Is Using Cardboard as Mulch Under Trees an Effective Practice?

The simple answer is NO. not really. Let's dive into some reasons why.

1. All Cardboard is Not Equal

Most 'cardboard' is made up of two smooth layers outside of an inner spongey layer. Sometimes the materials are limited and somewhat sustainable and will biodegrade, BUT a lot of the time the materials are 'enhanced' with strengtheners and coatings. The smooth layers are designed with to be tough and weather resistant and often contain chemicals and waxes to help hold them together. The corrugated inner layer is actually treated pulp held together with a glue adhesive. In addition to the cardboard box material, the inks used for barcodes and labels are petroleum based. Most of this material takes a long time to break down and when it does, harmful substances are leached into the ground.

2. Soil Needs to Breathe

This is the most important factor for healthy trees needing healthy soil. Soils breathe and need pore space and the ability to release and take in various gases. For an example, compacted clay is terrible for tree health because it is impermeable, trapping gasses below and preventing water from percolating down. Cardboard diverts water and traps oxygen in the soil. Earthworms are often found just under cardboard installations because they are starved of oxygen and are trying to get to the surface (like after a rain). The diffusion rate of cardboard is not much better than plastic landscape fabric.

3. Cardboard is Candy to Unwanted Insects

There are many predator insects that thrive in cardboard environments and feed on soil-beneficial insects. Termites, cockroaches, and millipedes are among the worst. The spaces and tunnels created by flat sheets of cardboard are not natural and disrupt the efficient systems of healthy soil profiles.

4. Cardboard Layering Acts Like Drainage Tiles

Unless the cardboard is perforated with small holes throughout, rainwater is diverted and runs towards lower areas. This imbalance can starve some tree roots and oversaturate others. Once the cardboard breaks down enough this might not be an issue, but expect 1-2 years of water flow disruption and for young trees especially, that can be detrimental.

We suggest simply using a 4-6 inch layer of natural hardwood mulch to suppress weeds near vegetation (staying away from tree trunks and root flares) and if you MUST use cardboard, at least follow some simple steps to help your soil and trees handle the ecosystem disruption. Cut the cardboard into small pieces and leave plenty of space between sections. Remove any inked areas and tape before application. Punch holes in the cardboard to allow water and gas exchange.