If you’re here, it’s probably because you’ve heard about this famous anti-mosquito tip using burnt coffee grounds. Or maybe you’re just looking to give a second life to that residue you throw away every morning. After three summers of trying out this method on my terrace (and failing miserably at first), I’m going to share with you everything I’ve learned about the art of burning coffee grounds effectively.
Spoiler: it really does work, but not in the way you’d expect.
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Why burn coffee grounds? The real reasons (beyond the myth)
A natural mosquito repellent… but in moderation
The first thing you need to know: yes, burnt coffee grounds keep mosquitoes away. But no, it’s not a magic shield that makes them flee 50 metres away.
During my tests in the summer of 2024, I compared three evenings on my terrace: one without protection, one with burnt coffee grounds, and one with a classic citronella candle. The results? The coffee grounds reduced the number of bites by around 40-50%, comparable to a simple candle. Not bad, but not miraculous either.
The science behind this phenomenon is fascinating. When the pomace burns, it releases volatile compounds into the air, notably caffeine and diterpenes. These molecules disrupt the ‘olfactory radar’ of mosquitoes, which use their highly developed sense of smell to locate their targets. A study published in the Journal of Agricultural and Food Chemistry confirms that these compounds interfere with the olfactory receptors of mosquitoes.
But here’s the rub: coffee grounds contain much less caffeine than freshly ground coffee, since most of it has gone into your cup. That’s why it’s only moderately effective.
A natural deodorant that really neutralises odours
I was pleasantly surprised here. Unlike chemical sprays, which mask odours, burnt coffee grounds absorb them. Its combustion releases particles that capture the hydrogen sulphide molecules responsible for unpleasant odours.
I tested it in my kitchen after frying some fish (the smell was a nightmare). Result: within 20 minutes, the smell had practically disappeared. Much more effective than a scented candle or room spray.
An ecological alternative to chemical incense
If you like burning incense but are concerned about the toxic substances in commercial products, coffee grounds are an excellent option. 100% biodegradable, with no chemical additives, it produces a fine smoke with a warm, woody fragrance.
The problem nobody is telling you: why your pomace won’t burn
You’ve probably already tried it. You spread some marc in a bowl, confidently approach a match… and nothing. It smokes a little, gives off a strange smell, then goes out. Frustrating, isn’t it?
I went through this at least ten times before I realised the three fatal mistakes.
Mistake No. 1: The pomace is not dry enough
This is THE main reason for failure. Coffee grounds fresh from your coffeemaker still contain 60-80% moisture. And wet grounds don’t burn, they smoke.
At first, I let my marc air-dry for 24 hours. Big mistake. Even after two days, the marc was still too moist. The result? A thick, black smoke that smelt of burnt coffee rather than roasted coffee.
The solution that really works:
- Spread the marc in a THIN layer (2-3mm maximum) on a baking tray covered with greaseproof paper.
- Leave to dry for 3-4 days in a dry, ventilated place (near a window is perfect).
- Or use the oven: 60°C for 30-40 minutes, stirring every 10 minutes
To check that it’s ready: the pomace should be completely crumbly between your fingers, like dry sand. If it sticks or clumps together, it’s still too wet.
Error no. 2: The grounds have already been “burnt” in your coffee maker
Here’s something that few articles mention: coffee grounds have already undergone a high-temperature extraction during the preparation of your coffee. This “first firing” creates carbonised solids that act as a thermal insulator.
In simple terms: some of the most combustible elements have already disappeared into your cup. That’s why grounds alone are so difficult to ignite.
Barista tip: mix 50% dry grounds with 50% freshly ground coffee (buy the cheapest from the supermarket, a 2-3€ packet will do). This mixture ignites much more easily and burns longer.
Mistake no. 3: Applying too much at once
Instinctively, you think that a big pile of pomace will burn better. The opposite is true. A thick pile blocks the air circulation needed for combustion, and only the surface burns while the bottom remains intact.
The right technique: spread a layer no thicker than 5mm in your container. If you want it to last longer, prepare several small containers rather than one large one.
The complete method I use (tried and tested)
After dozens of attempts, here’s my routine that works every time.
What you need
- Perfectly dry coffee grounds (3-4 days drying time)
- A heat-resistant container: tin can, terracotta dish, or old metal dish
- Aluminium foil (to protect the bottom and make cleaning easier)
- Long matches or a lighter
- Optional but recommended: a small natural fire starter (rolled up newspaper)
The step-by-step process
1. Prepare the container
Line the base with aluminium foil. This protects your container and creates a flat surface for even combustion.
2. Form a “nest
Arrange your dry grounds in a circle with a hollow in the centre (like a miniature volcano). This shape allows air to circulate and makes lighting easier.
3. Intelligent lighting
Here’s my secret technique: don’t try to light the grounds directly. Place a small piece of crumpled newspaper in the centre of your “nest”, light the paper, then let the pomace gradually ignite from the centre.
If you have mixed with fresh ground coffee (my preferred method), you can light directly with a long match.
4. Manage combustion
Once lit, the marc should burn slowly, giving off a fine, greyish smoke. If you see large flames or thick black smoke, something is wrong (the pomace is too moist or there is too much air).
In this case, blow gently to extinguish the flames and leave just the embers. The aim is to get a slow burn, like incense, not a campfire.
