Mushroom Grow Bags and Filter Bags
A mushroom grow bag is a polypropylene bag with a microporous filter patch: mycelium breathes through it, contaminant spores do not get in. Two things determine whether a bag suits your job - the micron rating of the patch and whether it has a self-healing injection port. Get either wrong and you are opening the bag when you should not have to.
Choosing a bag
| Micron | Use for | Why |
|---|---|---|
| 0.2 micron | Grain spawn, agar work | Tightest filtration - grain is nutrient-dense and contaminates easily |
| 0.5 micron | Grain and general purpose | Good balance of gas exchange and protection |
| 5 micron | Bulk substrate, fruiting blocks | Faster gas exchange, which colonising substrate needs |
- Injection port - a self-healing rubber patch you inject through. Essential if you are inoculating with a syringe rather than working in front of a flow hood.
- 3 mil polypropylene is the standard. It survives autoclave and pressure cooker cycles; polyethylene bags melt.
- Gusseted bags stand up on their own and hold substrate in a usable block shape.
- Size to the job - small bags for grain, large gusseted bags for 5-10 lb fruiting blocks.
What to skip
- Any bag not rated autoclavable. It will deform or melt in a pressure cooker.
- Taping over a torn filter patch. A compromised patch means the bag is compromised.
- Using 5 micron bags for grain. The gas exchange is generous enough to let contaminants in on a substrate that cannot fight back.
Frequently asked questions
What micron filter do mushroom grow bags need?
0.2 to 0.5 micron for grain spawn, 5 micron for bulk substrate and fruiting blocks. Tighter filtration for the more contamination-prone material.
Can mushroom grow bags be autoclaved?
Yes, if they are polypropylene rated for it - typically 3 mil. Check the listing; polyethylene will not survive.
Do I need an injection port?
If you are inoculating with a spore or culture syringe and do not have a flow hood, yes. It lets you inoculate without opening the bag.
Can grow bags be reused?
No. The filter patch degrades and the bag is a disease vector between grows. They are consumables.
