Why Is My Spawn Bag Not Colonising?
You inoculated your spawn bag, placed it somewhere warm, and now you're waiting. Days pass. Nothing happens — or something unexpected appears. This guide covers the most common reasons a spawn bag fails to colonise, how to tell what's going wrong, and what to do about it.
First: How Long Should You Actually Wait?
Before troubleshooting, check your expectations against realistic timelines. If you're within these windows, the bag may simply need more time — spores in particular take longer to germinate.
| Inoculation method | First signs of mycelium | Full colonisation |
|---|---|---|
| Liquid culture (millet) | 5–10 days | 2–3 weeks |
| Liquid culture (corn) | 7–14 days | 3–4 weeks |
| Spore syringe (millet) | 10–21 days | 3–5 weeks |
| Spore syringe (corn) | 14–21 days | 4–6 weeks |
If you're well outside these windows and seeing nothing, read on.
The 6 Most Common Causes
Temperature Is Too Low
Most commonSymptoms
Very slow colonisation or no visible growth at all, but no signs of contamination.
Mycelium is sensitive to temperature. Below 18°C (65°F), colonisation slows dramatically. Below 15°C, it may stop entirely.
Fix
Move the bag to a warmer location. Most cubensis and gourmet species colonise best at 21–26°C (70–79°F). A heat mat with a thermostat is the most reliable solution for cool homes — especially in winter. Use a folded towel as a buffer between the bag and the mat to prevent hot spots at the base.
Contamination
Symptoms
Green, black, pink, or yellow patches appearing in the bag. Sometimes a sour or musty smell.
Contamination is the most common reason grows fail — and the most important to act on quickly. Mould (most often Trichoderma, which appears green) competes aggressively with mycelium and usually wins once it takes hold.
How to tell contamination from healthy mycelium
Fix
Remove the bag from your grow space immediately. Seal it in a bin bag before disposal — don't open it indoors, as mould spores will spread. Do not try to cut out the contaminated section and continue — it's rarely worth it.
Prevent it next time
- • Work in a cleaner environment (still air box, wiped-down surfaces)
- • Flame sterilise your needle and clean the injection port properly before inoculating
- • Use liquid culture instead of spores — faster colonisation leaves less window for contamination
- • Check your substrate wasn't compromised before inoculation
Poor Inoculation Technique
Symptoms
No growth at all, even after several weeks, with no visible contamination.
If the inoculation didn't deliver viable mycelium or spores into the substrate, colonisation simply won't start. Common mistakes:
Fix
Review the How to Inoculate a Spawn Bag guide and check your technique against each step. When in doubt, inoculate a second bag using a fresh needle and careful technique.
The Liquid Culture or Spores Weren't Viable
Symptoms
No colonisation despite correct technique and good temperature. No contamination visible.
Liquid culture and spore syringes can lose viability over time, especially if stored incorrectly. Signs your liquid culture may not be viable:
- Completely clear with no visible mycelium clumps
- Stored at room temperature for several months
- Exposed to heat or direct sunlight
Fix
Check the storage conditions and age of your culture. Fresh liquid culture stored in the fridge at 4–6°C remains viable for 2–3 months. If yours is old or was stored poorly, the culture may be dead. If you suspect a quality issue with a Cloud920 product, contact us — we stand behind what we sell and will help you work out what happened.
The Substrate Was Too Wet or Too Dry
Symptoms
Slow, uneven colonisation, or contamination appearing early.
Substrate moisture affects how well mycelium can travel through the bag. Too wet creates anaerobic conditions that slow mycelium and encourage contamination. Too dry, and mycelium can't move through the substrate efficiently.
Fix
If you're using a pre-sterilised bag from Cloud920, moisture is handled for you — every bag is hydrated to the correct field capacity before it's sealed. If you're making your own substrate, use the squeeze test: a handful should release a few drops of water when squeezed hard, not a stream.
The Bag Was Compromised Before Inoculation
Symptoms
Contamination appears very early — within days of inoculation — or in areas far from the injection point.
If contamination appears faster than mycelium could have introduced it, the bag may have been contaminated before you inoculated it — from a manufacturing issue or transit damage (a small puncture, compromised seal).
Fix
Inspect bags carefully before inoculating. If you notice any damage to the seal or filter, or if the bag looks wet or discoloured before you've touched it, don't use it. Contact us and we'll sort it out.
Quick Diagnosis Guide
| What you see | Most likely cause | Action |
|---|---|---|
| Nothing after 2–3 weeks, no contamination | Temperature too low | Warm up the bag |
| Nothing after 3+ weeks, good temperature | Failed inoculation or dead culture | Re-inoculate with fresh culture |
| Green, black, or pink patches | Contamination | Remove from grow space, dispose |
| Yellow or brown liquid pooling | Normal metabolite secretion | Leave it — this is fine |
| Slow, patchy white growth | Temperature low or substrate issues | Raise temp, do break and shake |
| White and fluffy spreading evenly | Healthy colonisation ✓ | You're on track — keep waiting |
When to Reach Out
If you've worked through this guide and still can't work out what's happening, we're here. Take a clear photo of the bag — hold it up to a light source for the best view — and send it to our support team. We've seen most things that can go wrong in a grow.
Our support is personal, fast, and honest. We'll tell you straight whether the bag is salvageable or whether it's time to start fresh.
Frequently Asked Questions
Yellow or brown discolouration is often metabolite secretion — a normal response to stress or environmental changes. It's not contamination. True contamination is usually green, black, pink, or has a slimy texture. If in doubt, send us a photo.
Rarely. Once contamination is established, it almost always outcompetes mycelium. The safest approach is to remove and dispose of the bag immediately to protect your other grows. Starting fresh with a clean bag is faster than trying to rescue a contaminated one.
A break and shake at 30–40% colonisation often restarts stalled growth. Also check temperature — a drop in ambient temperature is the most common reason for a grow to stall partway through.
The entire substrate should be covered in white mycelium with no brown or uncolonised patches visible. Grain bags will feel firm and solid when you squeeze them. For CVG bags, the entire surface should be white.
It happens — even experienced growers have failed runs. Contact us with details of your setup (temperature, inoculation method, substrate, storage) and we'll help work out what went wrong. If the issue was with our product, we'll make it right.
Start Your Next Grow Right
A failed grow usually comes down to one of three things: temperature, technique, or substrate quality. Our bags handle the substrate side — every bag is prepared and tested by our mycologist before it ships.