The short version: Trench composting means digging a trench about a spade deep in a garden bed, filling it with kitchen scraps and garden waste, and covering it back over with soil. There is no bin, no turning and nothing to spread later, because the compost forms right where the roots will find it. Keep meat, dairy and cooked food out at any depth, dig no deeper than about 35 cm, and give a freshly filled trench a few months before you plant hungry crops over it.
Not everyone wants a compost bin. Some yards have no spare corner, some gardeners worry about smells and rodents, and some of us never get around to turning a heap. Trench composting sidesteps all of that. It is the oldest method there is, and it suits the way most of us produce waste: a bowl of peelings every day or two, a bucket of prunings on the weekend.
What trench composting is
Trench composting is in-ground composting. You dig a trench in a bed you are resting, drop in your organic waste, and backfill it with the soil you removed. Soil organisms and worms break the material down over the following months, and the bed usually ends up richer in organic matter and better at holding water, right in the root zone rather than on the surface.
Because it is buried, the process is slower and quieter than a hot heap. You give up speed and the chance to kill weed seed with heat, and in return you get a method with almost no ongoing work.
Before you dig: two safety notes
Garden soil and compost can carry Legionella longbeachae, the cause of a form of Legionnaires' disease, so wear gloves and a mask, dampen dry soil and compost before you handle it, and wash your hands afterwards. And a spade can reach buried gas, water and power services, so before digging deeper than a spade or anywhere near the house, get a free check from Before You Dig Australia. Its plans show utility mains, not private lines such as garden irrigation or the power run to a shed, so dig carefully by hand once you are near anything you suspect.
How to dig and fill a trench
Choose a bed you can leave alone for a season, or a strip between rows, and dig when the soil is moist rather than bone dry or sodden, which protects the soil structure, especially in clay.
- Dig no deeper than about 35 cm in most soils, and only about 25 cm in heavy clay. Two spade widths across is a good start, and as long as you like.
- If the soil is dry, water the base of the trench before you fill it.
- Lay a few broken twigs in the bottom for air, then add material in layers, alternating moist greens (scraps, clippings, soft weeds without seed) with dry browns (leaves, straw, torn cardboard, shredded paper), roughly half and half.
- Fill the whole trench in one go, or work from one end over weeks, always covering exposed scraps with soil before you walk away.
- Leave the top 10 to 15 cm for soil, backfill, and mound an extra few centimetres on top, because the trench will sink as the material breaks down.
What to bury, and what to keep out
- Bury: fruit and vegetable scraps, coffee grounds, tea leaves, eggshells, lawn clippings in thin layers, soft prunings, spent annuals, autumn leaves, straw, plain paper and cardboard.
- Keep out: meat, fish, bones, dairy, cooked food, fats and oils, dog and cat droppings, diseased plants, seeded weeds, and anything that regrows from a fragment, such as couch, kikuyu, nutgrass and oxalis.
Ignore the old advice about burying meat if you go deep enough. Australian gardening authorities and councils exclude meat and seafood from home composting outright, because they draw rats and foxes and no depth a home gardener digs is a reliable defence. Citrus and onion scraps are fine in ordinary quantities.
When can you plant over it?
Seeds and shallow-rooted seedlings can go into the layer of soil on top of a well-covered trench straight away, because by the time their roots reach the buried material it has usually broken down. For hungry, deeper-rooted crops, wait a few months.
The reason is nitrogen drawdown. As soil microbes break down carbon-rich material such as straw, leaves and paper, they borrow nitrogen from the surrounding soil, and for a while there is less for the plants. It is temporary, but a pumpkin planted into a trench of fresh straw and cardboard can sulk and yellow until it passes. Mixed kitchen scraps cause less of a lag than pure browns. A cover crop sown over the trench while it settles is a good use of the wait, and our guide to cover crops for winter explains which ones to sow.
