What can you (really) put in your compost?

You’ve installed a compost bin in your garden or kitchen, and now every meal raises the same question: ‘Can I put this in the compost?’ Avocado skin, eggshells, melon rinds, tea bags, paper towels, cheese rinds… There are many questions, and the answers aren’t always clear.

Because composting isn’t just about emptying your kitchen bin into a corner of the garden. To compost properly, you need to understand how the process works and, above all, what the microorganisms that transform your waste into humus are looking for — or avoiding. Good compost depends on a certain balance. And while some organic waste integrates easily, other types can slow down decomposition, disrupt the process or even cause problems.

This article aims to help you understand the situation clearly, without jargon or moralising. You will find:

• Key principles for understanding what happens in a composter (the version for beginners and the version for experts)
• What you can put in it, and why
• What you should avoid, and why
• A comprehensive list in alphabetical order of the main types of everyday waste, with a clear answer: compostable or not?

Whether you’re new to composting or already an expert, this guide will help you improve your practices — or simply stop asking yourself the question every time you peel a carrot.

Part 1 :

What makes good compost?

Why can’t everything be composted?

Composting is based on a well-established biological process: the decomposition of organic matter by micro-organisms (fungi, bacteria) and macro-organisms (earthworms, insects, woodlice, etc.). These living organisms gradually transform your waste into humus, a rich, stable material that can be returned to the soil to nourish life. However, this process only works well under certain conditions. And not all waste meets these conditions.

Three main factors influence the quality of compost:

• The balance between carbon and nitrogen
• Compost aeration
• Moisture

An imbalance in any of these three areas can slow down decomposition, create unpleasant odours, or even render the compost unusable.

The carbon/nitrogen ratio: the key to balanced compost

We often talk about “dry” and “wet” waste, or “brown” and “green” waste. Behind these terms lies a fundamental ratio in composting: the C/N ratio, i.e. the proportion of carbon (C) to nitrogen (N) contained in the composted material.

• Carbon is the energy source for microorganisms. It is found in dry materials such as dead leaves, shredded branches, untreated paper, brown cardboard, sawdust, etc.
  Nitrogen accelerates microbial growth. It is mainly found in moist, rich materials such as peelings, food scraps, grass clippings, coffee grounds, etc.

To work properly, compost needs a C/N ratio of between 25:1 and 30:1. In other words, it needs much more carbon than nitrogen. If this ratio is unbalanced:

• Too much nitrogen? The compost becomes too wet, smells like ammonia or rot, and fermentation takes over decomposition.
• Too much carbon? The compost dries out, slows down, and microorganisms lack nutrients to grow.

Key point: if your compost is too wet, add brown materials (dead leaves, cardboard). If it is too dry or slow to break down, add fresh materials.

Aeration: compost needs oxygen

Home composting is an aerobic process, meaning that it requires oxygen to function properly. In the absence of air, other microorganisms, known as anaerobic microorganisms, take over… but their activity produces methane, fermentation odours, and slows down decomposition.

Good aeration allows:

• Prevent unpleasant odours
• Activate the transformation of materials
• Prevent the formation of juice or compacted areas

In practical terms, this means that you must:

• Turn your compost regularly (every 10 to 15 days)
• Do not overload with very wet waste or thick layers (fresh grass, for example)
• Alternate brown and green layers to promote an aerated structure

How can you manage the moisture content of your compost?

Moisture is essential: microorganisms work in a moist environment, not a dry one. But too much water can displace air and create an anaerobic environment. The goal is to maintain a moisture level similar to that of a wrung-out sponge. Compost that is too dry will not break down. Compost that is too wet will smell bad.

Common sources of excess moisture:

• Too much fruit, vegetables or cooked food thrown away without adding dry materials
• Lack of absorbent brown materials (paper, cardboard, leaves)
• Poorly designed lid on the composter (poor water drainage)

Good habit: every time you add fresh material, remember to add some brown material. If you empty a bowl of peelings, add a handful of cardboard or dead leaves on top.

What types of waste can cause problems when composting?

Not all organic waste is equal when it comes to composting. Some is too fatty, too acidic, too slow to break down, or poses health risks.

Here are a few examples:

• Meat, fish, dairy products: rich in animal fats and proteins, they attract pests and do not decompose well at room temperature.
• Citrus fruits, garlic, onions: highly acidic or antibacterial, they can slow down the work of microorganisms.
• Diseased plants: they can transmit their pathogens to the compost.
• Printed or bleached paper: contains ink, glue or chemical treatments.

