I’m not a fan of using corrugated cardboard as a mulch, which like other sheet mulches creates problems for the underlying soil. Long-time readers of this blog may remember several previous posts (1, 2, 3 and 4) on this topic and I won’t belabor the points made in those posts. Instead, today I’m doing to focus on cardboard itself.
First, cardboard is a generic term that can refer to many types of manufactured paper. The box you see delivered to your front door is more properly called corrugated board or containerboard. It consists of two layers of linerboard sandwiching a layer of accordion-like fluting material. The linerboard is made from sheets of pulp that may be coated to improve smoothness (more about this later). The finished linerboard is laminated using adhesives to both sides of the fluting material.
These boxes are made to withstand rough handling and to protect the contents from the external environment. It’s tough stuff: while you might be able to bend a piece of corrugated board fairly easily, it’s more difficult to tear it in half. The more heavy duty the box, the more difficult it is to bend or tear its walls.
So let’s now consider using this tough material in your garden as a mulch. It may be coated as mentioned earlier to improve smoothness. That’s going to prevent it from absorbing moisture. The coating also reduces the ability for gases to move between the soil and the atmosphere. In fact, smoothness is measured using an air leak method – the smoothest materials have the least air leakage.
A garden or landscape mulched with cardboard (or heaven forbid several layers of cardboard as part of the science-free lasagna mulch method) is now covered with a tough, relatively gas- and water-impermeable material that will take some time to break down. It’s hardly a mulch that’s going to nurture soil life.
But cardboard mulch fans swear that they find more earthworms under cardboard than anywhere else in their garden. This is almost always the first response I get from gardeners who don’t believe that cardboard causes problems. And this is where it’s important to consider earthworm behavior.
We’ve all observed that earthworms crawl to the soil surface during heavy rains; this is due in part to water filling their burrows and reducing oxygen availability (Chuang and Chen demonstrated this nicely in 2008). Likewise, the reduction in oxygen movement from the atmosphere into cardboard-covered soil would cause worms to crawl upwards in an effort to find oxygen at the soil surface.
So don’t assume your lasagna mulching draws earthworms to your garden. It’s more likely that you’re smothering their habitat.
***An update on cardboard gas permeability. We’ve just published an article comparing diffusion rates of different mulches. You can find the article here but it is behind a paywall. Here is a graphic comparing diffusion rates of various mulches. This is a logarithmic scale.
Now, until cardboard proponents publish evidence to the contrary, it’s pretty obvious that cardboard mulch interferes with gas diffusion.
***And another update on how our blog works. This post, by far, is the most popular. It generates a lot of comments. All comments must be approved before they’re posted, and I don’t approve comments that are derogatory or promote a belief in the absense of supporting science. If you want your comment to be published, be polite and provide evidence to support your statements. Otherwise, you are wasting your time.
***Another update on cardboard in your garden: A recent paper reports on PFAs (aka “forever chemicals”) in various products used for poultry bedding (among other things). Cardboard was one of the worst. The article is behind a paywall but I have access to it and was able to find the table shown here. So if you need yet another argument to NOT use cardboard as a mulch (like in “lasagna gardening), maybe “keep forever chemicals out of your garden” will do it.