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How to Grow Bulk Oyster Mushrooms on pasteurised straw

Updated: Jul 21, 2023

This post will give step-by-step instructions on how to reliably pasteurise wood or straw for growing oyster mushrooms using grain spawn. We will cover how to grow mushrooms in plastic bags and reusable buckets. This is a reliable method that will give consistent results, teaching you how to grow bulk oyster mushrooms for your family and friends -- and even enable you to scale-up to a level to be able to sell at farmer's markets.

At the bottom of this blog post there are a few photos to help visual-learners. They show what to expect and include an alternative zero-plastic, outdoor-option!

This method only works for oyster mushroom varieties: Grey Oyster (Pleurotus Pulmonarius), NZ Native Oyster (Pleurotus Parsonsiae), and Pink Oyster (Pleurotus Djamor).

Equipment Needed:

If there are any terms/ jargon in this blog post that you don't understand check out this blog covering >> Common mushroom terminology.

Note, we recommend that beginners start with these simpler Oyster Mushroom Kits:

>> Ready-to-grow Grey Oyster Mushroom Kit - (Easy/ Beginner)

>> 5 Step Grey Oyster Mushroom Kit - (Learn How to Grow Your Own)

>> Reuseable Bucket Oyster Mushroom Grow Kit - (Learn How to Grow Your Own)

(It's usually best to start with the Grey Oyster mushroom -- as it is the easiest to grow, produces the most and is one of the best for eating):)

Step-by-step Guide to Growing a Massive Oyster Mushroom Crop

Step 1: Clean Equipment

As we want to reduce the risk of contamination in the mushroom growing process, simple cleaning goes a long way. You don't have to be over the top, just a quick spray down and rinse is generally fine unless equipment is very dirty. Clean all equipment and work area with some soapy water - 5% hydrogen peroxide or 5% bleach spray - and then rinse with clean water.

Step 2: Mix Lime-water

Hydrated lime dissolved in cold tap water for bulk oyster mushroom cultivation
Hydrated lime dissolved in cold tap water

Fill vessel with cold tap water to about 2/3-3/4 full of water. Weigh out the hydrated lime needed, you want to add 1.5-2g lime per L of water. (A 44gal (200L) drum would take about 300-400g of >> hydrated lime. )

Add the lime to the water and stir for 2 minutes with your pitchfork, the water will turn a milky white colour. CAUTION: Be careful not to inhale hydrated lime dust (masks and eye protection can be a good idea) & protect eyes and skin as it can be very irritating. If you get some on your skin simply wash off with cold water.

NOTE: If your water is chlorinated, fill vessel and leave open for 24-48 hours for the chlorine to evaporate out before mixing in lime and beginning the soaking process.

Step 3: Add Straw to Lime-water

Prepare straw to be soaked. Straw can be chopped up, which will give a slightly better yield and will be easier to pack, but this is optional -- for most people it will be more trouble than it is worth.

Stir your drum one more time, and then begin submerging straw into the lime-water solution.

Stuff as much straw into the drum as you can. The water-lime solution is safe to touch now without causing irritation, it is only the concentrated hydrated lime powder that can irritate the skin over time.

Once the vessel is full of straw, sit your bricks or weights on top the straw to keep it fully submerged.

Step 4: Wait!

Pasteurizing straw for mushroom growing
**Straw soaking in hydrated lime water solution.

Leave straw to soak anywhere for 6-24 hours (We have had the best results with a 12-18 hour overnight soak). The hydrated lime raises water PH to 12-13 which kills most micro-organisms and potential fungal competitors. Over the soak time, the PH will slowly come down and finish around 8 which is ideal. Slightly alkaline straw is excellent for Oyster Mushroom mycelium to rapidly grow through the straw and stops Trichoderma (green mould) spores germinating. The pasteurisation process will give us a 10-14 day window for the spawn to colonise the straw -- from that point on the mushroom mycelium will hold back any competitors.

