High-Yield Straw Mushrooms, Part 7: Grade II & III Spawn

In Part 6, VNmushrooms Farm walked through how to isolate mother spawn (Grade I spawn), and from those mother spawn bottles, we can produce Grade II and Grade III spawn bottles through propagation. So how exactly is this straw mushroom propagation method carried out? Let’s dig into it in this guide.

Grade III straw mushroom spawn — also called production spawn — can be packaged in either bottles or plastic bags. Bottled spawn has higher purity, while bagged spawn is more convenient for cultivation and saves labor. Each method has its own advantages.

Grade II and Grade III Spawn Culture Media

To produce standard-grade straw mushroom spawn, VNmushrooms Farm walks through the 5 media formulas we’ve used to effectively culture Grade II and Grade III straw mushroom spawn in the table below:

Formula	Main Ingredients
Formula 1	Rice straw 90%, wheat bran (or rice bran) 9%, gypsum powder 1%
Formula 2	Cottonseed hulls 94%, wheat bran 5%, gypsum powder 1%
Formula 3	Wheat grain 98.5%, gypsum powder 1%, calcium carbonate 0.5%
Formula 4	Sugarcane bagasse 100 kg, wheat bran 15 kg, NPK compound fertilizer (nitrogen, phosphorus, potassium) 1 kg, lime 5 kg, water 150 liters, pH adjusted to 8–9
Formula 5	Dried cow manure 48%, chopped wheat straw (or rice straw) 50%, lime 2%, water as needed

Formula 1: Rice straw 90%, wheat bran (or rice bran) 9%, gypsum powder 1%

Mixing method:

Step 1: Prepare and process the raw material

Choose dry, golden-colored rice straw with no mold or damage. Cut the straw into roughly 3 cm segments, then soak it in clean water for about 12 hours so it fully absorbs water.

Step 2: Mix the ingredients

After pulling the straw out, wring off the excess water, then mix it thoroughly at the right ratio with wheat bran (or rice bran) and gypsum powder.
The right moisture level is reached when squeezing the material tightly produces a steady drip of water.

Step 3: Fill the spawn bottles/bags

Load the material into the spawn bottles or bags, using a wooden rod to pack it moderately firm — not too loose, not too tight.
Each 750 ml spawn bottle should hold about 75 g of dry straw. Only fill up to about 5 cm below the bottle/bag opening — don’t overfill.
Once filled, use a pointed wooden rod to poke a small hole in the center, going about halfway down the bottle, to make later inoculation easier.

Step 4: Clean the spawn bottles/bags

Wipe or rinse away any residue clinging to the inside and outside of the bottle/bag opening. Once the interior is dry, plug it with a cotton stopper, wrap the opening in paper, and take it for sterilization.

Formula 2: Cottonseed hulls 94%, wheat bran 5%, gypsum powder 1%

Mixing method:

Pre-moisten the cottonseed hulls.
Then mix in the remaining ingredients.
Once moisture reaches about 60%, proceed to bottling. Bottling follows the same method as Formula 1.

Formula 3: Wheat grain 98.5%, gypsum powder 1%, calcium carbonate 0.5%

Mixing method:

Soak the wheat grain in 12–15% lime water for 4 hours, then boil it for 15 minutes.
Drain and let it dry off slightly; once the material has cooled, mix it thoroughly with the gypsum powder and calcium carbonate, then move straight into bottling.

Requirements for bottling with this formula:

First, add a thin layer of rice husk at 30–40% moisture to the bottom of the bottle.
At the neck of the bottle, add a thin layer of cottonseed hulls or rice husk as a plugging material to reduce contamination risk.
Fill the rest of the bottle/bag using the same method as Formula 1.

In our experience, this method works very well for producing Grade II straw mushroom spawn. Using it for Grade III spawn tends to invite contamination easily, since the nutrient content is too rich.

Formula 4: Sugarcane bagasse 100 kg, wheat bran 15 kg, NPK compound fertilizer 1 kg, lime 5 kg, water 150 liters, pH adjusted to 8–9

Mixing method:

Grind the sugarcane bagasse finely and sift it. The rest of the process follows Formula 1.

