Cardboard waste makes up a shockingly large percentage of landfills, with over 31% of municipal solid waste being cardboard boxes, tubes, and packaging materials. With cardboard being so ubiquitous, composting it instead of sending it to the landfill is an impactful way to reduce waste and create nutrient-rich compost for gardens and landscaping.
But not all cardboard can be composted. The type of cardboard, preparation, and proper composting process are key factors that determine if cardboard will successfully break down into rich humus or end up matting down and ruining the compost pile.
What Type of Cardboard is Compostable?
The most common question about composting cardboard is – what types can be added to a compost bin or pile? Not all cardboard is created equal when it comes to compostability.
Plain, Corrugated Cardboard
Plain cardboard boxes made of corrugated cardboard are perfectly compostable. This includes the typical brown cardboard shipping boxes, moving boxes, Amazon delivery boxes, cardboard tubes from paper towels or toilet paper rolls, cereal boxes, egg cartons (without plastic dividers), and other clean cardboard packaging. Corrugated cardboard has a rippled texture and is constructed using a layered structure with the corrugated layer in the middle.
The key is that the cardboard does not have any coatings, glossy finishes, or heavy printing with dyes and chemicals. Plain brown corrugated cardboard just contains cellulose and carbon which make it an ideal composting material.
Avoid Wax-Coated Cardboard
Some cardboard packaging has a wax coating applied to make it moisture resistant. Common examples are frozen food boxes, fruit and produce crates or boxes, cup sleeves from coffee shops, milk cartons, and disposable cups. The wax coating prevents the cardboard from properly breaking down during composting. Any waxed or coated cardboard should not be added to compost piles.
Avoid Dyed/Glossy/Laminated Cardboard
Heavily printed cardboard with dyes or other chemicals should also be avoided for compost piles. Examples include cereal boxes, soda boxes, glossy packaging with lamination or plastic-like coatings. Even if the dyes and inks are labeled non-toxic, they can still impede the composting process. Stick to plain, undyed cardboard boxes.
Avoid Grease-Stained Cardboard
Pizza boxes and other grease-stained cardboard are not suitable for backyard composting. The oils and fats cannot easily breakdown which can attract pests to the compost pile. Best to keep oily cardboard out of compost bins.
Avoid Cardboard With Staples, Tape, Glue
Any cardboard with staples, clear tape, packaging tape, glue or sticky residue should not go straight into backyard compost piles. These materials will not break down properly. Staples and tape can be picked off first, but glue and adhesives often leave a residue behind that persists.
Preparing Cardboard for Composting
Proper preparation is crucial for cardboard to compost successfully. Whole sheets or large flattened boxes will mat down, shed water, and make poor compost. Cardboard must be shredded, chopped, or torn into smaller pieces first.
Shredding Cardboard
The best method is shredding cardboard into thin strips or pieces no more than 1-2 inches in size. Using an electric chipper/shredder machine makes fast work of transforming large piles of cardboard into perfectly sized composting material.
For small amounts of cardboard, it can also be hand-shredded by tearing it into thin strips. Tearing along the corrugated grooves helps break it into smaller bits.
Chopping Up Boxes
If no shredder is available, cardboard boxes can be chopped up with a knife or machete. Flattened boxes should be chopped into 2-4 inch pieces. This smaller size helps the pieces break down faster versus composting a whole large sheet or box.
Tearing by Hand
For very small quantities of cardboard, it can be manually torn up by hand into 1 inch pieces. Dampening the cardboard first makes it easier to tear.
Avoid Overly Large Pieces
Avoid adding full sheets of cardboard or whole boxes flattened or intact into the compost pile. The large surface area will shed water, mat down, and impede airflow in the compost. Always shred or break down cardboard into smaller bits before composting.
Composting Cardboard in Piles or Bins
To compost cardboard successfully, it needs to be layered and mixed properly with other composting materials. Cardboard is a “brown” high-carbon material, so it needs to be balanced with “green” high-nitrogen materials. Here are some tips:
Layer Cardboard in 4 Inch Increments
Shredded cardboard should be layered in approximately 4 inch increments, mixed with high-nitrogen materials. Avoid thicker layers of just cardboard, as this can mat down.
Alternate Cardboard Layers with Greens
Alternate cardboard layers with fresh green material like grass clippings, yard trimmings, vegetable scraps, manure, etc. This provides a balance of carbon and nitrogen.
Follow Proper Carbon-Nitrogen Ratios
A C:N ratio of 25-30:1 is ideal. For each 4 inch layer of high-carbon cardboard, add 1 inch of green nitrogen-rich waste.
Use Cardboard in Sheet Mulching
Cardboard is excellent as a high-carbon brown layer when sheet mulching gardens or building new raised beds. Simply wet and layer sheets of cardboard directly on soil and cover with compost and mulch.
