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.
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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.
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.
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.
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” 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.
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 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.
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.