Yard notes
Mold on pallets: what it means and what it does not
Mold on a pallet is not a wood problem. It is a water problem that happens to be visible on wood.
Request a quote
Talk to the people who wrote this
Same form, same inbox. Technical questions get answered whether or not there is an order attached.
In short
Mold grows on pallets when wood stays wet. It needs moisture above roughly 20% content, a temperature above about 40°F, and time. Wood supplies the food; the only variable you control is water.
It matters most in food, beverage and pharmaceutical handling, where a moldy pallet is an audit finding. Surface treatment does not fix it — dry storage and prompt turnover prevent it.
The conditions, and which one you control
Mold needs four things: a food source, oxygen, a suitable temperature and moisture. Wood is the food source and it is not going anywhere. Oxygen and temperature are not practically controllable in a yard.
That leaves water, and water is entirely controllable. Every mold problem we have ever traced ended at one of four causes: pallets stored uncovered outdoors, pallets built from green lumber and wrapped before they dried, wet product loaded onto dry pallets, or a leaking roof over a storage bay.
This is why the prevention advice is so boring. It is all about keeping wood dry, because that is the only lever there is.
Where it actually causes damage
For general industrial freight, surface mold on a pallet is mostly a cosmetic and housekeeping problem.
For food and beverage, it is a hygiene finding, and a repeat finding is a supplier relationship problem.
For pharmaceutical and medical, it is a serious deviation.
For any shrink-wrapped load, it is worse than it looks, because a wet pallet inside a sealed wrap creates a small humid chamber. Mold grows on the pallet, on the carton bases, and sometimes on the product, and nobody sees it until the wrap comes off at the far end.
That last one is the case that catches people, and it is why we reject visibly wet stock at intake rather than routing it to repair.
Why treatment is not the answer
There is a persistent belief that mold can be sprayed away. Two problems with that.
First, mold that has penetrated the wood is not reached by a surface treatment. You remove the visible growth and leave the colony.
Second, and more important, the wood is still wet. Treating a moldy pallet without drying it treats the symptom and leaves the cause. It will regrow.
Heat treatment does not solve it either, and this is a common confusion. ISPM-15 heat treatment is a phytosanitary process targeting insects and larvae. It reduces moisture somewhat as a side effect but it is not a mold remedy and does not prevent regrowth in wood that gets wet again.
Prevention, in order of effect
- Turn stock over. Pallets that do not sit do not grow mold. Scheduled removal is the highest-leverage measure there is.
- Store under cover and off the ground. Half-covered is close to useless — water runs in.
- Use kiln-dried lumber on new builds. Green lumber wrapped in a warm warehouse is a mold incubator.
- Do not wrap wet. If pallets have come in from an outdoor stack in winter, break the bands, let them stand and dry, and inspect before use.
- Inspect at receipt. Two minutes with a torch at the dock beats discovering it in a customer's building.
More on the seasonal pattern in what an Ohio winter does to a pallet stack.
Takeaways
- Water is the only one of mold's four requirements you can control.
- A wet pallet inside a shrink wrap creates a humid chamber and grows mold invisibly.
- Surface treatment removes visible growth and leaves the cause.
- Heat treatment is a phytosanitary process, not a mold remedy.
Questions
Asked and answered
Can moldy pallets be cleaned and reused?
Not reliably. Surface treatment does not reach mold that has penetrated the wood, and the wood is usually still wet, so it regrows. For food or pharmaceutical handling it is not worth the risk.
Does heat treatment prevent mold?
No. ISPM-15 heat treatment targets insects and larvae. It reduces moisture slightly as a side effect but does not prevent mold from returning if the wood gets wet again.
Written by
Dale Whitmer
Yard Manager
Runs the grading line and the repair bay. Has graded more pallets than he can estimate and still argues about deck coverage.
Disagree with something here? That is genuinely useful to us — several of these articles exist because a customer told us we had it wrong. Use the form at the top of the page or write to the yard directly.
Terms used here
Definitions live in one place so they cannot drift. Full list in the pallet glossary.
- Base
- The lowest structural layer of a pallet — bottom deck boards or, on a skid, the runners themselves.
- Green lumber
- Freshly milled wood at 25–40% moisture content, which shrinks and loosens as it dries in service.
- Heat treatment (HT)
- Raising wood core temperature to 56°C for 30 continuous minutes to satisfy ISPM-15.
- ISPM-15
- The international standard governing treatment and marking of wood packaging in international trade.
Keep reading
Related notes
Blue stain is not rot, and rejecting it costs you money
Every year somebody rejects a load of perfectly sound pallets because the pine has gone grey-blue. It is one of the better bargains in this industry.
Yard notesWhat an Ohio winter does to a pallet stack
A pile worth real money in October is worth a fraction of it by April. The mechanism is entirely predictable and almost entirely preventable.
Yard notesHow to stack pallets outdoors so they are still worth something
Fifteen minutes of housekeeping routinely moves a load from 'we will remove this for free' to 'we will pay you for this'.