How Long Do Grain Jars Take to Colonize? (2026 Timeline & Speed Tips)

Wondering how long it takes for your grain jars to fully colonize? In this post, I break down the average colonization timeline, what factors can speed it up or slow it down, and how to know if something’s gone wrong. Whether you’re growing gourmet or medicinal mushrooms, getting your timing right can make or break your grow. I also share a few tips I’ve learned from experience to keep contamination low and growth steady.

GROWING

6/22/20253 min read

🧠 How Long Do Grain Jars Take to Colonize? (And What to Watch For)

If you're just getting into mushroom growing, grain jars can feel like a mystery. You inoculate the jar, store it somewhere warm and dark, and… then what? How long until it’s fully colonized and ready for the next step?

Let’s break it down.

"I remember my first jar of grain spawn, it was a gifted jar of Wild Bird Seed (WBS). I found myself checking it every few hours, wondering: 'Is it ready yet? How about now? Should I shake it?' I was instantly hooked on watching that white mycelium slowly claim the grain. If you're currently in that 'waiting game,' here is exactly what I've learned over the years about the timing and the science of colonization."

🕒 Typical Colonization Time

Most grain jars take 10 to 21 days to fully colonize, but that depends on a few key factors:

Grain type: Rye and wheat berries usually colonize a bit faster than brown rice or oats.
Spore vs. liquid culture: Spore syringes take longer (often 2–3 weeks or more). Liquid culture usually speeds things up (sometimes 7–14 days).
Temperature: Ideal temp is 70–78°F (21–26°C). Too cold slows growth. Too hot = risk of contamination.

👀 What to Look For

Here’s what you’ll typically see during the colonization process:

Day 3–7: Small white spots (mycelium) start growing around the inoculation points.
Day 7–14: Mycelium spreads across visible grains.
Day 14–21: Full jar coverage, including the bottom.

If it’s taking longer than 3 weeks and you don’t see strong white growth, something may be off (like contamination or weak spores).

🔁 Do I Shake the Jar?

Yes — shaking around 25–30% colonization can help speed things up by redistributing the mycelium. After shaking, it may stall for a few days, then bounce back strong.

Tip: Don’t shake more than once unless absolutely necessary.

Want a deeper look at the 'Break and Shake' technique? Check out my full guide on When and How to Shake Grain Jars for a step-by-step breakdown of exactly what to look for before you mix things up.

⚠️ Signs of Trouble

Green or black spots = mold
Wet, slimy grain = bacteria
Sour or funky smell = contamination
Think your jar might be a goner? Read my detailed guide on How to Identify Grain Jar Contamination to see photos of mold vs. healthy mycelium.

If in doubt, toss it. Contamination spreads fast.

💡 Pro Tip

Label your jars with the inoculation date. It helps you track colonization speed and notice slow jars early on.

Frequently Asked Questions

Can I colonize grain jars in the dark? Yes, mycelium doesn't need light to grow. A dark cupboard at the right temperature is perfect.
Why is my grain jar stalling? Usually, it's either too cold or there's a lack of gas exchange. Check your filter patch or lid holes.
Does shaking hurt the mycelium? It looks like you're "killing" it for a day, but it's just recovering. It will come back twice as fast.