By Martin Zorrilla

Growing gourmet mushrooms can be surprisingly easy. While serious myco-nerds obsess over sterile conditions and glove boxes, you don’t need any of that to enjoy fresh oyster mushrooms throughout the year. This is Martin’s method for growing oyster mushrooms on coffee grounds without sterilization or special equipment. The results are tasty, rewarding and strangely beautiful- the perfect solution for those with green thumbs who are stuck in the doldrums of winter.

You Will Need:

• A steady supply of coffee grounds from the household coffee drinker
• One mouth ball jar or washed yogurt container
• 5-2.4 lb Oyster mushroom spawn (see below)
• Cardboard, scissors and a spray bottle

Step 1. Procure Spawn

To grow mushrooms you need mushroom spawn. In this case spawn is nothing more alarming than ‘starter’ we can use to grow more mushrooms. Spawn is active mycelium, a web of cells growing and feeding on a substrate, such as wood or plant matter. The ‘mushrooms’ we eat are merely the reproductive structures that emerge from the mycelium network when conditions are right. For our purposes, spawn consists of bags of sawdust fully colonized by oyster mushroom mycelium. By giving this spawn new substrate and more moisture, we will make it grow and produce reproductive structures that we can eat.

Here are my favorite online spawn providers in the US:

1. Fungi Perfecti has a limited selection of strains but at reasonable prices and the website contains valuable information for new growers. $20 per 5 lb bag.
2. Field and Forest have an unparalleled selection and will let you buy 2.5 lb bags – more than enough. $24 per 5 lb bag and $18 per 2.5 lb bag.
3. The Mushroom People offer a great selection of strains as well as supplies for ambitious growers. $18 for a 5 lb bag.
4. Others include: AshvilleFungi, SporeTradingPost, and Territorial Seed

Note: Due to the importance of properly maturing spawn, it may take 2-3 weeks for delivery, particularly from smaller producers.

Most strains of Oyster mushrooms will work for indoor production. However, if you are growing in the winter you may want a cold-weather strain such as ‘Blue’ or ‘Blue Dolphin’. Likewise warm-weather strains such as the gorgeous ‘Golden’ will produce more in summer months.

The beauty of spawn is that it can be stored in the refrigerator for months at a time. I have had bags of spawn keep for up to 6 months (although viability does decline over time). You only need small amounts of spawn, and having it around can lead to some innovative uses!

Step 2. Innoculate Coffee Grounds

Inoculation happens when spawn is mixed with a substrate, something the mycelium can grow on and use for food. Coffee grounds are a fantastic substrate for mushrooms for two main reasons: the process of making coffee sterilizes the grounds and the acidity of coffee staves off spoilage bacteria. My method of growing oysters on coffee grounds involves adding small amounts of spawn to a container of coffee grounds as they accumulate from daily use.

Start with a wide mouth ball jar or yogurt container, cardboard and roughly tablespoons of spawn:

Pile cardboard snippets at the bottom of jar to prevent moisture from accumulating over time. Then, reserving 3-4 tablespoons, add however much freshly used coffee ground you have available (the grounds should not be too hot to touch).

Add your spawn, crumbling any chunks, evenly across the surface of the coffee. If either the coffee grounds or the spawn feel dry to the touch (the grounds should be black with moisture), hit them with a spray bottle. Cover the jar with a lid left partially ajar to allow some oxygen flow.

Step 3. Add Coffee & Wait

Keep your jar accessible and out of direct sunlight. Every time you make coffee  add your spent grounds to the jar. Adding them hot is fine, since they won’t directly touch the spawn. Once the jar is 1/2 half full add another layer of spawn. Continue filling the jar until the grounds are 3/4 inch from the top- then add a final layer of spawn. To maintain the humidity, top this final layer with a fine layer of moist grounds. 

Throughout the process you can watch the mycelium grow into the grounds, each layer growing towards each other. If you see green mold appear, don’t worry – oyster mushrooms are voracious and will often defeat and consume these invaders.

Once the jar is full, all you have to do is wait. Leave jar out of direct sunlight- and in the meantime you can start a fresh one! In 2-3 weeks the mycelium should spread throughout the grounds. If the top grounds dry out (turn light brown and dusty) before the mushroom gets to them, spray them lightly with tap water.

Step 4. Fruit and Harvest!

Your jar will be ready to fruit once the grounds are completely white with mycelium and the top of the jar displays a carpet of mycelium as in the image below:

Often your jar will fruit on its own once it reaches this stage. If this is the case all you have to do is give the baby mushrooms space to grow and enough moisture (spray bottle) to prevent them drying out. However, your jar may need a small nudge to encourage this process. The nudge should take the form of increased oxygen flow- the trick is to do this without drying the mushroom out. If the air in your mushroom garden is not overly dry you can do this by removing your lid completely. If your air is dry (less than 60%) try cutting a cheap plastic container such as a used tupperware – cut several holes in it and place it over the jar mouth. 

A happy carpet of mycelium will begin to fruit by forming adorable mini-mushrooms call “Pins.” Most of these will quickly grow into mushrooms- but they are most vulnerable to drying out in their early stages. Once the mushroom is larger than 3-5 cm the fungus is committed and generally will not dry out. 

   

Oyster mushrooms can be harvested when the cap edges flatten.  Do not discard your jar after harvesting! In 2-3 weeks you can get another flush. Our favorite ways to eat Oyster mushrooms are in Risotto  or simply fried in butter with salt. Happy Growing!