**Please note the pictured straw not submerged in the water was not used and was discarded as it is not pasteurised.**

Grey Oyster Mushroom Spawn
3KG Grey Oyster Spawn

Step 5: Drain Straw

Remove the straw from the drum and allow it to drain/ drip for about 2-3 hours. After 2-3 hours of draining we will have wet yellow-ish straw ready to be inoculated with our >> Oyster Mushroom Spawn.

Step 6: Mix Oyster Spawn with Straw

Measure out roughly 40-50L of straw per kg of spawn you have. Break up the >> Oyster Mushroom spawn by rolling the spawn through your hands (with spawn still inside the bag) -- you want all of the grains broken back up in to mostly free grains.

Spread out the straw on the tarp/ table and spread the spawn evenly overtop of the straw. Mix spawn in by hand.

Mushroom spawn added to straw substrate
Mushroom spawn broken up and added between bale books

The higher the spawn ratio to straw, the faster the straw will colonise and produce mushrooms - but you will have less final fruiting substrate overall. A minimal spawn ratio will take slightly longer to colonise and produce mushrooms but you will have more substrate and get more mushrooms over a longer time-period -- albeit with a slightly higher risk of the grow failing due to contamination. In other words, more spawn gives you mushrooms faster but slightly less mushrooms overall; less spawn will take longer and give you more mushrooms overall with a slightly higher risk of contamination.

A general rule of thumb is about 3-6kg of spawn for a full 44gal drum straw soak. You will be better off having more spawn than not enough, especially while learning the process.

For smaller batches 300-500g spawn per 20L bucket of straw works well, roughly 10-20% spawn to substrate ratio.

Step 7: Bag up your Straw Substrate

Your mushroom growing substrate spawn mix is now ready to be bagged up to incubate. Take your >> mushroom-growing plastic bag or bucket and begin stuffing the straw into it.

Once it is full, compress the straw down gently by hand and stuff in some more -- then repeat this a few more times. The straw should be tight enough that you had to work to compress it, but not so tight that you've strained and worked up a sweat to get it in!

Step 8: Tie-up your Bag

Once your bag is 80% full, squish out the air at the top, hold the top plastic and spin the bag to twist it up tight. You want as little an air-gap at the top of the bag as possible. Then take your cable-tie and tie if off close to the straw -- so the bag cannot unwind.

If using a bucket rather than a bag, your bucket should be filled level with the lid -- to avoid an air-gap at the top -- and the lid snap-locked on.

Label your bag/bucket with the mushroom type and the date.

Step 9: Cut Breathing Holes in your Bag

If using the >> bags from our shop (shown above) they already have pre-cut holes so you can skip this step.

Mushroom fruiting bag hole made with arrow head
Perfect hole made with 3-pronged arrow-head

When using other bags, holes need to be cut in the bag to allow the mycelium to breath as it grows through the straw and provide a place for the mushrooms to grow out of when they are ready to fruit.

Take a sharp knife and cut X shaped holes around the bag, you want the holes to be spaced about 10cm (4 inches) apart. Cut your first line of holes vertically from top to bottom of the bag. Then start the next vertical line of holes 10cm (4 inches) around the bag -- keep going until you have rows of holes all the way around the bag, usually 4-6 lines of holes.

Each X cut should be cut so each line of the X is about 30-40mm long.

Alternatively, you can use a 3-pronged arrow-head to easily stab perfect holes as pictured above.

If >> making/using your own bucket you follow the same pattern for the bag drilling 20mm holes. It is best to use food-grade plastic buckets for mushroom cultivation.

Mycelium growing off mushroom grain spawn through substrate
Mycelium beginning to grow off the grain spawn on to the substrate

Step 10: Incubation

From here the bags/ buckets will need to incubate for 7-12 days (Pink Oyster), 15-20 days (Grey Oyster/Native Oyster).

Incubation time can vary depending on ambient temperature. Pink Oyster is best grown during the summer if in the South Island of NZ -- as it won't grow in cooler temperatures. The ideal incubation temperature, for pretty-much all mushrooms, is between 15-25 degrees Celsius. Try to keep the bags out of direct sunlight to avoid moisture loss.