In an industrial composting facility, where temperatures can reach up to 70°C, this waste can be processed. However, caution is advised when composting at home.

The version for experts

Diving into compost: behind the scenes of a microbial banquet

Before looking at what can and cannot be put in compost, it is important to understand one essential thing: compost is not a rubbish bin, it is a living ecosystem, a veritable open-air feast for billions of invisible mouths. And there are many guests at this banquet: bacteria, fungi, actinobacteria, springtails, worms, mites, nematodes, etc. But let’s start with the chefs of this great buffet: microorganisms.

Microorganisms, the invisible workers of compost

Bacteria: the first to eat

These are the stars of composting. They arrive in large numbers as soon as new organic waste is added. There are three main categories of bacteria, depending on the temperature they prefer:

• Mesophiles: these develop between 10 and 40°C. They are the first to appear. They start the decomposition process as soon as you put peelings or coffee grounds in the bin.
• Thermophiles: these take over when the compost heats up, usually between 40 and 70°C. They accelerate the breakdown of the toughest materials, such as cellulose from paper and plant fibres. In home compost, they are most active in summer or in well-balanced volumes.
• Actinobacteria: these rather special bacteria like dry, woody materials. They break down carbon-rich materials (such as dead leaves and cardboard). They are often responsible for the pleasant smell of ripe compost.

Fungi: patient decomposers

Unlike bacteria, fungi are less hurried: they thrive in drier, more acidic conditions. Their speciality? Breaking down complex structures such as lignin and cellulose (found in wood, brown paper and certain plant fibres). When you see white filaments in your compost, these are often fungi.

They are like meticulous craftsmen dismantling a piece of wooden furniture to recover every screw. Slow but incredibly efficient.

And the others?

Earthworms, springtails, woodlice and rose chafer larvae also join the party, especially during the maturation phase. They mix, chew, digest and transform everything into humus. They are called macro-organisms.

Where do these bacteria and fungi come from?

This is one of the most beautiful lessons of composting: nothing is ever completely sterile. Microorganisms are everywhere around us. They end up in your compost:

• From the waste itself (fruit, vegetables, coffee grounds, egg shells, etc., all of which already carry bacteria on their surface)
• From the soil (if your composter is in contact with the ground, worms and bacteria can access it freely)
• From the air (fungal and bacterial spores fly around and settle naturally)

In other words, you don’t need to ‘seed’ compost for it to work. It populates itself, provided you give it a welcoming environment.

Nitrogen and carbon: fuel and structure

Many guides talk about ‘green materials’ and ‘brown materials,’ but the real distinction lies in their carbon and nitrogen content.

Here’s a simple metaphor: imagine that compost is a big barbecue party.

• Carbon (C) is coal, the fuel that feeds the fire.
• Nitrogen (N) is the meat you put on the grill: it’s what the microbes want to eat.

But be careful: without charcoal, the fire won’t catch. And without meat, it’s useless. So you need a good balance between the two.

Specific examples:

• A banana peel: rich in nitrogen → ‘green’ material
• A dead leaf: rich in carbon → ‘brown’ material
• A piece of torn cardboard: very carbonaceous
• Coffee grounds: despite their colour, very rich in nitrogen
• Dry bread: somewhere in between, but rather rich in carbon (especially once hardened)

Key points:

➤ Microorganisms eat nitrogen…
➤ …but they need carbon as an energy source to do so.

Why do some foods behave differently?

Take a look at your leftovers. A piece of bread will dry out and harden if left on the table for a few days. A banana will soften, turn black and then give off a fermented smell. Leftover meat will smell very bad. Why is this?

It’s all down to structure and composition:

• Bread is dry, low in fat and rich in starch → it loses moisture and becomes hard
• Bananas are rich in sugar → bacteria feast on it, the cells burst and it “melts”
• Meat is rich in protein → it attracts specific bacteria (often anaerobic) that produce very smelly sulphur compounds

Foods rich in nitrogen (proteins, sugars, animal matter) are the first to rot. Dry, fibre-rich foods (paper, leaves, bread, cardboard) degrade more slowly, or not at all if they are too dry or compacted.

What are the risks of unbalanced compost?

If you throw away all your food scraps without adding dry matter, you create an environment that is too rich in nitrogen and too moist. The result:

• Aerobic microorganisms suffocate
• Anaerobic bacteria take over
• The result: it ferments, it smells bad, it becomes sticky, and it attracts gnats.

