Can I put cardboard down on my soil?

Cardboard has become the standard tool for two jobs — killing grass and weeds under a new bed, and lining paths under wood chip — and the questions people ask about it are almost always about whether the inks and glues are a problem. Mostly they are not, and the exceptions are easy to spot and easy to remove.

What you'll need

• Plain brown corrugated cardboard
• A hose or watering cans
• Compost or wood chip to cover it
• A knife or scissors (optional)

Step by step

1. Choose plain brown corrugated board.Delivery boxes, appliance boxes, bike boxes. The corrugation matters — it holds moisture and creates the air gaps worms colonise. Thin card and cereal packets break down in weeks and do not last long enough to kill grass.
2. Strip everything that is not paper.Plastic packing tape, plastic address labels, staples, and any glossy or waxed coating. Plastic tape is the big one: it does not break down, it fragments, and you will be picking shreds of it out of that bed for a decade. Paper gummed tape is fine and can stay.
3. Reject the glossy printed sleeves.Heavily printed, shiny outer sleeves on white board are often plastic-laminated. If it feels slippery or tears with a plastic sheen at the edge, it is not cardboard for this purpose. Ordinary brown box printing is water-based ink and is not a concern.
4. Lay it with generous overlaps.At least 150mm at every joint, and run it beyond the edge of the area you are covering. Grass and weeds find every gap, and the joints open as the board wets and settles. There is no such thing as too much overlap.
5. Wet it thoroughly before covering.Soak it until it is limp. Dry cardboard under a mulch stays dry, repels water, and can stop rain reaching the soil for months. This step takes ten minutes and it is the one most often skipped.
6. Cover it with something.Compost if you are planting into it, wood chip if it is a path. Bare cardboard blows about, dries out and looks awful. It also breaks down far more slowly uncovered.
7. Expect it to be gone within three to six months.Faster in warm, moist, worm-rich soil; slower over winter or on poor ground. Once it has gone, the layers above and below have joined and the soil below is usually noticeably better — worms congregate under cardboard in remarkable numbers.
8. Cut through it to plant anything with a substantial rootball.Small plants and seedlings root down through softening cardboard within weeks. For a shrub or a large plant, cut a cross through the board and plant into the soil beneath.

Tips

• Worms mass under cardboard. Lift a sheet that has been down a month and you will see why the technique works — it holds moisture, moderates temperature and gives them food, all at once.
• On a path, cardboard under 75mm of wood chip stops weeds coming through for a season and turns into soil. Relaying it annually is a five-minute job.
• Newspaper works but needs eight to ten sheets, blows about while you are laying it, and lasts a fraction of the time. Cardboard is better in every respect except availability.
• Perennial weeds — bindweed, couch grass, ground elder, mare's tail, docks, brambles — come straight through cardboard. It kills grass and annual weeds and nothing else.
• Supermarkets, bike shops and furniture retailers will usually give you as much as you can carry, and are pleased to.

Questions people ask