Can I compost weeds without spreading them round the garden?

Which weeds go straight on the heap, which need drowning first, and which must never go near it at all — sorted by what actually survives a cold garden heap.

Difficulty: beginner
Time: 30 min
Read: 4 min
Safety: warning

Short answer

Annual weeds without seed heads go straight on. Weeds already in seed, and the roots of perennial weeds like bindweed, couch grass and ground elder, survive a cold heap and come back with the compost. Drown them in a lidded bucket of water for four to six weeks first — the sludge that results is safe to compost and the liquid is a free feed. Invasive listed species never go on a heap.

The rule that you cannot compost weeds costs gardeners an enormous amount of free organic matter. The truth is narrower: a cold heap does not kill viable seed or root fragments, and both then travel back to the beds inside your compost. Kill them first and the material is as good as any other green waste.

Safety

Never compost, shred, move or put in a green bin any listed invasive species — Japanese knotweed and giant hogweed among them. Causing them to spread is a legal matter and disposal is a licensed route. Giant hogweed sap causes severe burns in sunlight and must never be handled without full cover and eye protection. Wear gloves for all weeding, and keep tetanus cover current.

What you'll need

A bucket or old dustbin with a lid
Gloves
Black plastic sheet or old compost bags (optional)

Step by step

Sort as you weed, not afterwards.Three piles: safe to compost, needs drowning, must not go near the heap. Doing it at the point of pulling takes no extra time and is the whole system.
Put annual weeds without seed heads straight on.Chickweed, groundsel, hairy bittercress, annual meadow grass, fat hen, before they set seed. They are soft, nitrogen-rich and among the best green material a garden produces.
Drown anything with seed heads.Even a fortnight of hot weather turns a pulled weed's flower into viable seed. Once it is in seed it goes in the bucket, not on the heap.
Drown all perennial roots.Bindweed, couch grass, ground elder, creeping buttercup, dandelion, docks, horsetail. Every one regrows from a fragment, and a cold heap propagates them beautifully.
Use the bucket properly.Fill with water, submerge the weeds completely, weigh them down, put a lid on and leave for four to six weeks. It becomes evil-smelling black sludge — that is the point. Nothing survives it.
Use the liquid as a feed.Diluted to the colour of weak tea it is a genuinely useful nitrogen-rich liquid feed. Nettles in particular make an excellent one. Keep the lid on for your neighbours' sake.
Compost the sludge.Once drowned, tip it onto the heap with plenty of brown material to soak it up. It is a strong activator.
Or solarise instead, if you have space.Seal perennial roots in a black plastic bag or under sheeting in full sun for a whole season. It works, but slower and less certainly than drowning.
Bin or burn diseased material.Clubroot, honey fungus, blight-infected haulm and white rot survive home composting. Council green waste is processed hot enough; a garden heap is not.
Handle listed invasives separately and legally.Japanese knotweed and giant hogweed do not go on a heap, in a green bin, in a shredder or in household waste. They go through a licensed waste route, and knotweed spreads from a fragment the size of a fingernail.

Tips

A lidded bucket by the compost bin, kept permanently topped up with water, makes this effortless.
Nettles drowned in water make one of the best free nitrogen feeds available, and comfrey makes the best potassium one.
If a heap does get properly hot and you turn the outside in twice, seed and roots in the centre are killed — but do not rely on it in a cold bin.

Common mistakes

Composting bindweed or couch grass roots directly — They survive, multiply and are then distributed evenly across every bed you mulch. It is the most efficient method of spreading them ever devised.
Composting weeds that have flowered — Seed ripens after pulling. A flowering weed on the heap is a seeding weed a fortnight later.
Shredding Japanese knotweed — Every fragment is a new plant and you have just multiplied it and spread it across a wide area. It is also legally serious.

If it doesn't work

Bindweed emerging wherever compost was spread

Cause: Live root fragments composted — Fix: Dig out what you can, and drown all perennial roots in future rather than composting them.

Carpet of seedlings after mulching

Cause: Weed seed survived a cold heap — Fix: Hoe them off small, and keep seeding weeds out of the heap in future.

Drowning bucket smells appalling

Cause: Working correctly — Fix: Keep the lid on and site it away from doors and neighbours.

Questions people ask

How long do weeds need to be drowned for?

Four to six weeks fully submerged with a lid on. When it is black sludge that no longer resembles a plant, nothing in it is viable.

Can I compost grass and moss from the lawn?

Clippings yes, mixed with plenty of brown material. Moss and scarifying debris will compost but slowly, and moss can survive a cold heap, so drown it or bin it.

What about weeds treated with weedkiller?

Leave them where they are until they have fully died off, and then compost them only if the product label permits it. Some residues, especially in manure and hay, survive composting and damage sensitive plants — the label governs.