Conversely, if you only add cardboard or dead leaves, your compost will be:

• Too dry
• Slow to start
• Unattractive to bacteria
• Result: it stagnates, it crumbles, and nothing really happens

  

Element	Role in compost	Common example
Carbon (C)	Energy / structure	Cardboard, paper, dead leaves
Nitrogen (N)	Nutrients for microbes	Peelings, coffee grounds
Mesophilic bacteria	First agents of degradation	Fruit, fresh vegetables
Thermophilic bacteria	High-temperature accelerators	Well-established compost
Actinobacteria	Degrade wood and paper	Cardboard, branches
Mushrooms	Secondary decomposers	Wood, paper, hard plants

Part 2 :

What can (and can’t) you put in your compost?

Industrial or domestic compost: what’s the difference?

First of all, a useful reminder: not everything that is ‘compostable’ can necessarily be composted at home.

There are two main types of composters:

Home composters

• Garden composter
• Worm composter
• Apartment composter with or without bokashi

Variable temperature (15 to 50 °C), low volume, no sustained heat build-up

More fragile, sensitive to moisture, sorting errors and pests

Industrial composters

• High-temperature treatment of biowaste (up to 70°C)
• Rapid degradation, destruction of pathogens, acceptance of certain compostable packaging

The instructions are not the same. Packaging that is ‘compostable’ in industrial compost may never break down in a domestic compost heap.

Compostability: What are the main types of waste?

Raw plant waste: basic allies

Organic waste from plants is a must for home composting. Easy to manage, well known and effective, it is the ideal basis for healthy, balanced compost.

These include:

• Vegetable peelings
• Damaged fruit
• Coffee grounds
• Tea bags (without staples or plastic)
• Eggshells (crushed if possible)
• Dead leaves
• Wilted flowers
• Stems and tops (carrots, radishes, leeks, etc.)

Tip: Cut into pieces if the stalks are thick or fibrous (e.g. leeks). Citrus peel is compostable, but only in small quantities: it is acidic and breaks down slowly.

Why should raw food waste be handled with care?

This is a point that many people get wrong. Contrary to popular belief, some cooked foods can indeed be added to compost, but only under strict conditions. Only low-fat vegetarian leftovers without sauces are suitable. Dishes containing cheese, meat, excess fat or seasonings that are too salty or sweet should be avoided at all costs, as they upset the balance of the compost and attract pests. Leftovers such as pasta, rice or plain potatoes, dry bread, steamed vegetables or pizza crusts without cheese are perfectly acceptable in domestic compost.

On the other hand, rich or processed dishes such as lasagne, quiches, gratins or cream soups should be avoided at all costs. Too fatty and too salty, these foods slow down decomposition, generate unpleasant odours, ferment quickly and attract pests such as midges and rodents.

Tip: bury these cooked leftovers in the centre of the compost and immediately add brown matter (dead leaves, cardboard).

Why avoid animal waste?

Animal waste is rich in protein and fat, making it attractive to rodents and flies, and particularly difficult to compost cold.

• Raw or cooked meat
• Fish, crustaceans, shellfish
• Dairy products (butter, cheese, yoghurt)
• Whole or broken eggs
• Fats, sauces, cooking oils

However, a few exceptions are possible, such as eggshells (crushed) and small dairy product leftovers in a well-controlled or industrial composter.

When composted at home, this waste does not break down properly and creates health risks.

Why are paper and cardboard essential to your compost?

Dry materials are essential for balancing out wet waste. They absorb moisture, promote aeration and feed actinobacteria. It is advisable to use :

• Used paper towels (unscented)
• Paper tissues
• Cardboard egg cartons
• Newsprint, small quantities
• Unprinted brown cardboard
• Kraft paper

However, certain components should be avoided, such as :

• Glossy, plastic-coated, bleached paper
• Perfumed, coloured towels
• Packaging printed with chemical inks

Tip: tear into small pieces to accelerate degradation

Bioplastics, ‘compostable’ paper, microplastics: beware of false good ideas

The proliferation of so-called “compostable” or “bio-sourced” packaging often leads to confusion. Cardboard cups, Kraft bags, salad trays and “organic” tea bags all promise to be “eco-responsible”… but not all of them are welcome in the home compost.

Waterproof” paper and cardboard

A lot of ‘cardboard’ packaging is actually coated with a thin plastic film (polyethylene, polypropylene, etc.) to make it watertight. This is the case, for example, with :

• Hot drink cups
• Food trays
• Fast food boxes
• Certain tea bags and infusions

However, these films do not degrade in a garden composter. They can fragment into microplastics, polluting the final compost.

Bioplastics: what are they really worth?

