Cardboard Mulch – What Are You Actually Putting on Your Soil?
Sheet mulching with cardboard is growing in popularity: it suppresses weeds, retains moisture and breaks down over time, enriching the soil with organic matter. But before you lay another delivery box between your tomato rows, it's worth knowing what cardboard is actually made of – and what comes with it into your garden.
What's in ordinary brown cardboard?
Corrugated cardboard is essentially wood pulp bonded with starch-based adhesive – typically derived from maize, wheat or potato. These components are completely safe and biodegradable. The problem lies elsewhere – and it's not the printing inks or shiny coatings that are easy to spot and avoid. It's something invisible to the naked eye.
PFAS – the chemicals that never go away
PFAS (per- and polyfluoroalkyl substances) are a family of more than 15,000 synthetic compounds known as "forever chemicals". The carbon-fluorine bond at their core is virtually indestructible – the estimated half-life exceeds one thousand years.
PFAS have been used in paper packaging since the 1950s, primarily as coatings that repel water and grease. Fast-food wrappers, microwave popcorn bags, pizza boxes – these are the main carriers of PFAS into the paper recycling stream.
A Norwegian study published in the Journal of Hazardous Materials Letters analysed PFAS in various paper products and in fractions from a corrugated cardboard recycling facility. It found that PFAS enter cardboard as unintentionally added substances through the recycling process – mixing paper of different origins, including food packaging deliberately coated with PFAS, contaminates the entire pulp stream. Detected levels ranged from 0.4 to 971 µg/kg, with the highest values in food-contact packaging rather than plain corrugated board.
Heavy metals – a legacy of previous lives
A study published in BioResources found that the recycling process itself increases heavy metal content in cardboard. The source is printing inks and dyes from previous incarnations of the raw material – zinc, lead, cadmium and nickel enter the pulp together with the waste paper. Even plain brown unprinted cardboard, if it comes from recycled fibre, carries the chemical history of the materials it was made from.
Should you be worried?
The scale of the problem needs context. The amount of contamination introduced into soil by a typical shipping box is – based on available data – comparable to other popular mulching materials, including wood chips. Plants can take up PFAS from soil, but at the concentrations typical of ordinary corrugated cardboard the scale of this is small.
It's also worth keeping the bigger picture in mind: PFAS are now present virtually everywhere – in soil, water and air. A layer of cardboard in the garden is not their primary or sole source. That's not an argument for ignoring the issue, but for placing it in the right position in the hierarchy of risks.
How to choose safer cardboard
- Choose plain brown, unbleached cardboard with no coloured printing or shiny coatings.
- Remove tape, labels and staples before use – these don't biodegrade.
- Avoid food packaging cardboard – pizza boxes, fast-food containers, microwave packaging – these have the highest likelihood of containing intentionally added PFAS.
- Ordinary shipping boxes (online orders, Amazon) are a better choice than food packaging.
Using cardboard in the garden is a decision that deserves awareness – not panic, but not blind dismissal either. A well-chosen box is still a sensible and eco-friendly mulching material. It's just that not every box is equally good.
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