One thin layer of plain cardboard is not bad for most soil. The honest verdict is that the evidence on cardboard soil gas exchange is mixed but reassuring. A single sheet acts far better than the thick stacks you picture when you hear the warnings. Stack many wet layers and you can choke a bed. But that is a different setup from the way you would sheet mulch a real garden.
The worry makes sense at first glance. Cardboard is a flat, dense sheet that lies hard against the ground, so it looks like a seal. Loose wood chips are the opposite. Air, rain, and worms move through the gaps between chips with ease. A solid panel seems like it should trap everything below it. That picture is where your fear starts.
The lab numbers tell a more careful story. A 2019 study in Soil & Tillage Research measured cardboard at about ten times less gas-permeable than wood chips (Shahzad et al., 2019). That sounds alarming on its own. But the team also tested the soil oxygen in the ground under each mulch. The soil under cardboard was not meaningfully different from bare soil. The only cover that starved the ground was solid plastic film.
So the cardboard let enough air through to keep your soil breathing. The sheet itself resisted gas far more than chips did, yet the ground below stayed fine. Edges, seams, and the slow soak of rain give a single layer room to swap air. A buried plastic tarp has none of that. That is why it was the one material that did real harm in the test.
Not every expert is sold, and the cardboard controversy is worth taking seriously. Doctor Linda Chalker-Scott is a horticulturist at WSU. She argues that layered cardboard blocks both gas and water, and she warns it can smother soil life when you pile it on thick (Garden Professors). Read that as careful expert analysis, not a closed case. Her point lines up with the study in one key way. The problem is layers and thickness, not a thin sheet on its own.
A single layer of plain cardboard topped with wood chips is the evidence-backed choice. Stacking many thick layers is what slows air and water, so keep it to one.
Put both views together and the advice gets simple. Use a single cardboard layer, not a thick wad of three or four sheets stacked wet. Strip off any glossy print, plastic film, and tape first. Plain brown corrugated breaks down cleanest. One layer holds back your weeds for a season and still lets the ground breathe. If you need more weed control, overlap the edges of your sheets a few inches rather than doubling up the thickness.
Top that sheet with two to three inches of wood chips, the most gas-permeable mulch in the whole study. The chips protect the cardboard, feed the worms, and keep air moving down to your roots. Then keep the bed damp, and water it like you would any garden bed. Moist cardboard softens, draws in worms, and rots into the soil within a few months. A dry mat just sits there and sheds rain, which is exactly the sealing problem you want to dodge.
So skip the thick stack and skip the buried plastic. Lay one plain sheet, soak it, cover it with wood chips, and your soil keeps breathing while the weeds give up. The science and the skeptics agree on that much, even if they still argue about the rest. Keep it thin and keep it wet, and you get the weed barrier without the smothering.
Read the full article: Sheet Mulching: A Complete How-To Guide
