. If you’re hungry for deeper knowledge or just want to grow even more mushrooms, rotterzwam offers several learning opportunities:
Growing mushrooms at home might seem like a small act, but it’s part of a much larger shift toward regernative, circular, local food systems.
Every time you save coffee grounds, every harvest you share with friends, every meal you cook with your home-grown oysters, you’re proving that sustainability can be delicious, accessible, and fun.
