Cardboard might be unsafe to use in your garden. Sometimes.
Nothing inflames the passions of an online gardening group as much as…cardboard. Some of us gardeners love the stuff. We shred it, compost it, cover weeds with it; everything short of simply eating it instead of bothering with the gardening thing at all. The other group of us, however, insists that cardboard suffocates the soil and introduces a host of nasty industrial chemicals into your garden and ultimately into your body.
One of the things we have in common is that we love to yell about this while failing to cite our sources. There is lots of handwaving about toxins in cardboard glue and talking at length about how important it is that the soil breathe, or how sheet mulching with cardboard will double your soil biota and supercharge your vegetables, all free of any pesky scientific evidence.
This is actually important stuff. Cardboard is becoming more popular as a mulching material, especially as permaculture becomes more well-known, and municipal composting facilities often including cardboard in their compost mixes. If cardboard is actually introducing persistent environmental toxins into the soil, then, we need to know.
What harmful chemicals could be in cardboard?
Cardboard is a deceptively complicated material. Rather than being simple wood pulp, most cardboard in use today is a composite material composed of paper, petroleum byproducts, and various organic and inorganic inks and waxes. There are four main parts of cardboard that pose a potential threat when it comes to environmental toxins: glue, ink, coating, and paper.
The nature of these substances varies. According to the studies I’m listing here, Bisphenol-A (BPA), dibutyl phthalate (DBP), and di-2-ethylhexyl phthalate (DEHP), other plasticizers, aromatic amines, and mineral oil hydrocarbons seem to be the most concerning. These chemicals have been linked to reproductive dysfunction and cancer, and some of them are persistent in the environment.
Is all cardboard created equal with equal types and amounts of harmful chemicals? No.
The studies I could find seem to focus on food packaging, as dangerous chemicals in food packaging present the most direct danger to humans. It’s hard to imagine that standard corrugated cardboard is just as dangerous as color-print glossy food packaging, but studies have not been done. Either way, not all cardboard was found to be contaminated and the issue was most commonly associated with recycled cardboard packaging.
Are these chemicals bioavailable to plants and animals? Sometimes.
According to some studies, BPA is not particularly bioavailable in soils and was found to dissipate after a few days. Other pthalates may behave differently, though, and seem to last longer but will still degrade over time. Aromatic amines, which are sometimes present in dyes, are yet more persistent in soil to the point of the soil needing remediation to be non-toxic again.
This, of course, is ignoring the elephant in the room: plastic itself. Cardboard is often coated in a thin layer of plastic, often thin enough that an unsuspecting gardener might be unable to tell what the material is. I made this mistake myself, laying down a layer of plastic-coated cardboard as mulch. It was only several months later that I pulled a paper-thin layer of plastic out of my soil, the exact same shape as the cardboard I had laid months previous. If I had not removed it in a timely fashion it would have slowly broken apart further, through the action of the gardener, the sun, the rain, and the living soil itself, and would become a constellation of tiny microplastic fragments. These microplastics are impossible to remove and are harmful to both soil fertility, human health, and ecosystem health.
I have yet to see convincing evidence, though, that covering small sections of soil with plain cardboard or paper actually does anything actually negative beyond slightly reducing gas flow across the cardboard. In fact, sheet mulching has been investigated as a method to protect and encourage the growth of vulnerable conifer seedlings and increase vegetable yields. There is plenty of real world evidence that disturbing the soil decreases soil carbon, disrupts the soil food web, and promotes compaction over the long term.
If you are having second thoughts about using cardboard, follow a few simple rules: no glossies, no colors, and no coatings. If you avoid glossy, colorful cardboard you will avoid the lions share of the risks cardboard can pose. If you suspect an unusual type of cardboard, tear a small piece and make sure there is no plastic embedded inside. Plain cardboard without special coatings or colors is essentially harmless in the garden.
Sources
https://www.sciencedaily.com/rel…/2007/11/071128113022.htm
https://www.foodpackagingforum.org/…/bioassay-based…
https://www.tandfonline.com/…/10…/19440049.2021.1930200
https://acsess.onlinelibrary.wiley.com/…/sssaj1999…
https://cdnsciencepub.com/doi/abs/10.4141/p03-160
https://www.sciencedirect.com/…/pii/S0167198718305245