Cardboard is the cheapest useful material in gardening. It arrives at your door every week, it costs nothing, and laid properly in October it will hand you a clean, weed-free bed in March.

It is also routinely oversold. Cardboard does not kill everything, it does not improve your soil on its own, and laid badly it produces a bed with grass marching up through every seam. Here is what it actually does.

How does cardboard suppress weeds?

By excluding light. That is the entire mechanism, and understanding it tells you exactly where cardboard will work and where it will not.

A plant with no light cannot photosynthesise. It burns through its stored energy trying to reach the surface, and when the store runs out, it dies. Grass and annual weeds hold very little reserve, so they die in a few weeks. A dandelion with a long taproot holds a lot more and takes months. Bindweed holds an enormous amount, spread through metres of root, and it will simply grow sideways until it finds an edge.

Cardboard is also a physical barrier while it lasts. Weed seed already in the soil stays in the dark and does not germinate, and seed blowing in from outside lands on cardboard rather than soil. But the barrier is temporary - it rots - so the light exclusion has to do its work within a few months.

What cardboard is safe to use in the garden?

Plain brown corrugated cardboard. That is the one you want, and the boxes your deliveries come in are exactly right.

Take off everything that is not cardboard before you lay it: packing tape, plastic labels, staples, and the plastic windows out of envelopes. Tape does not rot. Lay a bed with the tape still on and you will be picking strips of polypropylene out of the soil for years.

Avoid: glossy or waxed cardboard (drinks cartons, some food packaging - it is plastic-coated and will not rot), heavily printed colour packaging, and anything with a plastic film laminated to it. If it shines, leave it out.

The ink question. Most modern cardboard printing in the UK and EU uses soy or water-based inks, and the corrugated brown board that packaging is made from is generally fine. The precautionary approach - which costs you nothing - is simply to prefer plain, unprinted board and to cut off any large blocks of colour print. Do not lose sleep over a printed logo.

The glue. Corrugated board is bonded with starch. It is one of the more benign things you will ever put on a garden.

How do I lay cardboard properly?

Five steps, and the two people usually skip are the two that matter.

  1. Cut the vegetation down first. Mow or strim as low as you can. Long grass under cardboard lifts it and holds it away from the soil.

  2. Water the ground. This is the step everyone skips. Cardboard is very good at keeping rain out. Lay it on dry ground in a dry autumn and the soil underneath stays bone dry until spring, the worms leave, and nothing rots.

  3. Overlap every sheet by 15cm minimum. This is the other step everyone skips, and it is the one that decides whether the job worked. Weeds do not come through cardboard. They come through the gaps between the sheets. Overlap generously, like roof tiles.

  4. Soak the cardboard once it is down. Wet cardboard hugs the soil, stays put in wind, and starts rotting immediately. Dry cardboard is a kite.

  5. Cover it. Compost is best - 5 to 10cm - because you end up with a bed you can plant into. Woodchip works for paths. Even grass clippings and autumn leaves will do. Uncovered cardboard blows away, dries out, and looks awful.

Done this way in October, the cardboard is gone by March and the grass under it is gone with it. Done without the watering and the overlap, you will spend next summer pulling couch grass out of the seams.

How long does cardboard take to break down?

Two to six months in a normal British autumn and winter, if it is wet and covered.

Laid in October, wetted and covered with compost, it is usually unrecognisable by March - the worms will have taken most of it down. Laid dry in a hot summer with nothing on top, it can still be sitting there intact a year later.

The three things that decide the speed: moisture (dry cardboard does not rot at all), contact with soil (it needs to be flat against the ground, not bridging over lumps), and worms. If the cardboard is still there after a full year, the reason is almost always that it never got wet.

What will cardboard NOT kill?

The perennial weeds with deep root reserves, and it is worth naming them so you are not disappointed:

What cardboard is genuinely excellent at: grass and turf, annual weeds (chickweed, groundsel, fat hen, annual meadow grass), and the seed bank sitting in the top few centimetres of soil waiting for light.

For the perennials, cardboard is one weapon in a longer campaign. Cover the ground, then snap off every shoot the moment it appears. Each shoot costs the plant root reserve, and each one you remove before it makes a leaf is energy it never recovers. Do that consistently for a season and even bindweed runs out (RHS weed advice). Do it when you remember, and it wins.

Is cardboard better than weed membrane?

For beds, yes. For long-term paths, membrane has a case - but a weak one.

Cardboard rots down into the soil and feeds the worms. Plastic membrane does not rot: it shreds. Anyone who has taken over an allotment that had membrane down for a decade will tell you what you find - a soil full of black plastic threads that can never be got out, and a compacted, lifeless layer underneath where nothing has been able to work.

Membrane is also worse at its actual job over time. Weed seed blows onto the top of it, roots into the mulch above, and grows perfectly happily with its roots woven into the fabric - which then makes it impossible to pull out.

If you want a permanent path, use woodchip on cardboard and top it up. Better surface, better soil, and nothing to dig out in ten years.

Where should I use cardboard this autumn?

Three jobs, all of which are best done in October:

Do it now, while the soil is still warm enough for the worms to be working. Lay it in January and it just sits there, cold, until spring.