How to make compost — hot pile, cold pile, or bin in 90 days

How to make compost — hot pile, cold pile, or bin in 90 days

Compost is the single best soil amendment a home gardener can produce — better than any bought fertiliser, free, and a closed loop for kitchen + garden waste. The science is straightforward: get the carbon-to-nitrogen ratio right, keep the pile moist and aerated, and microorganisms do all the work. This guide covers the three main methods (hot pile, cold pile, dedicated bin), what materials to use + avoid, how to fix common problems, and how to test when it's finished.

Try Growli: Add your compost system to Growli and the app sends a turn reminder every 7-14 days for hot piles, plus seasonal tips for what to add (autumn = leaf mould layer; spring = grass clippings season).

The 3 main methods compared

For most home gardeners: a cold pile or stationary bin is the realistic choice. Hot piles are satisfying but require commitment to turning + monitoring.

The science in one paragraph

Aerobic decomposition breaks down organic matter via bacteria and decomposer fungi that need four things: carbon (energy), nitrogen (protein synthesis), moisture (40-60% by weight — roughly a wrung-out sponge), and oxygen (aerobic = with oxygen, not anaerobic / stinky). The pale threads and occasional mushrooms you sometimes see in a maturing pile are these saprophytic fungi at work, not a sign anything has gone wrong. When all four are balanced, the pile heats to 55-70°C and decomposes in weeks. When any one is off, the pile stalls or smells.

The most important balance is the C:N ratio: roughly 30 parts carbon to 1 part nitrogen by weight is optimal. By volume that's roughly 2 parts "browns" to 1 part "greens" — the practical rule home composters actually use.

Greens vs browns — the practical guide

Greens (nitrogen-rich, wet, smell when fresh)

• Kitchen scraps: fruit + vegetable peels, coffee grounds + filters, tea bags (paper only — avoid plastic-mesh teabags), eggshells (slow to break down but fine)
• Fresh grass clippings (don't dump 30 cm thick — layer thinly or mix with browns)
• Fresh garden trimmings (annual flowers, vegetable plant remains)
• Spent cut flowers
• Plant-based food waste from cooking prep
• Hair (human, pet) — surprisingly high nitrogen
• Manure from herbivores: chicken, rabbit, horse, cow — but COMPOSTED MANURE only; fresh manure should age 6 months before garden use

Browns (carbon-rich, dry, structural)

• Dry autumn leaves (the single best brown source for most gardens)
• Straw + dry hay
• Cardboard (corrugated, plain — remove tape + glossy labels)
• Plain paper, paper towel rolls, newspaper (avoid glossy magazine paper)
• Wood chips + sawdust (untreated wood only — never pressure-treated)
• Dry plant stalks (broad bean haulms, corn stalks)
• Pine needles (slow + slightly acidic — use in moderation)
• Spent peat-free compost from old pots

What NOT to add

• Meat, dairy, cooked food, oil — attract rodents, slow to break down, smell
• Dog + cat waste — pathogen risk for edible gardens
• Diseased plants — many disease spores survive (e.g., late blight on tomatoes, club root on brassicas, powdery mildew on cucurbits)
• Weeds with seeds or perennial roots — bindweed, ground elder, couch grass will regrow from the pile. Drown in water 6 weeks first, then add.
• Glossy magazine paper + receipts — chemical-coated, possible BPA
• Pressure-treated wood, sawdust, or chips — older CCA-treated wood (banned US 2003, UK 2005) leached arsenic + chromium
• Coal ash, charcoal briquette ash — too alkaline + may contain additives. Wood ash in moderation is OK.
• Citrus peels — fine in small quantities but slow to break down; large volumes acidic and may slow pile

Method 1 — Hot pile (4-8 weeks)

The fastest method. Needs commitment.

Build:

1. Locate a flat spot 1×1 m minimum, partial shade, near where you'll use the finished compost
2. Optional: drop a single layer of twigs at the base for drainage + air
3. Build the pile in alternating thin layers — 10-15 cm browns, 5-10 cm greens, repeat
4. Wet each layer as you go — squeeze test, should drip a few drops only
5. Build to full 1 cubic yard (90×90×90 cm) — smaller piles don't heat properly
6. Cover with a tarp to retain heat + moisture (optional but speeds up)

Maintain:

1. By day 3-5 the pile should be steaming and hot inside (55-70°C is ideal). If it's not heating, you need more nitrogen — add greens or sprinkle bonemeal
2. Turn the pile fully (outer to inner) every 1-2 weeks
3. Check moisture each turn — re-wet if a squeezed handful releases no drops
4. After 3-4 turns, the pile stops re-heating — that's when active decomposition is finishing
5. Let cure for 2-4 weeks (let earthworms move in, microbiology stabilises)

Done when: dark, crumbly, smells like earth (not ammonia, not rot), original materials no longer recognisable.

Method 2 — Cold pile (6-18 months)

The lazy gardener's method. Almost no work; takes much longer.

Build:

1. Pile in a corner of the garden, any shape (heap, contained bin, three-sided pallet enclosure)
2. Add materials as they appear — kitchen scraps, autumn leaves, garden trimmings
3. Try to balance greens + browns roughly 1:2 by volume but don't stress
4. Cover top with a piece of carpet or tarp to keep rain off (reduces leaching of nutrients)

Maintain:

• Nothing. Just keep adding to the top.

Done when: dig from the bottom — bottom layers (oldest) are finished compost. Top of pile is still recognisable materials. Sift through a 1 cm mesh to separate finished from unfinished.