Bioplastics are plastics made from plant materials (maize starch, sugar cane, etc.), some of which are certified as compostable under standard EN 13432.

But be careful:

• This standard applies to industrial composting, with high temperatures and strong mixing
  In a domestic composter, these materials break down badly, slowly, or not at all
  Result: they remain visible for a long time, or fragment without being truly digested.

Microplastics: why degraded does not mean digested

Some materials fragment under the effect of heat or humidity, but are not digested by micro-organisms. They become microplastics, invisible but persistent. These residues can contaminate your compost, then your plants, and ultimately the soil and the food chain. Little is known about their ecological impact, but it’s best to err on the side of caution.

What you can compost without risk

Type of packaging	Domestic compost?	Why is this?
Tea bag with clip	No	Metal + plastic
Unbleached paper tea bag	Yes	Biodegradable and harmless
Cardboard coffee cup	No	Plastic film
Kraft” burger box	Caution	Check: often laminated
Untreated, non-greasy kraft sack	Yes	Good source of carbon
Compostable bags EN 13432	No	Industrial compost only
OK compost bags HOME	Caution	Worth testing, but sometimes disappointing
Starch-based film (maize, manioc, etc.)	Caution	Degrades slowly, not always digested

Plants, leaves and flowers: why pay attention to exceptions?

When composting, it’s a good idea to include the following plants:

• Dead leaves
• Garden waste
• Withered plants
• Bouquet flowers

However, the following should be avoided:

• Diseased or infested plants (risk of spreading pathogens)
• Thick or waxy leaves (e.g. laurel, oak): very slow to break down
• Plants treated with pesticides

Tip: if in doubt, leave the plants to dry out for a few days before composting them.

However, there are some special cases:

Element	Compostable?	Remarks
Citrus	Yes, in moderation	Too much acid
Oignons, ail	Yes	Antibacterial → limit
Avocado skins, pits	No, or very slow	Too tough
Fruit stones	No, or very slow	May take 2 to 5 years
Wood ash	In small doses	Very alkaline
Wood chips	Yes	If not treated
Succulents (aloe type)	With caution	Very damp, slow decomposition
Frying oil	No	Smothers micro-organisms
Natural textiles	Caution	Very slow to cut

Part 3 :

Alphabetical list: What can you really put in your compost?

This list is for anyone who has ever stood in front of their rubbish bin and wondered:

‘Can I put this in the compost?’

