The Ultimate Guide to Mushroom Grow Bags for Fruiting and Spawn (2024)

What is a Mushroom Grow Bag?

A mushroom grow bag is, quite simply, a bag designed for growing mushrooms[i]. Many kinds of mushrooms can be grown in a bag, depending on what kind of substrate is put in the bag. There are several brands and types, but all good grow bags have several features in common.


Grow bags are made of polypropylene plastic designed to withstand high temperature. The plastic is durable, flexible, clear enough to make monitoring growth easy, and relatively low-cost. There are bags being developed using “greener” materials, but none of them have come to market yet.


The reason temperature resistance is important is that both the substrate and the inside of the bag must be sterilized before inoculation. A convenient sterilization method is to stick the bag and its contents in an autoclave—except that most plastic bags would melt in such conditions. “Autoclavable” just means that the bag can be safely used in an autoclave. An autoclave is a machine that uses steam under pressure to kill bacteria.

Filter Patch

A filter patch is a small area of the bag that allows air in through an extremely fine mesh. The mesh is so fine that airborne bacteria and the spores of unwanted fungal species (weeds) can’t get through. Unlike a vegetable garden, a mushroom grow set-up can’t be weeded; the weeds have to be kept out from the beginning or the batch must be abandoned. Mushroom grow bags needs filter patches because while it is important to keep out weeds, the mycelium must be able to breathe (another thing, besides the need for food, that fungi have in common with animals).


A gusseted bag has folds in the sides so that a large, very roomy bag can still fold nice and flat when not in use. The mushrooms don’t care whether grow bags are gusseted, but the feature makes things easier for growers be organized.

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Why Use Mushroom Grow bags?

It’s possible to grow mushrooms without bags. Of course, some substrates, such as logs, don’t need and can’t fit in bags, but even substrates often used with bags, such as coffee grounds or sawdust, don’t actually require them. Alternatives include wide-mouth glass jars and plastic buckets and tubs (these are sometimes available used from restaurants at no cost), provided the containers in question can be sterilized and fitted with air filters[ii]. There are also low-temperature sterilization methods and ways to grow mushrooms that don’t require sterilization at all—such methods dramatically broaden the range of set-ups that can be used.

Some growers seek to avoid grow bags specifically to avoid creating so much plastic waste, since the bags are generally single-use. There may be other reasons for avoiding grow-bags, too.

However, grow bags offer certain features that are difficult to access otherwise:

  • It’s easy to cut holes in a bag, allowing the fungus to fruit out in all directions, instead of just upwards. The same can’t be said of glass.
  • The clear plastic makes it easy to monitor the mycelium as it colonizes the substrate. With buckets, it’s harder to tell when colonization is complete and when the fungus is ready to fruit.
  • The spent substrate—or the fully colonized spawn, in the case of spawn bags—is easy to remove from a bag. Because the mycelium tends to hold the substrate together in a clump, getting the clump out of a jar or even a bucket can be a real challenge.
  • The flexible bag makes it easy to mix the spawn thoroughly through the substrate; just squeeze. The same can’t be said of a hard-sized container.
  • Start-up costs are low, even for large operations, since bags are cheap.

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What Are Mushroom Grow Bags Used For?

Mushroom grow bags are used for growing mushrooms, that much is clear, but there are multiple stages to the mushroom-growing process, and grow bags can be used at several of them.

Spawn Bags

“Spawn” is material growers use to multiply up the amount of mycelium they have quickly, so as to be able to grow a much larger crop[iii]. Rye and other grains are popular spawn media, though there are others. Generally speaking, the procedure begins with the germination of spores on a Petri dish of nutrients, where a small, baby mycelium begins to grow. The baby in the dish is then transferred to a larger quantity (up to several pounds) of sterilized spawn substrate. Once the spawn is fully colonized, it can be used to inoculate the substrate used for growth and fruiting—the spawn can also be used to inoculate more spawn. Many people buy bags of spawn ready to go, but some growers produce their own.

Because spawn needs sterile conditions and must be monitored closely, clear polypropylene bags are a good option.

Fruiting Blocks

A fruiting block is a unit of some mushroom substrate that does not naturally hang together, such as sawdust. The grow bag serves both to neatly contain the material and to keep out weed spores and other contaminants. Once the mycelium has fully colonized the block, weeds can no longer gain a foot-hold and the container can be opened to allow the fungus enough room and air to fruit. Again, the clear bag makes it easy to see whether colonization is complete, and the plastic can be cut open to allow more fruiting without risking damage to the mycelium below.

