Composting Cardboard
Those boxes piling up by your recycling bin are a free, high-carbon brown that pulls double duty in a pile — they add carbon, and their ridges prop the heap open so air can move through it.
Cardboard is the brown that's hiding in plain sight. Between shipping boxes, cereal cartons, egg trays, and toilet-roll tubes, you're probably generating a steady supply of it without trying — and it's one of the highest-carbon materials you can put in a pile. Used well, it does two jobs at once: it balances out all your nitrogen-rich greens, and its corrugation props the pile open so air can move through. Used badly, it just sits there in wet, unbroken slabs for a year. The difference comes down to a few minutes of prep.
A very high-carbon brown
Corrugated cardboard's C:N ratio swings widely — anywhere from about 150:1 to 500:1 — depending on how much the fibers have been bleached and printed.[1] Whatever the exact number, it's carbon-dominant, in the same league as newspaper and office paper. That makes it a strong counterweight to greens: a little cardboard goes a long way toward offsetting a lot of grass clippings or food scraps. Just don't build a pile out of cardboard alone — with nothing but carbon on hand, it'll sit there going nowhere for want of nitrogen. Pair it with greens and check the blend in the C:N Ratio Calculator.
Shred it and wet it
Two prep steps make all the difference. First, break it down small — tear it by hand or run it through a shredder. Smaller pieces give microbes more edges to work on and keep sheets from forming a layer nothing can get through. Second, just as important, wet it thoroughly. Dry cardboard sheds water; drop a dry slab into a pile and it'll sit there dry and intact for months. Soak the torn pieces in a bucket first, or hose down each layer as you build, so they can absorb moisture and actually start breaking down. Wet cardboard is also floppy, which means it packs into the pile far better than a stiff dry sheet.
Strip the tape, skip the coatings
Cardboard usually shows up with things that don't belong in soil. Remove plastic packing tape, shipping labels, and staples before composting — none of it breaks down, and otherwise you'll be picking plastic strips out of your finished compost later. Ordinary printed ink on plain cardboard isn't something to worry about; today's inks are mostly soy- or water-based and non-toxic. What you do want to avoid entirely is wax-coated cardboard (think produce boxes and freezer cartons) and glossy, plastic-laminated board — their coatings resist breaking down and can leave residue behind. If a box has a shiny plastic film or a waxy feel to it, leave it out of the pile.
Beyond the pile: sheet mulching
Cardboard has a second life outside the compost bin, too. Lay it flat over weeds or grass, wet it down, and top it with compost and mulch, and it smothers whatever's underneath while breaking down in place — that's the whole idea behind sheet mulching (sometimes called lasagna gardening) for starting a new bed without digging. Worms come up to feed on the softening cardboard and end up tilling the soil for you in the process. It's the same material doing the same job — high-carbon, slow to break down, good for structure — just applied to the ground instead of a heap. Stick to plain corrugated for this, with the tape and labels stripped off.
Frequently Asked Questions
Is cardboard a brown or a green in composting?
It's a brown — a very high-carbon material. Corrugated cardboard runs anywhere from about 150:1 to 500:1 depending on bleaching and printing, which makes it one of the strongest carbon sources you can add. Balance it with nitrogen-rich greens to keep a pile heating up.
Do you have to shred cardboard before composting?
Not strictly, but it helps a lot. Tearing or shredding it into small pieces gives microbes more edge area to work with and lets the pieces wet all the way through. Soaking them first is the other key step, since dry cardboard sheds water instead of absorbing it.
Can you compost cardboard with tape, labels, or ink on it?
Remove plastic tape, labels, and staples first — none of that breaks down. Ordinary printing on plain cardboard is fine; modern inks are mostly soy- or water-based. Just avoid wax-coated and glossy laminated cardboard, since those coatings won't decompose.
How long does cardboard take to compost?
Shredded and moistened cardboard in an active, balanced pile breaks down over a few months. Whole dry sheets can hang around for a year or more. Keeping the pieces small, wet, and mixed in with greens is what speeds things up.