Composting Cardboard: How to Do It Right

Cardboard is one of the most valuable brown materials available to home composters, and most households generate it in significant quantities. Corrugated cardboard has a very high carbon-to-nitrogen ratio (often cited at 400:1 or above), which makes it an excellent balancing material for nitrogen-rich kitchen scraps and grass clippings. Used correctly, it speeds up decomposition by providing structure, absorbing excess moisture, and maintaining airflow in a pile that might otherwise compact.

Which Cardboard Is Safe to Compost

Corrugated brown cardboard is ideal: the fluted interior layer creates air channels that benefit pile aeration even as the cardboard breaks down, and it wets and degrades relatively quickly once soaked. Plain brown packaging cardboard, paper grocery bags, and cereal box card (inner layer, not the glossy exterior) are all good compost material.

Cardboard to avoid or use with caution includes: heavily waxed produce boxes (paraffin-coated; does not biodegrade), foil-lined cardboard (juice boxes, some frozen food cartons), and heavily printed cardboard with dense ink coverage. Modern water-based inks used on most packaging cardboard are not a meaningful concern in household quantities. Remove tape, staples, and plastic labels before composting.

How to Prepare Cardboard for the Pile

Large flat sheets of cardboard create an impermeable barrier in the pile if added whole. The barrier effect prevents water from moving through the pile and blocks airflow. Tear or cut cardboard into pieces no larger than about six inches square before adding it. Smaller pieces decompose faster and integrate better with the pile.

Wet the cardboard before or as you add it. Dry cardboard initially resists moisture and can take a long time to soften in the pile. Soaking pieces in a bucket of water for a few minutes before adding them, or adding dry pieces in thin layers that get wetted by the moisture already in the pile, both work well.

Adding Cardboard in a Pile with a Lot of Nitrogen-Rich Material

When a pile has accumulated a large volume of nitrogen-rich material (food scraps, grass clippings) and needs carbon to balance, cardboard is one of the most practical corrections. Add torn, pre-wetted cardboard in layers throughout the existing pile material as you turn. The high carbon content acts quickly to bring the ratio back toward the target range.

Shredded cardboard is even more effective than torn pieces because the shredding increases the surface area available to microbial breakdown. A cardboard shredder (or a paper shredder rated for cardboard) makes this fast and practical for households generating large amounts of delivery packaging.