Type of waste in alphabetical order	Compostable?	Remarks
Citrus	In moderation	Acid, slow degradation
Garlic	Yes	Antibacterial effect, limit quantity
Pineapple	Yes	Acidic, fibrous but compostable
Avocado (skin, stone)	No, very slow	Very tough, takes years to degrade
Very ripe bananas	Yes	Perfect for launching a downgrade
Banana	Yes	Softens quickly, good for compost
Cooked beetroot	Caution	Sugar → fermentation possible
Egg cartons (cardboard)	Yes	Perfect as a carbon source
Compostable coffee pods	Caution	Check: OK compost HOME recommended
Brown cardboard	Yes	Good dry matter, tear into small pieces
Coloured cardboard	No	Industrial inks, risk of residues
Natural wood ash	Caution, in small quantities	Highly alkaline, dose finely
Coal ash	No	Contains heavy metals, toxic to soil
Mushrooms	Yes	Very good decomposers
Charcuterie	Non	Too fatty and salty, harmful attractions
Hair	Very slow	Rich in keratin, very long degradation time
Cabbage (flower, red…)	Yes	Odour possible but compostable
Lemon	Yes, in moderation	Acidic, degrades slowly
Raw wood chips	Yes	Slow but useful dry matter
Varnished wood chips	No	Chemical treatment
Eggshells	Yes, crushed	Grind to speed up
Camembert crusts	No	Fat + fermentation
Cheese crusts	No	Odour risk, greasy, very slow
Brioche bread crusts	Caution	Too sweet, limit it
Watercress	Yes	Very wet, mix well
Paper towels (unscented)	Yes	Absorbs moisture, ideal for
Radish tops	Yes	Ideal green waste
Carrot dewlaps	Yes	Very quick to compost
Oak leaves	Caution	Acid, slow decomposition
Bay leaves	No	Very tough, rich in essential oils
Walnut leaves	No	Contient juglone, toxique pour plantes
Plane tree leaves	Caution	Very thick, slow degradation
Wilted flowers	Yes	Conventional green waste
Processed flowers (bouquets)	No	Pesticides, dyes, toxic to microorganisms
Damaged raspberries	Yes	Degrade very quickly
Mouldy fruit	Yes	Excellent nitrogen, be careful with seeds
Cake	Please note: very low in fat and vegetable-based	Avoid if there is cream/butter
Freshly mowed lawn	Caution, use in small quantities	Very moist, rapid fermentation
Sprouted seeds	Caution	Risk of germination if not buried
Green beans	Yes	Fully compostable
Cooking oil	No	Stifles microorganisms
Cooked lentils	Caution	Small quantity, very moist
Animal litter (mineral)	No	Not biodegradable, risk of heavy metals
Animal bedding (plant-based)	Please note, depending on composition	Okay if no additives or animal waste
Coffee grounds	Yes	Rapid nitrogen supply
Melon (seeds)	Yes	Very small, no worries
Tissues	Yes	Good source of carbon
Fruit pits	No, very slow	Very hard, takes 2-5 years to decompose
Onions	Yes, to a limited extent	Contains natural antibacterial substances
Nails	Caution, very slow	Same problem as hair: biodegradable but slow
Oranges	Yes, in moderation.	Same logic as other citrus fruits
Bred	Yes	Okay in small quantities.
Baking paper	No	Often siliconised
Glossy paper	No	Plastic film, non-biodegradable
Newspaper	Caution, use in small quantities	Okay in small quantities.
Colour printed papers	Caution	Potentially toxic ink
Watermelon (skin)	Yes	Very moist, balance with brown
Cooked pasta	Yes	Okay, without sauce or meat.
Citrus peel	Yes	Limit for acidity
Kiwi skins	Yes	Thin and biodegradable
Potato peelings	Yes	Good degradation
Peas (shelled)	Yes	Rich in fibre, crush if possible
Scented paper towels	No	Chemical scent, non-biodegradable
Succulents	Yes, with caution	Very damp, slow decomposition
Leeks	Yes	Fibrous stems, cut into pieces
Fish	No	Fibrous stems, cut into pieces
Dairy products	No	Risk of odour and fermentation
Grape	Yes	Very good for bacteria
Leftovers	Please note, if you are vegetarian and avoid fatty foods	Okay if low in fat, buried + brown
Cooked rice	Caution, yes if natural	If natural, avoid sauces.
Tea bag (without clip)	Yes	Compostable without staples
Tea bag (with clip)	No	Contains metal, to be sorted
EN13432 bioplastic bags	No	Not for home composting
Kraft bags	Yes, if untreated	Unprinted, non-bold only
OK Compost HOME bags	Caution	Moderately reliable
Salad (leftovers)	Yes	Breaks down very well
Unscented paper towels	Yes	Absorbs moisture, ideal
Artichoke stems	Caution	Very fibrous
Corn stalks	No, very slow	Fibrous, difficult to compost
Sunflower stems	Caution	Difficult to grind, slow decomposition
Tomatoes	Yes	Be careful with the seeds if not hot.
Mowing dry grass	Yes	Good source of nitrogen
Meat	No	Too much protein attracts pests
Plain yoghurt	No	Dairy product, avoidable
Loose leaf herbal tea	Yes	Ideal for humidity

Knowing what to put in your compost isn’t just a matter of ticking boxes; it’s about understanding an ecosystem. An ecosystem of microorganisms, carbon/nitrogen balances, moisture and time. By following a few key principles (not too much fat, no plastic, vary what you put in), we enable these invisible decomposers to transform our leftovers into a living resource.

Composting isn’t just about sorting: it’s about learning to observe how matter evolves, moulds and transforms. It’s a simple but demanding, humble and effective gesture. And it starts with a daily question: ‘Can I compost this?’Frequently asked questions:

What can you put in a garden composter?

You can put kitchen waste (peelings, fruit, vegetables, coffee grounds, etc.), dry materials (cardboard, dead leaves), but avoid dairy products, meat, fish or plastics.

What foods should not be put in compost?

Avoid meat, fish, dairy products, very fatty or salty cooked dishes, and anything that is chemically treated or contains plastic (e.g. tea bags with staples).

Can citrus fruits be put in compost?

Yes, but in small quantities. Their acidity can slow down decomposition and disrupt microorganisms. Cut them into pieces if possible.

What should be done with paper towels and tissues?

If they are not scented or coloured, you can compost them. They provide carbon and help balance the moisture content of the compost.

Are compostable bags (EN13432 or OK Compost) really compostable?

Some are, but only under industrial conditions. Only bags certified OK compost HOME will break down in a home compost heap — and even then, slowly.