The bean trench, translated for Australia
English allotment gardeners have filled a "bean trench" for generations, digging it in autumn so it is ready for runner beans in spring. It is a Northern Hemisphere habit, so the timing needs turning around for Australia.
In temperate gardens, dig and fill the trench in late summer or autumn, or through winter, so it has a few months to settle before spring and summer planting. Then sow the crops that most reward rich, moisture-holding soil along the line of the trench: climbing and bush beans, pumpkins, zucchini, melons, cucumbers and sweet corn. The buried material holds water as well as feeding, which is exactly what those thirsty plants want in a hot January.
Pros and cons
There is no bin to buy, nothing to turn, no finished compost to barrow around, and no open heap to smell or attract flies. Buried organic matter also helps sandy soil hold water, and the digging itself opens up compacted ground.
The drawbacks are the digging, the wait before planting, and the space the trench takes out of use for a season. Because it breaks down with little air, a trench works more slowly than a hot heap and does not kill weed seed or disease, so it needs cleaner ingredients. You may read that buried compost produces more methane than an open heap. A small, well-drained garden trench is likely a much smaller source than the same scraps rotting in landfill, so it is probably still one of the better things you can do with them.
Trench composting across Australia's climate zones
- Tropical (Cairns and far north Queensland, Darwin and the Top End of the Northern Territory, the Kimberley in Western Australia): a trench can waterlog and turn sour in the wet season. Dig and fill in the dry months and keep the site well drained.
- Subtropical (Brisbane and coastal Queensland, Coffs Harbour and northern New South Wales): fast and forgiving. Inside South East Queensland's fire ant biosecurity zones, check the National Fire Ant Eradication Program's rules on fireants.org.au before moving soil, compost or mulch off your property.
- Arid and semi-arid (inland Queensland, Broken Hill and Dubbo in New South Wales, Mildura in Victoria, inland South Australia and Western Australia, Alice Springs in the Northern Territory): nothing rots dry. Water the trench when you fill it and through the season, and mulch the surface.
- Temperate (Sydney and New South Wales, Melbourne and Victoria): fill in late summer to autumn, or over winter, for spring and summer crops.
- Mediterranean (Perth and southern Western Australia, and Adelaide and the Adelaide Plains in South Australia): water-repellent sands gain a great deal from buried organic matter. Fill in autumn ahead of the winter rain, and repeat over several seasons.
- Cool and alpine (Hobart and Tasmania, Canberra and the ACT, Ballarat in Victoria, Orange and Armidale on the New South Wales tablelands): breakdown almost stops over winter. Fill in autumn and plant the hungry crops in late spring once the soil has warmed.
Frequently asked questions
How deep should a compost trench be?
About a spade deep, no more than roughly 35 cm in most soils and about 25 cm in heavy clay. Deeper trenches are more work, break down more slowly, and are more likely to meet buried services.
Will a trench attract rats?
Far less than an open heap, provided you keep meat, dairy, bread and cooked food out and cover every scrap with soil before you leave it. Rats and foxes dig for food they can smell.
Can I put lawn clippings in the trench?
Yes, in thin layers mixed with browns. Leave out clippings from any lawn treated with a broadleaf or residual herbicide in the past several months, since some of these chemicals stay active in clippings, compost and manure for up to a year.
How long before I can plant?
Shallow-rooted seedlings and seeds can go into the covering soil straight away. Give hungry crops a few months, longer over a cool winter, so the nitrogen drawdown has passed.
Is a hole easier than a trench?
Often, especially among established plants or in a small bed. Our guide to dig and drop compost holes covers that version of the same idea.
A trench asks for an hour with a spade and then very little else, and it puts the goodness exactly where next season's beans and pumpkins will look for it. ABC Gardening Australia's own trench composting guide is a good companion read.
Related growing guides
- Composting for beginners
- Dig and drop compost holes
- Sandy, clay or loam? Understanding and improving your soil type
Ready to plant along the trench? Browse our bean seeds and vegetable seeds.