During the incubation period you will see white strands beginning to grow off of each grain kernel, this is the mycelium growing through the substrate to break it down to produce mushrooms. In 7-14 days, the bag should be almost fully white and will continue to get whiter and whiter. Please note: Grey Oyster mycelium is much thicker - and denser - than Pink Oyster mycelium.

Step 11: Mushroom Pinning

Oyster mushroom pins growing off straw NZ
Mushroom pins growing out of the holes we cut.

Pinning, the bags will begin to pin (produce baby mushrooms) after the incubation period is finished. From here, the mushrooms will draw moisture and nutrients from the straw and grow rapidly.

You can actually see this process beginning with the mycelium getting much thicker around the holes in the bags a few days before the mushrooms show up. Keep the bag out of direct sunlight and move to a humid area to produce mushrooms.

From this point, mushroom growth will be rapid with mushroom's almost doubling in size every 24 hours.

Step 12: Rapid Mushroom Fruiting

If you are growing the mushroom outdoors, you can hang the bag up from a tree or inside a shed. You need reading-level light for the mushrooms to grow well.

Outdoor grown mushrooms can attract bugs, so pick mushrooms early, after 4-5 days when edges are still curled down to get them before the bugs!

If you are going to fruit the bag or bucket inside, bathrooms can be great as your shower will provide some humidity helping them grow.

Alternatively, you could setup a wee >> mushroom green-house-tent and spray water on the sides each day to keep humidity high: leave the zipper unzipped to allow for fresh air to get in.

PROTIP: If you want to grow higher quality mushrooms, you can make a >> misting unit laid out in this blog here -- this can be attached to the grow tent, automating the mushroom fruiting environment!

Step 13: Harvest Your Oyster Mushroom Crop

Perfect oyster mushrooms ready to harvest NZ
Perfect oyster mushrooms ready to harvest

Harvesting your hard work! Your mushrooms are ready to pick when the edges begin to flatten out. If you are not sure if they are ready it is best to pick Oyster Mushrooms earlier rather than later -- before the edges begin to turn upwards!

To harvest the mushrooms grab the entire cluster at the base, twist and pull. The cluster will pop off the bag.

Oyster mushrooms drop a lot of spores once the edges turn upwards, this will make a mess if inside the house and can potentially cause an allergic irritation to some people.

Once harvested the bag will sit dormant for 1-4 weeks, gathering up nutrients and then more mushrooms will grow (this is called a 'flush'). You can easily get 6+ harvests off some bags!

Mushrooms in the picture here are at the ideal harvest time, 12-24 hours later and they will be a bit over ripe! If your not sure always pick slightly early!

Closing Thoughts:

Once you have done this process a couple of times, it becomes very simple and takes up little time. You will be able to grow more mushrooms than you are able to eat in no time. Making bags up every 2-4 weeks will give you a constant supply of fresh nutritious mushrooms.

Many issues that may happen during the growing process are covered in one of my other blog posts: >> "Troubleshooting Oyster Mushrooms" -- which shows and discusses common issues when growing oyster mushrooms.

I like to advise people to start with the grey oyster mushroom as it is generally the easiest to grow, produces the most and is one of the best for eating.

If you are wanting to go a little further and start growing to supply your local market and community please feel free to fill in a contact form and we can discuss weekly commercial spawn supply with some better rates!

Outdoor method:

This method can be done for whole bales to be grown outdoors also, this bale was soaked hole, strings cut off and filled with spawn, then tied back together and left sitting out under garden growth and foliage. It produced masses of oysters all summer long.

Oyster mushroom's grown outdoors on a hay bale - ready for harvest
Oyster mushroom's grown outdoors on a hay bale - ready for harvest
Mushroom pins finding their way to the holes in pre-punched bags - bags are available from our online shop. The plastic these bags are made of are very thin and the mushrooms will stretch the hole open as they push through!
Mushroom pins finding their way to the holes in pre-punched bags - bags are available from our online shop. The plastic these bags are made of are very thin and the mushrooms will stretch the hole open as they push through!

Oyster Mushroom Mycelium growing through substrate in a bag, about 5 days growth
Mycelium growing through substrate, about 5 days growth

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