Fruiting in the Bag

It is also possible to keep the bag closed during fruiting, though in this case the bag must be only partially filled to give the mushrooms plenty of room to grow. The bag then serves as a miniature fruiting room where humidity and other factors are easy to control. The trick is to open the bag frequently; the air filter alone doesn’t allow enough air, and the mature mycelium, like an animal in a similar situation, would gradually run out of oxygen if kept too enclosed.

Allowing the fungus to fruit entirely inside the bag means not needing a climate-controlled room. For some species, there is an added benefit. Even with opening the box often, there will be less oxygen and more carbon dioxide, in a bag than out of it, and the difference can sometimes force mushrooms into startlingly different growth forms.

Choosing the Right Mushroom Grow Bag

Not all mushroom grow bags are exactly the same, but choosing the right bags for a given operation means paying attention to just three variables: filter size, bag size, and bag thickness.

Filter Size

Filter sizes vary. That is, the size of the holes, or pores, in the filter varies, usually from 0.2 micron to 5 microns. The smaller the pore size, the less can get through. That means fewer spores and other contaminants, but it also means less air. The key is to strike workable a balance between the need for air and the need for protection. For most purposes, the filter should be no larger than 0.5 micron.

Bag Size

There is no hard and fast rule about bag size. Bags that are too small are inefficient; bags that are too big are inconvenient—but what sizes are too big or too small varies from grower to grower. Points to consider include the size of the sterilizer and the size of the hoped-for crop. Above all, how big will a bag of a given size be when full?

A large bag, one that is eight by five inches across and 18 inches tall, can hold about five pounds of inoculated substrate.


The thicker the plastic, the stronger the bag—but the stronger bags are also more expensive. The normal range is between 2.2 mil to 4.0 mil. Growers can start off with bags at the thin and inexpensive end of the scale, and if the bags rip too often (a ripped bag is vulnerable to contamination), step up to something thicker. The occasional rip is not considered a problem, just part of the cost of doing business.

How to Seal a Mushroom Grow Bag

Grow bags must be sealed after being inoculated and the bags themselves don’t come with seals. Wire ties and zip ties both work, but the time necessary to apply these ties, though brief on a per-bag basis, adds up for people who have to seal a lot of bags. Large-scale growers buy impulse sealers and find the greater expense is worth it.

Can I re-use a Mushroom Grow Bag?

Yes, mushroom grow bags are reusable, but not necessarily as mushroom grow bags. The plastic gets weaker with use, and many growers do not want to risk a bag coming apart—or developing tiny holes through which weed spores could pass—mid-cycle. But the bags can be reused in other contexts. For example, material that needs to be sterilized and then transferred to some other container can be boiled inside an old grow bag. Bags can also be used for making straw logs. Some growers are even experimenting with processing the material into mushroom substrate; there are fungi capable of eating certain plastics.

Polypropylene is also closed-loop recyclable[iv], meaning the recycled plastic can be used for the same types of products (including mushroom grow bags) as the virgin plastic. That is not true of all plastic types. Growers concerned about the issue should research the environmental impacts of several different grow set-ups to determine which is best in their situation.

[i] Mushroom Grow bags: The Ultimate Guide, accessed on 10 September, 2019.

[ii] The Permies website, accessed on 10 September, 2019.

[iii] Bradley, K. (2015). Mushroom Cultivation: Making Grain Spawn. Milkwood. Nov. 23, 2015.

[iv] LeBlanc, R. (2019). An Overview of Polypropylene Recycling. The Balance Small Business. May 9, 2015.

The Ultimate Guide to Mushroom Grow Bags for Fruiting and Spawn (2024)


What are the fruiting conditions for mushroom grow bags? ›

Fruiting in the Bag

The top of the bag creates a humid dome for the mushrooms to fruit. Keep in mind that there won't be a lot of fresh air inside the bag, so if you are using the bag to fruit your mushrooms, ensure that you open the top of the bag often in order to get adequate fresh air exchange.

