Cardboard, not bioplastic: what actually breaks down in a UK garden bin

The phrase "compostable" on a UK supermarket bag means the material will break down in an industrial composting facility running at 58 °C with constant aeration and 60 per cent moisture, over about twelve weeks. Your garden bin — a plastic dalek, a wooden slatted bin, a hotbin, it does not matter — reaches those conditions on precisely zero days of the year. A British garden heap, well maintained, holds 15 to 25 °C through summer and barely breaks above ambient for six months of the year.

That gap is the whole story. Cardboard — plain, brown, unlaminated — has evolved to break down at those temperatures because fungi and worms have been eating wood fibre for 400 million years. "Compostable" bioplastic, usually PLA from fermented sugar, has not. In a cold heap it behaves roughly like a very thin plastic bag: it goes slightly brittle, maybe develops a few holes, and is still there in two years.

What actually composts in a UK garden bin

The browns — the carbon-rich half of a good compost mix — are the pieces people tend to get wrong, so start there:

• Cardboard: brown delivery boxes, cereal boxes, egg boxes, kitchen-roll tubes, toilet-roll tubes. Shred or tear to roughly A5 size. Remove any plastic tape, glossy printed labels and clear windows. These are gone in six to twelve weeks in an active bin.
• Dried leaves: especially beech and oak, which rot more slowly and bulk out the heap. Horse chestnut leaves rot faster. Sycamore is fine.
• Newspaper: the ink is vegetable-based and has been for twenty years. Shred small. Avoid the Sunday supplements with glossy pages.
• Wood prunings up to pencil thickness: thicker than that goes in the garden waste collection or the chipper.
• Straw and sawdust: if you can get them, pet-shop straw and untreated wood sawdust are superb browns. Avoid cat litter.

The greens — the nitrogen-rich half — are usually fine and most people over-add them. Grass clippings, raw vegetable peelings, coffee grounds, used tea bags with the staple removed. Please note that the "biodegradable" plastic in many teabags is polypropylene; the WRAP guidance on compostable plastics is worth reading before you add bags whole.

What does not compost, despite the label

This is where the industry's language has caused real environmental damage.

"Compostable" supermarket bags, marked with a seedling logo or the EN 13432 standard, will not break down in your bin. The standard tests under industrial conditions — 58 °C for twelve weeks — and the bags are formulated to pass that test, not your garden. In a cold heap they persist. I have fished out whole "compostable" bags from a bin three summers after they were buried, still readable, still intact. UCL's 2022 research on home composting found that 60 per cent of samples labelled home-compostable did not fully break down in two years of real-world domestic bins.

"Bagasse" and "PLA" takeaway containers are the same story. The material will compost industrially; it will not compost in your bin. If your council collects food waste separately — most English councils will by March 2026, under the government's Simpler Recycling policy — those items may go in the food caddy, depending on your council's specific accepted-items list. Do not assume.

Tea bags: the pyramid-shaped ones are usually PLA (nylon in older stock) and do not compost at home. Teapigs' silk-look tea temples, for instance, are PLA. Supermarket round bags from most British brands have had plastic heat-seals until recently. PG Tips and Clipper have moved to fully plant-based; Yorkshire Tea has moved but check the box, as older stock still exists. Shred open and compost the leaves; bin the bag.

Finally, "oxo-degradable" is a scam of long standing. It describes conventional plastic with an additive that causes it to fragment into microplastic pieces faster than usual. The EU banned it in 2021 as part of the Single-Use Plastics Directive. If you see it on a UK product, that product is old stock or from an importer who has not updated. It has no place in a compost bin or anywhere else.

The three brown swaps that actually earn their place

Having said what does not work, the short list of things that do is encouragingly short and cheap:

One: stop recycling brown cardboard, start composting it. A British household produces roughly 15 kg of brown cardboard a month once you count delivery boxes and food packaging. If half of that goes in the compost bin instead of the kerbside recycling, you have your browns for a whole year without buying anything. Shred with the office shredder or tear by hand; the smaller the pieces, the faster they rot.

Two: a bale of straw is £6 at a feed merchant. Enough to bulk the heap for nine months. Straw in the top layer also deters flies and keeps the compost from going anaerobic. If you don't have a feed merchant nearby, a pet shop will sell rabbit-grade straw in small bags for the same purpose.

Three: leaves in autumn, taken seriously. A typical British suburban garden sheds 200 to 400 litres of leaves in November. Rake them into a dedicated leaf cage (four fence posts and a roll of chicken wire, £20 at any builders' merchant) and you have leaf mould in a year, pure brown mulch in two. Add a shovelful of finished compost on top to seed the fungi, cover with a sheet of old carpet or hessian to keep moisture in.

Why the bin gets cold, and the one fix that matters

Compost generates heat because bacteria respire. Respiration needs oxygen, nitrogen and moisture in approximately the right proportions. In most British garden bins the limiting factor is almost always oxygen — the mix compacts, air is squeezed out, the heap goes anaerobic and cool, and decomposition slows to a crawl.

The single fix that most matters is turning. Not weekly; that is more effort than needed. Three times a year is enough: once in April (to wake the heap from winter), once in July (to carry the summer temperature peak), once in October (to layer the new leaves). A garden fork, twenty minutes. The RHS composting guidance has photographs of what a properly aerated heap looks like in cross-section.

A hotbin — the insulated plastic bins sold in the UK for about £180 — short-circuits most of this. They hold 40-60 °C through most of the year, which is still below industrial-compost temperature but closer, and they will actually process some "compostable" packaging within a year. If you want to compost PLA at home, that is the honest route; a cold heap will not.

Everything else — the "activator" powders, the pineapple-enzyme concentrates, the compost tea bags — is trying to speed up the wrong variable. Oxygen is the missing bit in 90 per cent of under-performing garden bins. Get a fork, turn the bin, and leave the rest alone.

If you are starting out and want the beginner-level view of all of this with a UK-specific lens, our beginner's guide to starting a home compost bin walks through the siting, sizing and first six months.