Formula 5: Dried cow manure 48%, chopped wheat straw (or rice straw) 50%, lime 2%, water as needed

Mixing method:

Grind the sun-dried cow manure finely.
Cut the straw into roughly 5 cm segments.
Mix in the gypsum powder while piling the material into a heap and watering it as you go.
Compost for 3 days, turning the pile twice, until the straw softens — then proceed to bottling.

Sterilizing the Straw Mushroom Spawn Material

Once bottling/bagging the straw mushroom spawn material is complete, sterilization needs to happen the same day to prevent the medium from fermenting and spoiling. The sterilization method is the same for both Grade II and Grade III straw mushroom spawn.

If using a high-pressure steam autoclave, sterilize at 137–147 kPa for 1.5–2 hours. If using an atmospheric-pressure steam sterilizer, boil continuously for 7–8 hours, then let it sit sealed overnight.

Inoculating the Straw Mushroom Spawn

Once sterilization is complete, immediately move the spawn bottles/bags into the inoculation room or inoculation box, let them cool, then proceed with inoculation.

Using an inoculation tool, take a piece of fully developed mother spawn (Grade I) from the slant agar test tube, along with a small amount of agar medium, and quickly place it into the pre-made hole in the center of the culture medium bottle, then reseal with the cotton stopper.

Standard Propagation Ratios — A Benchmark for Buying Spawn

Spawn Grade	Propagation Ratio
1 tube of Grade I spawn	→ 4–8 bottles of Grade II spawn
1 bottle of Grade II spawn	→ about 60 bottles (or bags) of Grade III spawn

Incubating the Straw Mushroom Spawn

After inoculation, move all the straw mushroom spawn bottles/bags into the incubator or spawn run room, holding the room temperature at 30–32°C. Checking can begin after 24 hours.

Before the mycelium fully covers the medium’s surface, it’s best to check once a day so any contaminated bottles can be removed promptly.

Once the mycelium has fully colonized the bottle, continue incubating for another 3–5 days to reach the ideal spawn age. For a 750 ml bottle, total incubation time typically runs 25–30 days.

One thing worth stressing: the spawn run room needs to stay clean and well-ventilated, and the area just outside the room needs to be kept clean as well.

Straw Mushroom Farming: Frequently Asked Questions

How long does straw mushroom spawn need to incubate before it’s ready for production use?

Once the mycelium has spread across the entire surface of the medium, continue incubating for another 3–5 days. For a 750 ml bottle, total incubation time is typically 25–30 days before it’s ready for production.

Should one bottle of Grade II straw mushroom spawn be propagated into a large number of production-grade bottles?

No — one bottle of Grade II spawn should only be propagated into about 60 bottles/bags of Grade III spawn (production spawn) to avoid compromising quality and vigor.

Should small-scale straw mushroom growers make their own Grade II and III spawn?

Not unless you already have the proper equipment for quality spawn production. Buying spawn from a reputable supplier is generally more reliable for yield and more cost-effective overall.

How can you tell if straw mushroom spawn is healthy?

A healthy, standard-grade spawn bottle/bag should show mycelium growing evenly, spreading to fully cover the medium, with no unusual coloring, liquid seepage, or off odors.

Should I use Grade II or Grade III spawn for production?

It depends on your scale and goals. If you need a large volume of spawn for large-scale cultivation, propagating to Grade III cuts costs and saves time. For smaller-scale straw mushroom growing, Grade II spawn gives you better control over purity and lower contamination risk.

In this guide, VNmushrooms Farm walked through the correct technique for producing Grade II and Grade III straw mushroom spawn (production spawn). So what does a quality spawn bottle actually look like, and how should it be stored? Find out in “High-Yield Straw Mushrooms, Part 8.”

And if you’re running into challenges or want extra guidance to get more yield and quality out of your straw mushroom crop, don’t hesitate to reach out to the VNmushrooms Farm engineering team right away!

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