GROWING MUSHROOMS ON STRAW

Straw bags (or buckets) are a common way of cultivating gourmet mushroom species. The benefits of straw is that it has a quick turnaround, in some cases producing the first crop within two or three weeks of being made.
 

Straw bags (or buckets) are essentially an artificial log, and are therefore best suited to wood loving mushroom species. Oyster mushrooms are the most common species grown using this method. A straw-specific strain of shiitake is now also available. Enoki mushrooms are another species that are easy to grow on straw.
 

To grow mushrooms on straw, the straw must first be hydrated and pasteurised by soaking in either very hot water or water with lime dissolved in it, before being thouroughly drained. Spawn is then mixed with the straw before it's packed densely into tubular grow bags or reusable containers for incubation and eventually fruiting.

 

For more detailed information on this subject and growing mushrooms in general, we offer a printed hard copy mushroom growing booklet
 

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PASTEURISING STRAW WITH HYDRATED LIME

For the home scale hobbyist, the easiest way of preparing bulk amounts of straw is to soak it in water which has had hydrated lime mixed into it.

This works by tipping the pH (acidity / alkalinity) balance in favour of living mycelium, but inhibiting spore germination of molds. That means that when you inoculate the straw with your mushroom spawn (living mycelium) it will be able to grow, but any foreign spores (e.g. mold spores) landing on the substrate will be inhibited in their growth.

Pasteurising straw with hydrated lime is a simple process.

Materials needed:

  • Clean straw (not rotten, moldy or off-colour). Wheat, oat, and barley straw are the most commonly used types. Pea straw also works, but it is more prone to contamination as it is very nutrient rich.

  • A large barrel, or other large container like a wheelie bin or a watertight bathtub.

  • Hydrated lime (calcium hydroxide). Available cheaply at any hardware store. Usually found in the building section and sometimes in garden section.

Process:

  1. Make sure that your tub / barrel is nice and clean. Scrub off any visible dirt with soap and water then rinse thoroughly with water afterwards. A container with a lid helps a lot or at least having something you can use to weigh or strap down the straw if it tries to float. 

  2. Pack the dry straw tightly into the tub / barrel leaving about 1/4 of the space at the top. 

  3. Check the volume of your soaking container in litres. For a big batch, a 240L wheelie bin can fit up to one bale of straw and produce about 10 x 20L oyster mushroom buckets. For a small batch a 60 or 80L storage tote bin can pasteurise enough straw for 2-3 x 20L buckets.

  4. Dissolve your hydrated lime in a few litres of warm water. You need around 30-40g of lime (2 slightly heaped tablespoons) of hydrated lime powder per 10L of container volume that you're soaking in. So for a 80L container you'll need 8 x that amount or approx 250g (16 slightly heaped tbsp or one heaped cup). See chart. Measure your lime and then stir it into the hot water until its dissolved. Wear a mask when handling powdered lime and avoid breathing in its dust as it is an irritant. 

  5. Pour the lime solution over the dry straw. 

  6. Continue filling the container with plain water (e.g from the garden hose). It helps to have the lid on the container from the start and put the hose through a gap at the top, and to have the lid weighed or strapped down as the straw will inevitably try to float out of the liquid.

  7. When the container is totally full, put a weight on top to keep the straw submerged, and leave it submerged in the lime water for 6-12 hours.

  8. Drain the straw by tipping the container over (with lid still strapped on) and keep it tilted to drain for at least an hour.

  9.  The rest of the process is described below, after the hot water pasteurisation section, as the process will be the same from here on for both heat- and lime-pasteurised straw.

 

 
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PASTEURISING STRAW WITH HOT WATER

Using heat to pasteurise straw is very effective. It is the most commonly used method at a commercial scale, however it’s less commonly used at a hobby scale due to the need for access to large quantities of hot water, and a vessel (usually stainless steel) capable of holding it. At a home scale, many people use metal drums as a mid-size pasteurisation vessel.

Heat pasteurisation works by killing off any spores or mycelium of contaminating organisms that may be living in the straw. It is not the same as sterilisation - which kills everything with higher temperatures. Pasteurisation kills off most of the organisms that are a threat to your mushroom mycelium. Some particularly hardy types of bacteria etc. may remain, but they can often actually be helpful to the growth of your mushrooms and protecting the substrate from invasion from mold spores.

Process:

  1. Get a large, watertight and heat resistant vessel. An old bathtub, or a clean 44 gallon drum are common choices. A large stainless steel vessel is ideal. On a small scale, an old beer keg with the top cut off can work nicely.

