Can Mold Grow on Cardboard? Why Boxes Go Mouldy and How to Prevent It

Last Updated on October 26, 2026 by Umar Farooq

Can mold grow on cardboard? Yes: cardboard is paper fibre, which mould can eat, and it soaks up water from damp floors and damp air, so a box in a humid room can go mouldy while the outside still looks fine. It is the stored-box version of a sponge sitting on a sponge.

I have never met a basement that did not contain at least one box with a suspicious soft corner. Usually the one that has the wedding photos in it.

Why cardboard goes mouldy when plastic crates don’t

Mould needs moisture and something organic to eat. A plastic crate offers neither, which is why mould on plastic is really mould on the grime sitting on it. Cardboard is different: it is made of paper fibre, so the surface itself is food, and it is absorbent, so it draws in water vapour from the air and liquid water from whatever it sits on.

That combination is why a box can feel damp, go soft and smell musty long before you see a single speck. The EPA’s rule for this kind of material is blunt: absorbent or porous materials may have to be thrown away if they become moldy, because the growth can be difficult or impossible to remove completely.

How a box gets wet: floor, air or contents

Three routes, in roughly the order I’d check them:

1. The floor. Cardboard that touches a damp slab wicks water straight up. My page on mould on concrete explains why bare concrete floors stay damp for so long.
2. The air. Basements, garages, lofts and storage units sit damp for weeks at a stretch. The EPA says the key to mold control is moisture control, and advises indoor humidity below 60%, ideally 30-50%.
3. The contents. Damp towels, camping gear or a bag put away before it dried will wet the box from the inside.

If you are packing a storage unit rather than a spare room, how to prevent mould in a storage unit goes further on that case.

Can mold grow on cardboard that looks dry?

It can start before it shows. The early signs I would trust are a musty smell when you open the box, corners or the bottom edge that feel soft, and discolouration or speckling on the surface or flaps. If the box has been through a leak, assume it is wet inside even if the outside has dried.

How long it takes depends on how wet and how warm it is. I haven’t got a trustworthy number for cardboard, and I’d distrust sites that give one; how long it takes for mould to grow covers what the research does and doesn’t say.

Save it or bin it?

Wear gloves and a dust mask, and don’t shake or flap a mouldy box indoors. Open it outside if you can. This is housekeeping, not medical advice; if you are worried about your health, see a doctor.

For an ordinary packing box, bin it. It is cheap, it is porous, and the EPA says porous material may have to be thrown away. Slide it into a bin bag before you carry it through the house, so the spores stay in the bag rather than in the hallway.

For the contents, the answer depends on what they are:

• Paper and documents you care about. The Library of Congress advises to set out items to air dry immediately upon discovering wetness or mould. It adds that if newly wet or mouldy papers cannot be air dried within two days, they should be prepared and frozen, and points to specialist conservation guidance for how.
• Clothes and fabric. Mouldy fabric is a separate question, and the answer depends on the fabric.
• Anything irreplaceable. Stop, isolate it, and ask a conservator.

Four ways to stop mould on cardboard before it starts

1. Get it off the floor. Shelves, pallets or even a couple of battens leave an air gap, and cut the wicking route.
2. Swap valuables into lidded plastic crates. My opinion: cardboard is fine for moving house and a poor long-term store. A cheap crate costs less than the photographs inside it.
3. Keep the room dry. The Library of Congress suggests keeping relative humidity between 35 and 55% to stop mould stains returning on paper. A hygrometer tells you where you stand, and does a dehumidifier help with mold covers whether a unit is worth running.
4. Don’t pack anything damp. Dry towels, tents and shoes properly before they go in a box, and leave a little air around the boxes rather than wedging them against a wall.

Dry floor, dry air and boxes off the ground: cardboard is fine until the damp arrives

So, can mold grow on cardboard? Yes, readily, once it is damp. That means the box is rarely the problem; it is where you put it. Lift it, dry the room, and a stack of cardboard will sit there for years without a speck.

FAQ

Is it safe to reuse mouldy cardboard boxes?

I wouldn’t. The EPA says porous materials may have to be thrown away, because mould can be hard or impossible to remove completely. Use a clean box or a plastic crate.

Can mould on cardboard spread to other things in the room?

Mould can move to other damp, absorbent items stored beside it, so check anything nearby and keep the boxes apart. The usual route is shared damp air rather than a box infecting its neighbour, so fix the humidity as well as the one box.

How do I get mould out of cardboard?

For an ordinary box, you generally can’t. Cleaning reaches the surface of porous material but not the fibres. For valuable papers inside, follow the Library of Congress advice above: air dry quickly, or freeze if it can’t be done within two days.

Can mold grow on cardboard in a dry room?

Not in any quantity. With nothing damp, the fibre stays dry and mould has little to work with. Trouble starts when humidity climbs or the box sits on a wet floor.

Does a dehumidifier help with cardboard storage?

It helps by lowering the humidity that boxes absorb, which is the air route above. It won’t help if the box sits on a wet floor, so get them off the ground too.