Expect Cardboard to Take 2-6 Months to Break Down
Allow plenty of time for cardboard to break down. It will take a minimum of two months to compost, but thick corrugated pieces may take up to 6 months in cooler compost piles. Turning the pile weekly can help speed decomposition.
Avoid Overloading the Pile with Cardboard
While cardboard makes great compost, too much can overwhelm the nitrogen balance and slow down the composting process. Limit cardboard to no more than 25% of the total volume of the compost pile for optimal decomposition.
Use Coarsely Shredded Cardboard
Using cardboard shredded into 1-2 inch pieces provides more surface area for microbes to colonize and speeds the breakdown process. Avoid making cardboard pieces too small and fine, however, as this can restrict airflow.
Monitor Moisture Levels
Cardboard soaks up moisture, so compost piles with large amounts of cardboard will dry out faster. Watch moisture levels and water as needed to keep the pile evenly moist but not soaked. Turning the pile often can also improve internal moisture distribution.
Troubleshooting Cardboard Composting Problems
Composting cardboard does take some extra forethought compared to fruit and vegetable waste. Here are some common problems and solutions:
Matted Cardboard Layers
Shredding cardboard before adding it to compost is key. If whole sheets or large flattened boxes were added, the cardboard can become matted down, waterlogged, and impede airflow. Turn and fluff up the pile, remove the flattened cardboard, and replace with shredded pieces.
Slow to Break Down
Extremely thick cardboard or wax-coated pieces can persist for many months in compost piles without fully breaking down. Pick these pieces out and add additional nitrogen materials.
Dry and Compacted Pile
Too much cardboard can lead to a very dry and airless pile that stops decomposing. Turn the pile and add additional green waste like grass clippings to increase moisture and aeration. Water pile as needed.
Cardboard Attracting Pests
If unfinished compost containing cardboard is accessible to rodents, it can become an attractive nesting spot. Only add finished composted cardboard to gardens, keep compost bins closed off, and eliminate other food waste that may be attracting pests.
FAQs About Composting Cardboard
Can wax coated cardboard be composted?
No, wax coated cardboard cannot be readily composted at home. The wax prevents the cardboard fibers from being accessed by compost microbes. Avoid wax-coated boxes.
How small should cardboard be shredded for compost?
Shred cardboard into approximately 1-2 inch sized pieces for composting. Small shredded bits provide more surface area for microbial activity speeding decomposition.
Is cardboard a green or brown compost material?
Cardboard is considered a brown compost material due to its high carbon content. It should be mixed with nitrogen-rich greens like grass clippings, manure, and plant trimmings.
Can I put cardboard pizza boxes in compost?
Avoid composting grease-stained pizza boxes, as the oil and grease residue is difficult to break down and can attract pests. Use unsoiled plain cardboard boxes only.
How long does cardboard take to break down in compost?
Expect cardboard to take a minimum of 2-6 months to fully compost down. Thicker corrugated cardboard is on the higher end of that range. Turning the compost pile weekly can help speed decomposition.
Does cardboard acidify soil when composted?
No, cardboard has a near neutral pH when composted and will not significantly impact soil acidity or alkalinity. Finished composted cardboard provides beneficial organic material to improve all soils.
Can I put cardboard in my worm compost bin?
Yes, cardboard can be added to vermicomposting bins with worms. The key is shredding it finely and limiting thick layers. Worms will help break down thin cardboard scraps efficiently.
What can’t be composted with cardboard?
Avoid composting cardboard contaminated with food waste, oils, grease, tape, staples, or other residues. Do not add cardboard coated in plastic, wax, or glossy prints which will not break down.
Using Composted Cardboard in the Garden
Finished compost containing decomposed cardboard fibers makes a wonderful soil amendment with many uses:
Mix into Garden Beds
Blend about 2-3 inches of composted cardboard into the top 6-8 inches of any garden bed or potted plants to add organic matter and nutrients.
Top Dress Lawns
Spread a thin 1/4 inch layer over grass lawns as an organic top dressing to improve drainage and soil quality.
Mulch Trees, Shrubs and Landscapes
A 2-4 inch thick layer of compost makes an excellent mulch around trees, shrubs, and planting beds to suppress weeds and retain moisture.
Improve Soil for Vegetables
Stir 1-2 inches of composted cardboard into vegetable beds and rows to add carbon, and improve drainage and water retention. Great for tomatoes, peppers, and root crops.
Boost Soil Fertility
The carbon from composted cardboard combines with nitrogen to form humus, slowly releasing nutrients to plants and boosting microbial activity and fertility.
Retain Moisture in Soil
Compost binds to clay soils, opening them up to hold more water. It also helps sandy soils retain more water by adding organic matter.
Final Thoughts
Composted cardboard is a versatile soil amendment with multiple benefits for gardens and landscapes. It provides organic matter, nutrients, and carbon slowly over time as it continues to break down in the soil. Tap into this free resource and put waste cardboard to work enhancing your soil quality. Follow the tips here for successful cardboard composting.