Pallet mold: causes, prevention and what to do with moldy pallets

Mold is the most common quality complaint about wood pallets, and it usually shows up at the worst moment: when a container is opened at the destination, or when a retailer's receiving dock inspects a load. The cause is almost always the same, and so is the fix.

#What causes mold on pallets?

Mold on wood pallets needs four things at once:

Take away any one of them and mold struggles. Moisture in the wood is the one you control most easily.

#Why green pallets are the usual culprit

Most pallets are built from green (undried) lumber because it is cheaper and nails easily. Green hardwood can hold 50–80% moisture. When green pallets are stacked tightly, stretch-wrapped under a load or sealed in a container, that water evaporates into the enclosed air, the air saturates, and mold grows on the wood and often on the cartons too. See pallet lumber moisture.

#Container rain

In ocean containers, moisture from wet pallets, cargo and air condenses on the container roof as it cools at night and drips back onto the load. Pallets are often the largest single source of water in the box.

#Heat treatment does not prevent mold

ISPM 15 heat treatment (HT) heats the wood core to 56 °C for 30 minutes to kill insects. That removes little water. An HT pallet built from green wood is still green and can grow mold. Buyers who specify "HT" expecting dry pallets are often surprised. Specify moisture separately. See heat-treated vs kiln-dried.

#How to prevent pallet mold

#1. Specify dry wood

• Kiln-dried to 19% MC or less (KD19) is the usual standard for food, retail and export.
• For long ocean voyages, tropical destinations or sensitive products, 15% is safer.
• Costco's pallet policy caps pallet moisture at 19% when containerized. EPAL requires dried wood for Euro pallets.
• Ask whether the lumber was dried before assembly or the whole pallet was kiln-dried after assembly. Either works if the delivered pallet meets the target.

#2. Store pallets properly

• Keep pallets under cover and off the ground.
• Leave space between stacks for airflow; do not tarp tightly.
• Rotate stock: first in, first out.
• Keep pallets away from wash areas, coolers and dock doors in wet weather.

#3. Check before loading

• Measure moisture on a sample with a wood moisture meter; read stringers or blocks, not just deckboards.
• Reject pallets with visible mold, wet patches or a musty smell.
• Record readings for traceability. See pallet inspection.

#4. Control the container

• Use desiccant bags or strips sized for the voyage.
• Load dry cargo on dry pallets; do not load right after rain.
• Consider ventilated containers for some commodities.
• Keep dunnage and bracing dry too.

#5. Consider treatments or other materials

• Anti-mold or anti-sapstain dips and sprays are used on green lumber by some mills. Ask the supplier what chemical is used and whether it is acceptable for your product and destination; food and pharma buyers often prohibit them.
• Plastic, metal, presswood and paper pallets do not grow mold in the material itself, although dirt on them can. See plastic vs wood pallets.

#Which pallets are most at risk?

The riskiest combination is the most common export pattern: pallets built from green wood, heat-treated for ISPM 15 (which does not dry them), loaded with cartons, stretch-wrapped and sealed in a container for a long voyage.

#Mold, wood species and moisture numbers

Species matters less than moisture. Sapwood of many pines and hardwoods molds readily when wet; dense heartwood somewhat less. What reliably separates moldy loads from clean ones is whether the wood was dry when loaded and stayed dry. Useful numbers:

• Fiber saturation point: about 28% moisture content. Above this, wood holds free water.
• Mold threshold: roughly 20% moisture content and above, in warm conditions.
• KD19 target: 19% or below, the usual pallet spec.
• Green hardwood: often 50–80% at the sawmill.

See pallet lumber moisture for measurement and shrinkage.

#Who pays when a load arrives moldy?

Usually the shipper, at first. Retailers and importers reject or charge for moldy loads, and the product may be written off if it is food, pharmaceutical or cosmetic. Whether you can recover the cost from the pallet supplier depends on your contract. A spec that states the moisture limit, how it is measured and when (on delivery to you) gives you a basis for a claim. Without it, a supplier can reasonably say it delivered what was ordered.

#Mold vs sapstain vs weathering

Not every dark mark is mold. Knowing the difference avoids rejecting good pallets.

#What to do with moldy pallets

1. Isolate them from clean pallets and product.
2. Do not ship food, pharmaceuticals or personal care products on them.
3. Minor surface mold on pallets for non-sensitive use: brush or wash off outdoors, then dry the pallets in sun and moving air until they read below 19%. Expect regrowth if they get wet again.
4. Heavy mold, decay or odor: send them to a recycler for grinding or disposal rather than reuse.
5. Find the cause: check the pallet moisture on arrival from the supplier, your storage, and your transit conditions.

#Specifying it for a quote

Add these fields to your pallet RFQ:

• Moisture at delivery: KD19 (or KD15) measured at stringers or blocks
• Measurement method and sample size
• Treatments allowed or prohibited (for example "no anti-mold chemicals" for food)
• Storage and delivery: covered transport, no pallets delivered wet
• ISPM 15 HT, stated separately

For shipping pallets in containers, see pallets per container for load planning, and food-grade pallets for food supply chains.