To grow mushrooms, commercial products like sawdust, coffee grounds and straw, as well as birdseed, can be used as substrates (bases), as long as you provide the right conditions. Mushrooms require substrates that provide a porous, moist and warm environment, along with access to air warmth and circulation. It's important that you use an appropriate substrate in concert with a pressure cooker, which helps to develop the right growing conditions.
Pour birdseed or another substrate into a plastic pot or bucket and add tap water until the bucket is three-quarters full. Allow the birdseed or substrate to soak overnight to prepare it for mushroom growing.
Pour the mixture through a strainer to drain it. You don't need to remove any husks that have separated from the main body of the birdseed. Run water through the mixture to rinse it again.
Fill each jar three-quarters of the way full with birdseed or substrate. Put polyfill on top of the birdseed to fill the jar.
Drill holes in the tops of the jars. Holes should be one-quarter inch to 3/8 inch. Screw the lids on, then loosen them by a quarter turn. This will allow circulation inside the jar, which will help mushrooms grow.
Wrap coffee filters around the tops of the jars and use rubber bands to secure them. These will keep your new mushrooms sheltered during growth while still allowing circulation.
Cook the jars at 15 lbs, for 60 to 90 minutes in the pressure cooker. Always handle the pressure cooker with care, as it will get hot and steam.
Turn the pressure cooker off and allow the temperature to drop down to 0. Use oven mitts to remove the jars from the pressure cooker. Tighten the lids and shake the jars to break up any clumps in the birdseed or substrate. Put the jars in a safe place, like the oven, and let them cool overnight.
Purchase mushroom culture from a local nursery or home and garden shop. Unscrew the lids and inject the culture into your growing substrate, then replace the lids. Follow the instructions on the label of the mushroom culture in regard to quantity and injection.
Place the jars in a cupboard or cardboard box and allow the mushrooms time to grow. Read the label on your mushroom culture to get a good idea of how long your mushrooms will take.