The World is your Oyster mushroom

The World is your Oyster mushroom

Oyster mushroom gardening                                

There are many ways to incorporate growing edible mushrooms into your garden. You can grow mushrooms outdoors in beds, towers, containers, in the mulch on pathways or around trees or incorporate mushroom growing logs into your garden. For example, put a stack of shiitake logs in a cool, damp corner or use logs inoculated with oyster mushrooms as the edging for raised beds (see our blog post on inoculating logs here). You can also bury fully colonised fruiting blocks in the ground. You could buy a ready-to-fruit kit and once it has fruited indoors, remove the packaging and bury it in a shady spot in the garden. When the conditions are right (usually after rain) it might fruit again! Not all mushrooms are suited to outdoor growing. Some of the best ones to start with are oyster mushrooms. The best types to start with are grey oyster or elm oyster as these are very forgiving and fruit over a wide range of temperatures. Once you have mastered these you could try pink or gold (summer fruiting) or blue oyster (winter fruiting).


We recommend starting with an oyster mushroom tower or bed. The basic steps are:

  1. Choose a cool, shady, humid place for your installation
  2. Obtain some substrate (food for the mushrooms - usually straw for oyster mushrooms) and oyster mushroom spawn (the ‘seeds’)
  3. Pasteurise and hydrate the straw by either cooking it or soaking it in water with hydrated lime overnight
  4. Build your tower or bed structure
  5. Plant (or in more technical terms, inoculate) your mushrooms!


Choosing the site

Mushrooms need humidity. Look around your garden for places that are sheltered, shady and damp. Think about how the conditions in the spot will change with the time of day and the seasons (oyster towers can fruit for several months). Deep shade is best. Against a south facing wall or in a shady corner could work. Underneath other vegetation like trees, shrubs or bushes is good as they will help to hold humid air around the tower. You will need to water your installation and the area around it regularly, so choose somewhere that you can get to with a hosepipe or watering can, or even better, somewhere that gets automatically irrigated without you needing to remember!


Gathering materials

For the oyster tower/bed you will need:

  • Oyster mushroom spawn (included in the Afrifungi oyster tower kit available here) – 2kg of spawn is enough for one large tower OR two small towers OR 1 square meter of bed
  • Hydrated lime to pasteurise your substrate (included in the Afrifungi oyster tower kit) *Note: not all types of lime will work -  make sure it is calcium hydroxide and not calcium carbonate
  • Chipped straw (this will be the ‘food’ for the mushrooms) – about half a bale will be enough for a medium size tower
  • Wood chips, sawdust or cardboard as a buffer between the soil and the straw (Poplar shavings sold as horse bedding work well)
  • Wire mesh (optional)
  • Plastic wrap (optional)


Hydrating and pasteurising your substrate

Mushroom ‘food’ is called substrate. Oysters will grow on a wide range of woody substrates including wood chips, sawdust (preferably not pine), straw, seedpods, coffee, cardboard, paper and even cloth and cigarette butts. We recommend straw for this project because its relatively easy to get hold of (try horse feed or agricultural supply stores), easy to work with, resistant to contamination and oysters love it. It is best to chop it up a bit because shorter pieces colonise faster and pack better into the tower. You can do this by hand with shears or scissors (not recommended for large amounts), using a weed wacker/strimmer in a large bucket, or (best) with a garden chipper. 


No matter what substrate you use, it must be hydrated to the correct moisture content. For mushrooms, this means that it should hold as much water as possible without actually dripping. This is called ‘field capacity’. If you are using straw, the pasteurisation process will also hydrate it substrate by the correct amount, so you don’t need to worry about doing this in a separate step. If you need to hydrate any other substrate to field capacity, slowly add water while mixing well, until when you take a handful and squeeze it as hard as you can, one or two drops of water, but not more, come out.


We pasteurise the straw to reduce the number of contaminating mould and bacterial cells. This gives the mushrooms that we want to grow the best chance at taking over the substrate. You can pasteurise by heating e.g. hold it at a temperature above 70C for an hour. You can do this by placing straw in mesh bags in a cooler box, weight them down with a couple of bricks and cover with boiling water. An easier way for large quantities is the cold pasteurisation method. Adding hydrated lime (CaOH, also known as slaked lime or white hydrated lime used for water purification NOT builders lime which is calcium carbonate) to water creates a very high pH (12-12.5) which, given enough time, will kill most contaminants. You will need a large watertight container (large enough to hold the amount of straw you want to pasteurise – a garbage bin or big tote works well). You need to submerge the straw overnight in hydrated-lime water. There are two ways to do this:


  1. Fill the large container with water (leave some space at the top because the straw needs to fit in too). Add about 3 grams of hydrated lime per litre of water and stir to dissolve (this is about ½ to ¾ of a cup per 100L). If you have a pH meter you can add hydrated lime until the pH is around 12.5. Pack your chipped straw into mesh bags (like an orange bag or sew your own out of shadecloth) and push them under the water. Place something heavy on them to hold them underwater and leave for 12-24 hours.