Do mushroom grow bags work? ›

Some mushroom cultivators like to use mushroom grow bags because this method of growing offers several benefits, including: Mushroom grow bags are generally pretty easy to keep sterile, especially if they're equipped with a filter patch (more on this in a moment) Mushroom grow bags can help growers to conserve space.

How do you fill mushroom grow bags? ›

Fill the growing bag with the substrate mix Add oyster mushroom spawn to your hydrated pellets, mix everything together and add it into your plastic grow bag. Leave the top third of our straw pellet bags vacant to allow for air exchange through the filter when utilised using straw pellets.

Where is the best place to put a mushroom grow bag? ›

While this can be good, airflow is also important and small enclosed spaces can cause your mushrooms to suffer from lack of oxygen. We recommend placing your grow kit on your kitchen bench, out of direct sunlight.

What is the best substrate for mushroom grow bags? ›

Straw or hardwood sawdust are common substrates for growing mushrooms, but there are a variety of other good options to consider.

What are the disadvantages of grow bags? ›

What are the Disadvantages of Using Grow Bags? They can dry out quickly, especially in hot weather, requiring more frequent watering. They may not be suitable for plants that require deep soil. Growbags can be easily punctured or torn, which can cause soil to spill out.

How much does a mushroom bag yield? ›

While some growers report harvests of 10-15 grams of dried mushrooms per pound of substrate in these bags, others might achieve less, often from significant side pinning, and especially in the case of contamination or suboptimal growing conditions.

When should I mix my mushroom grow bag? ›

You can shake up your mushroom spawn bag when the mycelium has colonized about 40–50 percent of the spawn grain. As mentioned previously, this gives access to non-colonized patches of mycelium. You'll be surprised by how much this simple 30-second action can help your yield.

How often do you spray mushroom grow bags? ›

Drape the bag over your block and mist the inside of the bag every day. When you start to see baby mushrooms form (pinning), you can remove the bag and keep misting the block.

Do you water mushroom grow bags? ›

You don't need to water mushrooms like a plant.

The spray bottle is for creating humidity. Humidity is a key factor for successful mushroom growth. Spray the area around your kit with a few pumps of water whenever you have a chance, at least 2-3 times a day.

Do mushroom bags need light? ›

⇒ Typically most people have always colonized spawn bags in the dark like the jars. Recent testing and research has shown that light doesn't have any effect on colonizing. Since premature pinning before 100% colonization doesn't happen with spawn bags, do whatever is convenient for your setup.

What should grow bags sit on? ›

Grow bags can be used indoors like clay and ceramic pots. Be sure to put a saucer under the bag, just like if it's on a deck or patio. Grow bags can be cleaned. For best results, manufacturers recommend you hand wash with a gentle detergent and allow to air dry.

Do mushroom grow bags expire? ›

All in one mushroom grow bags: Leave in original packaging. Store your all in one grow bag in a cool and dry location. All in one grow bags can keep for up to 3 months in these conditions. There will be no need to rehydrated or sterilize the products a second time.

How do I know if my mushroom grow bag is contaminated? ›

Spend time getting to know what your mycelium is supposed to look like so you can better identify unhealthy or contaminated patches. While mycelium is typically white and filamentous, contamination generally takes the form of green, blue, gray, or black patches or discolorations in your substrate.

What are optimal mushroom fruiting conditions? ›

Humidity. The humidity with a fruiting chamber must remain high in order to encourage pinning and to all the growing mycelium to retain as much water as it can hold. The relative humidity(RH) of the air around fruiting mycelium should be at least 90%.

What temperature and humidity should a mushroom fruiting chamber be? ›

Button mushrooms, temperature of 75-77 degrees Fahrenheit for mycelium growth then once the mycelium is fully formed temperature range should be 53-60 degrees & 90% for air humidity level. For straw mushrooms it's 95 degrees wherein growth can occur between 59-113.

Why is my mushroom block not fruiting? ›

Severe lack of fresh air &/or light: otherwise healthy block! Mushrooms are the opposite of plants in that they 'inhale' oxygen and 'exhale' CO2: much like us humans. Oyster mushrooms in particular produce a lot of CO2 as they grow, and need a steady supply of fresh air to develop normal fruit bodies.

How do I know when my substrate is ready to fruit? ›

it usually takes about 2 weeks (10-16 days). Once the kit starts to fruit you will see tiny mushrooms or “pins” forming on the substrate.

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