Pallet mould in a humid port city
Pallets grow mould in Florida when the wood stays above about 20% moisture content. Wood below that line will not support mould, mildew, decay or fungus, and heat treatment does not change that.
Why Jacksonville is hard on wood
Summer afternoons bring rain, mornings bring condensation, and a dock door open to the river air lets humid air into a warehouse. Boards that sit through that take on water and stay wet if they are stacked tight.
| Season | What the air does to wood | Storage move |
|---|---|---|
| Winter | Cooler, lower humidity, slower drying | Keep stacks off the ground; watch condensation on cold mornings |
| Spring | Pollen, intermittent rain | Cover stacks; space them |
| Summer | Afternoon storms, high humidity | Under cover with air movement; check moisture before cold-room loads |
| Fall | Storm season risk, wet ground | Raise stacks; clear flooded yards |
What keeps stock dry
- Store pallets under cover and off the ground.
- Leave air between stacks: even a slight air velocity of a few feet per second helps prevent mould on stored pallets.
- Do not wrap or bag wet wood; plastic traps the water.
- Check moisture with a handheld meter before loading a cold room.
These come from Pallet Enterprise. A handheld meter reads wood moisture in seconds: push the pins into a deck board and a stringer, and anything near or above 20% needs drying before it enters a cold room. Wood that sat in summer rain gets back under the line open-stacked on spacers, under cover, and measured again. Stock that cannot be dried goes to recycling.
What a cold dock does with stained stock
| Sign | What a receiver does |
|---|---|
| Green or black fuzz | Rejects the pallet, sometimes the load |
| Dark staining, soft wood | Rejects or flags it |
| Musty smell | Rejects or asks for a dry swap |
| Surface dusting that wipes off | May accept, depending on the dock |
Early mould shows as grey or green fuzz at board ends and in the gaps between boards. Surface mould may brush off and dry, but deep stain means retiring the pallet. For grocery docks see grocery and frozen food.
Heat-treated pallets get mould too
Heat treatment under ISPM 15 takes the wood core to at least 56 °C for 30 minutes. That kills pests and drives off some water, but the wood re-absorbs moisture from the air afterwards, so a pallet dry out of the kiln can be wet again after a week in a humid yard. Pallets loaded into a trailer or container still hot carry warm, moist air into a closed box, and as the box cools the moisture condenses on the wood.
Picture a forwarder loading heat-treated pallets in July at a Northside warehouse. They came off a treatment line two days earlier and sat outside under a tarp. The box waits four days at the terminal in the sun, then sails, and the pallets sit in warm, humid air for weeks. The mould appears before the box is opened.
| Step | Why |
|---|---|
| Let treated pallets cool and dry before loading | No trapped warm air |
| Store them under cover and off the ground | Rain and ground moisture re-wet the wood |
| Check moisture content before stuffing | Under 20% will not support mould |
| Ask how long the box sits and sails | Long waits and voyages give mould time |
An export request should say dry wood, not only HT: "HT and DB marked, kiln dry or air dried under cover, under about 20% moisture content", with the days between delivery and stuffing, whether the box waits at the terminal, and the destination. See reading the stamp and heat-treated pallets.
What to tell a supplier
- Where the stock was stored and for how long.
- Whether it sat in rain.
- Whether it will travel in a closed trailer, which traps humid air.
- How soon it loads after delivery.
Stored well, wood stays under the mould line. Stored badly, it fails. No pallet is immune, treated or not, and none is promised mould-free.
Questions
Why do pallets get mouldy in Florida?
Humid air and rain keep wood above about 20% moisture content.
How do you stop mould on pallets?
Keep them dry, off the ground, and give them air.
Are heat-treated pallets mould-free?
No. They are pest-treated, not mould-proof.
Does fumigation change this?
No. Moisture decides mould, not the treatment type.
Can a mouldy pallet be cleaned?
Surface mould may brush off, but a cold or food dock will usually still reject it.
Keep reading
Sources, reviewed 12 November 2026: Mould control (Pallet Enterprise); ISPM 15 (Wikipedia).