Mushroom Grow Bags: The Ultimate Guide

What Is a Mushroom Grow Bag?

When someone says “mushroom grow bag” they are usually talking about a gusseted autoclavable polypropylene filter patch bag. OK, I’ll admit, that’s a lot of terms. Let me break it down.

Gusseted

Gusseted simply refers to the way the bag is folded, with the sides of the bag folded flat towards the inside. One reason for this is merely so that the bag folds small and lay flat, yet can open up wide to take on a large amount of grain or sawdust. This shape is also quite useful during sterilization, where the cultivator can slip a filter in between the gussets. This prevents contamination from the air that enters the bag during the post-sterilization cool down.

Autoclavable

Autoclavable just means that the bag is able to withstand the high temperatures of an autoclave, or sterilizer. Typically this is in the form of a pressure canner operating at 15PSI, which subjects the bag to a temperature of about 250 deg F, sometimes for hours at a time. Bags not specifically designed for this would likely melt.

Polypropylene

Polypropylene is the material in which the bag is made out of. It is a “thermoplastic” which won’t melt unless temperatures are sustained between 266 to 340 °F. This makes it an ideal candidate for holding your grain or sawdust during sterilization. A pressure canner wouldn’t see those temperatures unless the pressure got to be over 25 PSI, which is way over the safe limit for home cultivators.

Filter Patch

This refers to the filter on the front of the bag, and is one of the most useful aspects. The filter allows for the mushroom cultures inside the bag to receive fresh air without passing contaminants like mold spores and bacteria. Without fresh air, the mushroom culture would eventually stop growing, but if the bag were just left open after sterilization, contaminates would quickly take over. The filter patch is the perfect solution.

Uses for a Mushroom Grow Bag

There are many awesome ways that you can use mushroom grow bags for mushroom cultivation. The most common uses are spawn bags and sawdust fruiting blocks.

Spawn Bags

Mason jars are useful for making first generation grain spawn, or when only running test batches, but if you are producing large volumes of grain spawn it is much better to use a grow bag. Having your spawn in a grow bag allows you to break up your spawn easily in order to mix it and speed up colonization. It also makes it easier for spreading your spawn to a bulk substrate. Anyone who has tried to pull well colonized spawn out of a grain jar knows that sometimes it just doesn’t want to come out. You end up digging it out with a spoon, and risking contamination in the process. Picture trying to get that stubborn ketchup out of a glass bottle. Spawn bags solve this problem. Usually you can get 5lbs of spawn in a large spawn bag, which, in my experience is a perfect amount. It is enough to spawn a small straw log, or several fruiting blocks. You could even use it to inoculate ten more spawn bags!

Fruiting Blocks

Filter patch grow bags are an indispensible tool for making sawdust fruiting blocks. A large bag will typically make a 5 lb fruiting block, including the weight of your spawn. The bag allows you to easily mix your spawn after inoculation, and the filter patch allows your mushroom culture to breath while it grows throughout the sawdust. Once your mushrooms have fully colonized the bag, you can either cut the holes or “x’s” in the side of the bag, or cut the top of the bag off entirely in order to fruit your mushrooms. Cutting the bag with the edges at different heights will allow you some fine control over the humidity on the grow block.

Fruiting in the Bag

Another way to use a mushroom grow bag is to use it as a mini fruiting chamber! 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. Ensure that you don’t open the bag until the block is fully colonized, or else your bag is likely to get contaminated. Some mushrooms will grow inside the bag in interesting ways, notably the Reishi mushroom. In the high CO2 environment of the inside of a bag, the Reishi will grow long and twisted “antlers” instead of the more familiar conk. If you let Reishi grow long enough, it will eventually fruit right through the filter!

Reuse the Bag

Although you probably want to start with new bags when making fruiting blocks or propagating spawn bags, you can always reuse your bags for other purposes, as long has you haven’t cut the bag open for fruiting. For example, used spawn bags make for a great container to pasteurize small batches of casing layer on the stove. Simply fill the bag with your casing layer and set it in a pot of hot water on the stove. You may need to place something heavy on top of the bag to hold the bulk of it under the water. You can also reuse the bag to make mini straw logs instead of using poly tubing. Just fill the cleaned bag with pasteurized straw and grain spawn and you normally would when growing mushrooms on straw. You could use the bags again for spawn or fruiting blocks, but the integrity of the bag and filter patch decreases after multiple sterilizations, so you are better off using new bags to reduce the chances of contamination.

