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How to Make Grain Spawn Jars – The Complete DIY Guide

How to Make a Spawn Jar Lid
Updated on 09/17/22

One of the most critical parts of growing mushrooms is the spawn phase. This is when your mycelium culture is at its highest risk of contamination.

In this guide, we are going to look at a cheap and effective way to make lids for grain spawn jars that will let your mycelium thrive while protecting it from contaminates!

Let’s get to it!

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What is a Grain Spawn Jar

A spawn jar is a container full of nutritious grains that we use to inoculate and cultivate a mycelium culture. It is the medium that we use to start and colonize our culture.

While the container itself can be as simple as a mason jar, the lid has more strict criteria.

A good spawn jar lid includes two things:

  • An Injection Port: A hole where we can inject our spores or liquid culture that minimizes contamination.
  • Gas Exchange: A hole that allows ample CO2 and oxygen exchange that also minimizes contamination.

By including the two items above we can maximize the rate at which our culture colonizes and greatly reduce the potential of contamination.

So with that in mind, let’s start craft our DIY spawn jar lids!

Spawn Jar Lids – What You’ll Need

To make our spawn jar lids, we need three things: A lid, a gas exchange filter, and a self-healing injection port.

Most people use canning jars for their spawn, and in this guide, I’ll be doing the same. If you are looking to use another type of container, simply modify the steps below to your setup.


Alright! With all our materials, let’s get crafting!

How to Make Lids For Grain Spawn Jars

In this build, I’ll be using the alternative plastic mason jar lids rather than the original metal ones. I prefer these plastic lids for several reasons including:

  1. They don’t get impossibly tight on the jar after sterilizing.
  2. They are easier to work with than metal.
  3. They are cheap.
  4. They are easier to clean.
  5. They look cool.

Now don’t feel that you need to upgrade to these plastic lids. They are a little nicer to work with but don’t add any other real benefits to your spawn.

Step 1: Drill Your Holes

Alright, our first step is to drill three 1/4″ holes into the lid. One into the center for our gas exchange, and two on either side for injection ports. For reference, check out the photo below!

Spawn Jar Lids - Step 1: Drill Three Holes

Note: If you want, you can use one injection port instead of two. However, more injection ports allow you to inoculation more sections of the jar. This greatly increases the speed of colonization but also increases the chance of contamination. I recommend two injection ports as a great balance between speed and risk of contamination.

Step 2: Create Your Self Healing Injection Port

Alright, now that we have our holes, let’s make our self-healing injection ports.

We are going to be using a method referred to as the “self-healing injection port tek.

Spawn Jar Lids - Gasket Maker

Take your High Temp Silicone and place a bead on the hole large enough completely cover it. With your finger, flatten the silicone against the lid. When working with liquid gasket maker, I would highly recommend wearing gloves as this stuff is very very very messy.

Spawn Jar Lids - Flatten Top

Flip your lid over and also flatten any silicone that pushed through the back.

This will create a nice injection bubble on the front

Step 3 - Flatten the Back

Let your new injection ports dry for at least 12 hours and keep them in a safe place away from your animals….trust me on this one.

[Alternative] – Rubber Self Healing Injection Port

If you want to put a little more pep in your spawn jar’s step, you may consider using rubber self-healing injection ports. These awesome little things pop right into your holes and hold tight.

A 100 pack of these rubber injection port costs nearly the same as a tube of High Temp Silicone so they make for a cool alternative!

Spawn Jar Lids - Self Healing Injection Ports

Step 3: Create Your Gas Exchange

For the last step, we are going to create our gas exchange. The goal of the gas exchange is to allow fresh air in and old air out, but also restrict contaminants from entering.

One of the easiest and cheapest ways of accomplishing this by covering the hole with two layers of micropore paper tape as seen below.

Spawn Jar Lids - Micropore Tape

[Alternative] – Polyfill

An alternative to micropore tape is polyfill. You can find this stuff in nearly any craft store or even inside an old pillow and it is a cheap, but effective, filter for your gas exchange.

To use polyfill, simply grab a handful like seen below and cram it through your gas exchange hole. Cut the extra material off both sides to keep it clean and from rubbing up against your grain spawn.

[Alternative] – Syringe Filter

The last—and coolest—alternative for gas exchange are syringe filters. Similar to rubber self-healing injection ports, these filters are going to give you the best protection against contaminants.

Just glue it into your gas exchange hole and let it do the rest. Quick, effective, and just as cheap as the other gas exchange filter options.

Wrapping It Up!

I hope this little guide on making inoculation jars lids has answered all your burning questions! However, if there is something I missed or if you have any comments, be sure to leave those below!

Also, be sure to check out our other guides on mushroom cultivation here, and if there is anything you would like to see me cover about this topic, leave those suggestions in the comments below or throw me an email here!

Happy Harvest!