Q: Please settle an argument between my husband and I. He’s a fan of using cardboard as a temporary weed-suppressant mulch, but I’ve heard that it can contain chemicals harmful to the soil. Who’s right?
Maria F, Cork
A: Cardboard has been used by many organic gardeners for decades, both in the way you’ve described, as well as to quickly create new vegetable and flower beds, in which case it’s used as a weed-suppressant layer beneath an additional generous top layer of soil or compost. Made from trees and naturally rich in carbon, beneficial for the soil, it slowly rots down over time.
I’ve used both methods in my own garden to good effect, always making sure to first remove any colourful labels or sticky tape that could contain contaminants. For the same reason, I also avoid using any glossy or shiny kinds of cardboard.
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However, some organic gardeners argue strongly that even this kind of plain brown cardboard isn’t suitable as a garden mulch or for adding to the compost heap, as it potentially contains small amounts of glue as well as dyes (for example, if there’s a logo or signs printed on it). They also argue that cardboard mulch interferes with a healthy soil’s ability to breathe, adversely affecting the different kinds of micro-organisms essential to its health.
Countering these criticisms, others argue that the vast majority of glues and dyes used in brown cardboard boxes are, in fact, plant-based, so they don’t pose a risk to soil or plant health. As regards preventing soil from breathing, this is more likely to be a problem in drier climates than ours, where it would be slower to break down.
On top of this, I’d argue that cardboard offers a far more planet-friendly alternative to the use of environmentally harmful weedkillers, while it’s a much more affordable alternative to buying large quantities of organic mulch. It also minimises the need for digging, another plus from an organic gardening point of view, where regular disturbance of the different layers of soil should be avoided to support soil health. There’s also a very good argument to be made for the fact that you’re usefully repurposing something that would otherwise end up in a commercial incinerator or going to landfill.
All the above is unlikely to answer your question and instead probably just puts more fuel on the fire, so my apologies. But my own view would be that while cardboard is not perfect – and what is? – it’s still vastly preferable to most other options.