Filter Size

When you purchase spawn bags you need to pay attention to what is specified for the filter. Typically, the filter will be given a micron rating. This simply describes the pore size of the filter. The sizes typically range from 0.2 micron to 5 micron. If your bags are to be used for fruiting blocks or spawn bags, you’ll want to shoot for a small micron rating, no higher than 0.5 micron. This will be required to adequately protect your substrate from contamination. Mushroom bags with a 5 micron filter will allow for more gas exchange, but will likely lead to a much higher contamination rate and are not recommended for sterile cultivation.

Bag Size

These bags are generally available in a variety of sizes. The size of bag you want obviously depends on how much substrate of grain you want to put in your bag, and other factors such as the size of your sterilizer. In my experience, the large sized bags are by far the most useful. To give you an idea, a bag of size 8 inches wide by 5 inches deep by 18 inches tall will comfortably hold 5 lbs of supplemented sawdust and grain spawn. You can just fit 6 of these bags in a Presto Pressure Canner.

Material Thickness

The bags come in a variety of different thicknesses, with the typical range being 2.2 mil to 4.0 mil. Bags of thickness 2.2 mil work fine, but you can expect a small percentage of your bags to rip during normal handling. If your bag rips post-sterilization, there is good chance that your bag will get contaminated. Thicker bags are less likely to tear, but come at a higher price. My recommendation would be to go with thinner 2.2 mil bags, and take reasonable care when handling your bags.

Using a Filter Sleeve

A good trick to know when using grow bags is how to use a slip-in filter to prevent contamination during post-sterilization cool down. After sterilization, as the bags are returning to room temperature, room air will naturally be drawn into the bags. If this air is not filtered properly then there is a risk of contamination before you even get a chance to inoculate your bags. In industrial mushroom farms, this problem is eliminated by allowing the bags to cool down in a clean air environment. Often the autoclave will be cooled before it is opened, and the air is drawn in through a HEPA filter on the outside of the sterilizer. This is not that practical to do on a small scale. The way around this is to slip a small cloth filter into the bag between the gussets before sterilization. Tyvek works great for this. You can even use cut up pieces of a painter’s suit, which can be found at any hardware store or on Amazon. The filter should go between the gussets above the substrate. Make sure the inside walls of the bag are clean before inserting the filter. Then, simply fold the bag over a couple times and load it in to the sterilizer. There is no need to seal the bag before sterilization. Any air drawn into the bag during cool down will be passed first through this tyvek filter. Simply pull the tyvek sleeve out after the bag cools off, and right before inoculating your bag. I have used this method with almost a 100% success rate.

How To Seal a Mushroom Grow Bag

After you inoculate your grow bag, you’ll want to seal the top before you set it on a shelf for colonization. There are a couple easy ways to do this. The simplest method is to use small pieces of wire. After inoculating the bag with your spawn, quickly twist the top of the bag and tightly wrap the the wire around the twist. You can use something like this, or any steel wire found at your local hardware store. You could also use zip ties, which are quick and easy but much less cost effective. Of course, you could always step up your game and use an impulse sealer. If you are pumping through a high volume of bags, an impulse sealer will save you some time and effort, but for small scale grows, it is likely not worth the cost.

Biodegradable Grow Bags?

One concern about using mushroom bags, especially for large scale commercial operations is the environmental cost. Polypropylene bags are not all that environmentally friendly, and after a short while end up being a huge amount of waste. Some growers are investigating using biodegradable mushroom bags, although there is nothing currently commercially available that can compete with the effectiveness and utility of a autoclavable polypropylene filter patch grow bag.  

Best Mushroom Grow Bags

There are plenty of places that you can find grow bags online, although most of them come from a just a handful of small suppliers in China. If you are looking for large volumes of bags (1000 or more at a time) check out Unicorn Bags. They have all types of bags, in every size and shape, and all different filter sizes. I haven’t personally ordered from them, but they have been around for a while and many growers use them with success.

If you are looking just to get a smaller quantity of bags you can find them on Amazon here.

Good Luck With Your Bags

This may be more information than you ever wanted to know about grow bags, and if you read this far you just may be mushroom grower! Let me know in the comments below if you have any other questions or need any more clarification. As always, best of luck with your grows!