  2. Pack the vessel tightly with straw. Put a weight on top to keep it submerged once water is added.

  3. Fill the vessel with very hot, near boiling water. The target temperature for pasteurisation is between 70 to 80℃, with an ideal temperature around 75℃. However since there is a large mass of straw which is at ambient temperature, the water initially added needs to be around 90℃ and it will cool down to the desired temperature range as it mixes with the cooler straw.

  4. Leave for 1-2 hours. If temperature drops below 70℃, add more hot water to bring it back above 70℃.

  5. Drain the straw and allow to cool before spreading out and inoculating with mushroom spawn before packing into grow bags.

 

 
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MAKING YOUR STRAW BAGS OR BUCKETS

Once you’ve got your straw prepared, it’s time to spawn it with living mycelium and pack it into grow bags.

Materials needed:

  • Freshly pasteurised straw (drained and cooled)

  • Healthy fresh mushroom spawn (usually grain)

  • A clean surface to work on outdoors, such as a clean tarpaulin. Plain clean concrete etc. is OK too. But a tarpaulin makes clean up easier.

  • Grow bags. Bags made from fairly durable plastic work best. When laid flat, the width of the bag should be between 30-40cm. The height can be as high as you like, but a minimum of 60cm is recommended, to leave some room at the top to tie the bag closed. Some growers who want to avoid plastic have also been known to use modified (narrowed down) coffee sacks or similar natural fibre bags. You can also construct a re-usable mushroom fruiting bucket by getting a 10-20L food grade plastic bucket and drilling holes in it spaced 10-15cm apart and 8-10mm wide.

  • A kitchen knife, for stabbing slits in the bag after it’s packed. This provides gas exchange and its also where the mushrooms will grow from.

  • Some cable ties or string, to tie the bag closed at the top.


 

Process:

  1. Lay your pasteurised straw out on the tarpaulin. If it is compacted from the pasteurisation process, you can fluff it up so that it is loose.

  2. Get your spawn and gently shake it up a bit so that the grains are separated, rather than being stuck together in a block by the mycelium. For 20L grow buckets, 330-500g of spawn is required. So you can make 2 or 3 buckets from a kg of spawn. One whole bale of straw needs 5-6kg of spawn for a good inoculation rate. 

  3. Sprinkle the spawn evenly over the straw. For an average size grow bag, half a cup to one cup of spawn is about right. The higher the spawn rate, the faster the bag will colonise and the less chance of contamination. So you need to judge for yourself how many bags you’re making and how much spawn you have available. 

  4. With clean hands, begin picking up handfuls of the straw / spawn mixture and packing it into the grow bag. It is important that you pack the material into the grow bag as tightly as possible and also avoid leaving big air gaps along the sides of the bag. Air gaps inside the bag can cause mushrooms to fruit inside the bag rather than coming out of the slits.

  5. Once your bag is mostly full, pack the straw down even more using your body weight. Once its sufficiently compressed, twist the top of the bag closed, and secure it with a cable tie or sturdy string, as close to the top of the straw layer as possible.

  6. Using a clean kitchen knife, stab a series of vertical slits in the bag. These should be 2-4cm long (basically one knife width) and should be evenly spaced 10-15cm apart across the sides of the bag. Do not stab any slits in the top or the bottom of the bag.

 

 

INCUBATING YOUR GROW BAG OR BUCKET

Your bag is now ready to incubate so that it can colonise with mycelium. This should be done in a warm (16 to 28℃) place that is also dark and not too dry. Since many warm places are also dry and bright, it is often easiest to place your straw bags inside black rubbish bags or the like, but do not tie the black bag shut as some gas exchange is still needed so that the mushrooms don’t suffocate. The black bag will keep out light and also help retain moisture.
 

After a few days, if you check the bag, mycelium should be visibly ‘leaping’ from the grain spawn onto the straw. This is a good sign that things are going well.
 

You can monitor the progress of colonisation through the clear plastic of your grow bag. Depending on temperatures and how much spawn you used, a bag should fully colonise in between 7 to 20 days. The bag can be deemed fully colonised when all of the straw is covered in nice fresh white mycelium.

 

 

FRUITING YOUR GROW BAG OR BUCKET

Once your bag is colonised it’s ready to be placed in fruiting conditions.

This means hanging or placing the bag in a nice shaded, humid spot where the bag has at least 10 to 15cm of empty space all around it (to allow for big mushrooms to grow!)

Many people simply hang their grow bag from a tree in a damp shaded corner of their garden. If you don’t have this kind of area available and you want to grow it indoors, you can make a makeshift greenhouse type structure to house your mushroom bag. This is to retain humidity, and also prevent spores from spreading around inside once the mushrooms are fruiting.

 

After a week or so in fruiting conditions, check around the knife slits in the bag. You should soon start to notice small bumps of white mycelium forming at the slit entrances. Once this happens, they will start growing into mature mushrooms very quickly!

 

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