  1. Place a few cm of dry straw into the bottom of your container and then sprinkle some hydrated lime on top until the surface looks while – like icing sugar on a cake. Add another layer of straw and then more lime. Spray some water onto the mixture to wet it (this helps to pack it down, but not so much water that it starts to float. Continue to add more layers until you reach the top of the container. Place a heavy weight on top of the pile of straw to weight it down and then fill the container with water so that all the straw is submerged. Leave overnight.


After 12-24h, you need to drain the water off the straw. If it is in bags, pull them out of the lime water and hang/place them somewhere to drip dry. If the straw is packed loose into a tub, tip the water off and then leave it upside down to drain completely. Leave it to drain/dry for several hours. It is ready when it still feels damp but when you squeeze a handful no more than one or two drops of water come out. If some spots dry out too much, don’t worry, just mix your straw well before using it to even out the moisture content. Try to use your pasteurised straw the same day that its ready because, although the pH will remain high for a while, providing some protection, contaminants will start to move in so it’s best to inoculate with the oyster spawn as soon as possible.


Note: do not put the lime water onto the garden – it has a very high pH and may damage plants. Flush it down the drain or re-use it to pasteurise another batch.


Build your structure

Once you’ve chosen your shady, cool, humid spot and pasteurised your straw you are ready to go! The size and shape of your installation is up to you. For a tower, we recommend a diameter of 30-40cm and a height of around half a meter. You can go shorter and wider if you prefer. You can go also go taller (especially if you are using the plastic wrap) but we’ve found that the mushrooms like to fruit lower to the ground because the humidity is greater there. The higher your tower is the more it is exposed to air movement and dry air. Recently we have started sinking our towers into the ground to keep them cool and damp, with just about 10-20cm above ground. For a bed you can use any dimensions. A depth of 20-40cm is ideal. The bigger your installation the more mushrooms you will get. 


Option 1: above ground plastic wrap tower method

Form your wire mesh into a tube about 20-40cm and anywhere up to 1m high. Stand it on the ground to form a tower. Preferably put a layer of cardboard, wood chips or sawdust in the bottom to create a buffer between the soil and the contents of the mushroom tower, but this is not absolutely necessary. Wrap plastic wrap around the walls of the tower but leave the top open. This is to protect the tower from drying out. 


Option 2: sunk tower

Form your mesh into a tube with your chosen diameter and a height of up to half a meter. Dig a hole wider than the tower and deep enough that 10-20cm of the mesh will stick up above the ground. Place the mesh into the hole and pack some dry wood chips or sawdustinto the base of the tower and between the outside wall of the tower and the soil. This creates a buffer between the soil and the straw to prevent contamination and also helps to hold moisture and give a bit of extra food to the mushrooms. You can also line the bottom and walls of the tower with cardboard or newspaper. 


Option 3: sunk or raised bed

Either create a raised bed or dig a trench or hole in the ground large enough to contain your substrate. Two or three bricks high is perfect. You can also use inoculated logs to make the bed. Line the bottom of the bed with cardboard. 


Plant your mushrooms!

Before you start, take your bag of grain spawn (still sealed) and break it up with your hands until there are no large lumps left. Then you need to mix the spawn with the pasteurised substrate and pack it tightly into the installation. We usually do this using the lasagne method as follows: 

  1. Place a few big handfuls of straw into the bottom of the tower and pack it down firmly
  2. Sprinkle some grain spawn on top (a handful is enough. The grain spawn should be about 10-20% of the amount of the substrate (straw). The more grain spawn you use the faster it will colonise, up to around 25-30% grain at which point it is not beneficial to add more)
  3. Add another layer of straw and then more grain spawn. Pack it down well – you do not want any air gaps
  4. Repeat until you read the top of the tower/bed


If you have used plastic wrap, cover the top of the tower with plastic. If it’s a sunk tower or a bed, add a layer or dry wood chips, sawdust, newspaper or cardboard on top to prevent the straw from drying out.


How to care for you installation

For the first couple of weeks while the mushroom is colonising (growing through) the substrate, water your installation lightly every second day or so (like you would water seedlings, adjust according to the weather – you need to keep it damp but not soggy). After 10-14 days, you can gently lift some of the straw on top and if you see fuzzy white stuff (mycelia) everywhere, you know that your installation is fully colonised. Now it will wait for the right conditions to fruit (make mushrooms). These conditions are species dependent but usually include: a drop in temperature, a good soaking (e.g. rain shower), cool, humid conditions, lots of fresh air and little bit of light (never direct sunlight). You can try to induce fruiting by soaking your installation with water, but ultimately the fungi will decide when they are ready to gift you some mushrooms. Be patient and keep checking – they particularly like to fruit hidden at the back or bottom of the towers when you least expect it. Enjoy!

 Get your Oyster mushroom spawn to get going here!


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