Cold pile compost is excellent for general garden use (mulch, bed amendment) but not for seed-starting (may contain weed seeds or pathogens that hot composting kills).

Method 3 — Tumbler bin (4-12 weeks)

A drum bin mounted to spin. Two-chamber tumblers let you fill one side while the other matures.

Build:

• Buy or build a 200-400 L tumbler (Mantis, Joraform, ComposTumbler in the US; Hozelock, Garden Works in the UK)
• Start with 50% greens / 50% browns
• Add water until materials clump but don't drip

Maintain:

• Spin the drum 3-5 times every 2-3 days
• After 4-6 weeks contents should be dark + crumbly
• Empty, start again

Tumblers are best for kitchen-waste-heavy households + small gardens. Less effective for large quantities of leaves or grass clippings.

Method 4 — Stationary bin (3-6 months)

The compromise option. A 3-bay system (US Mother Earth News classic; RHS-recommended for UK allotments) lets you turn from active bay to maturing bay.

Build:

• Three side-by-side bays, each ~1×1 m
• Bay 1: actively adding new materials
• Bay 2: maturing (was bay 1 last quarter; turn fortnightly if energy permits)
• Bay 3: finished compost ready to use

The cycle rotates each quarter or season.

Troubleshooting common problems

Pile smells like ammonia

Too much nitrogen. Add browns (dry leaves, cardboard, straw). Turn to incorporate.

Pile smells like rotten eggs / sewage

Anaerobic — too wet, too compacted, or too much grass clippings layered thick. Turn to add oxygen. Add browns. Cover with tarp if rain is the culprit.

Pile is cold + dry

Too dry. Sprinkle water as you turn until you can squeeze a few drops from a handful.

Pile is cold + wet

Too much water + too much nitrogen too compacted. Turn + add browns.

Pile won't heat up

Either too small (under 1 cubic yard) OR too much carbon (browns). Add greens + water. Heat should arrive within 2-5 days of building correctly.

Flies + maggots

You added meat or dairy, OR pile too wet. Hot-compost-grade temperatures kill maggots; cold piles may have black soldier fly larvae which actually help decomposition (they're not house flies and don't bother you).

Rodents

Either you added meat / dairy / cooked food, OR the pile has too much undecomposed food on the outside. Switch to a covered bin if rodents are a regular problem (urban gardens especially).

UK + US specific considerations

UK

• Peat ban transitions affect bagged composts but not home composting. Per Defra's revised timeline, the full retail ban on peat in horticultural products applies from 2030. RHS retail is fully peat-free from January 2026. Home compost has never contained peat.
• Council green waste collection: most UK councils provide weekly or fortnightly garden waste collection in summer — feed your council bin too, or keep on-site.
• Common UK compost cultures: Bokashi (anaerobic kitchen pre-composter, Japanese origin) popular for flats without garden space.

US

• Municipal composting programs are growing — SF Bay Area, Seattle, Portland, NYC, Boulder all require or strongly encourage food waste composting curbside.
• Black soldier fly larvae are increasingly recommended for hot summers in zones 7+ — accelerate kitchen waste breakdown.

How to test when compost is finished

1. Look: dark brown to black, crumbly texture
2. Smell: earthy, like forest floor — NOT ammonia, NOT rotten, NOT manure-y
3. Touch: cool (not hot inside)
4. Bag test: seal a handful in a plastic bag overnight. Open + smell. Sour/ammonia = not ready. Earthy = done.
5. Bioassay: sow 5-10 radish or lettuce seeds in straight compost mixed with water. If germination + first 2 leaves look normal in 7-10 days, compost is finished.

How to use finished compost

• Top dressing existing beds: 2-5 cm layer over soil surface, let earthworms incorporate
• Mulch under trees + shrubs: 5-8 cm layer
• Potting mix component: 30-50% of homemade potting mix (combine with peat-free multipurpose, perlite or grit, optional coir)
• Side dressing vegetables: handful around base every 4-6 weeks during growing season
• Seedling mix: ONLY hot-composted material that reached 55°C+ for 3+ days; cold compost may carry pathogens or weed seeds dangerous for seedlings
• Lawn top-dressing: sift through a 1 cm mesh, broadcast 1 cm layer in spring (this is what professional groundskeepers do)

Common compost myths debunked

• "You must add starter / activator" — myth. Microbes are everywhere; a healthy pile starts on its own.
• "Eggshells lower acidity" — partially myth. Eggshells take 2-3 years to break down enough to release calcium. Crush them fine if you want them to help.
• "You can't compost in winter" — myth. Cold-pile composting continues slowly through winter; hot piles can stay warm with adequate mass + insulation.
• "All organic matter goes brown" — half-myth. Pile colour gives no information about whether composting is finished — bag test or bioassay is the only reliable check.
• "Compost tea is better than compost" — overstated. The science on compost tea efficacy is mixed; finished compost itself is more reliably beneficial.

Related

• Garden soil preparation — what to do with finished compost
• Mulching guide — using compost as mulch
• Types of soil — what compost adds to each soil type
• Types of fertiliser — compost as a fertiliser
• How to start a vegetable garden — using compost in a new bed
• Raised bed vegetable garden — soil fill with compost
• Vegetable garden layout — compost application timing
• Companion planting guide — compost-loving crops

Sources: Cornell Compost Science, University of Maryland Extension, RHS composting guide, Penn State Extension. Drainage layer myth context per glossary entry and 2025 PLOS ONE experimental research.