5. Keep a constant eye on
NEVER leave the pomace to burn unattended, especially the first few times. Keep a glass of water handy to put out the fire if you need to.
How long does it burn?
- Marc alone: 15-20 minutes
- Marc mixed 50/50 with ground coffee: 30-40 minutes
- Commercial compressed tablets: up to 60 minutes
Professional tips to boost efficiency
1. Add aromatic herbs
My favourite combination: dried lavender + rosemary. Add a pinch of these herbs to your grounds before lighting. Not only does it smell divine, it also enhances the insect repellent effect.
The dosage I use :
- 3 tablespoons dry coffee grounds
- 1 teaspoon dried lavender
- 1/2 teaspoon dried rosemary
2. The hole-in-the-can method
For outdoor use (terrace, camping), I have developed this ultra-efficient technique:
- Drill 3-4 holes in the side of a can (at the bottom) to create air circulation.
- Fill with dry grounds
- Light from above
This configuration creates a “chimney effect” that improves combustion and generates more repellent smoke.
3. Strategic location
For maximum effectiveness against mosquitoes, position your burning grounds downwind, so that the smoke drifts towards the area you want to protect.
On my 15m² terrace, I use two containers at either end. This creates a more complete olfactory barrier.
Alternatives if you’re fed up with the hassle
Let’s be honest: preparing and burning coffee grounds is a lot of work. If you’re looking for a simpler solution, here are the options I recommend.
Compressed coffee grounds tablets
This is the solution I now use 80% of the time. These little discs are already dry, pressed and ready to use. You light them like a candle and they burn cleanly for 40-60 minutes.
Advantages :
- Zero preparation
- Homogeneous, predictable combustion
- No flying residues
- Keep for months
Disadvantage: it costs a bit more (around €8-12 for 10 tablets), but given the time and trouble you save, it’s well worth it.
The ceramic incense burner
If you want an ultra-controlled, aesthetically pleasing burn, invest in a suitable incense burner (€15-25). You place your pomace in the upper cup, light a natural coal underneath, and the heat gently smokes the pomace without a direct flame.
This is my preferred method for indoor use (deodorisation, ambiance), as the smoke is very fine and the fragrance more subtle.
Electric smoke diffusers
The luxury option: a small electric diffuser for ground coffee (€30-50). It gradually heats the grounds without burning them, creating a gentle, long-lasting diffusion. No excessive smoke, no supervision required, no ashes to clean up.
Safety: the non-negotiable rules
After three years of regular use, here are my safety recommendations based on experience (and a few scares).
Outdoors
- Always on a stable, fireproof surface (stone, tiles, metal)
- Away from all flammable materials: at least 50cm away from tablecloths, dry plants, wooden furniture, etc.
- Never leave it unattended: even 5 minutes is not long for a gust of wind to fan the flames.
- Switch off completely before leaving: sprinkle with water or smother with sand
Inside
- Only in very well ventilated rooms: open a window
- Not in bedrooms: smoke, even natural smoke, is not good for the lungs while you sleep.
- Keep children and pets away: a hot pot is tempting for curious little hands
- Keep a fire extinguisher or water handy: you can never be too careful
Warning signs to watch out for
Stop immediately if you notice :
- Thick, black smoke (a sign of poor burning or too much moisture)
- Large flames (the marc must smoulder, not flare up)
- A pungent smell of burning rather than roasted coffee
- Sparks that fly
The truth about effectiveness: my assessment after 3 summers of testing
I want to be 100% transparent with you: burning coffee grounds is not a miracle solution.
What works really well:
✅ Deodorisation: Excellent for neutralising cooking, tobacco and musty smells. Fast-acting (15-20 minutes) and long-lasting.
✅ Atmosphere and fragrance: The scent of roasted coffee is warm and comforting, perfect for creating a cosy atmosphere.
✅ Moderate mosquito reduction: Around 40-50% fewer bites on my 15m² terrace, comparable to a citronella candle.
What doesn’t work :
❌ Total protection against mosquitoes: You’ll still get a few bites, especially if you’re in a heavily infested area.
❌ Easy to use: preparation takes time (3-4 days drying time), and lighting is not always obvious the first few times.
❌ Durability: a portion of pomace rarely burns for more than 20-30 minutes, so recharge regularly.
My personal verdict
Burnt coffee grounds are an excellent eco-friendly and economical back-up solution, especially if you already drink a lot of coffee. It’s perfect for :
- Occasional terrace parties
- Deodorise a room quickly
- Creating a zen and natural atmosphere
- Reduce your waste by giving pomace a second life
But if you’re looking for maximum protection against mosquitoes, combine it with other methods (long clothing, mosquito netting, skin repellent in high-risk areas).
Alternative uses for burnt pomace that I have tested
Garden fertiliser
Once completely cooled, coffee grounds ash is an excellent fertiliser, rich in nitrogen, phosphorus and potassium. I sprinkle it around my rose bushes and tomato plants (they love the acidity).
Dosage: maximum 500g of ash per m² per year, to avoid over-acidifying the soil.
Natural precursor
The still-warm (not hot!) ashes mixed with a little water form a gentle abrasive paste, perfect for cleaning hobs or removing grease from stubborn pans.
Moisture absorber
I left a bowl of ashes in my damp garage. The result: they absorbed the excess humidity and deodorised the space. Repeat every month.
FAQ : Your questions, my honest answers
Can you burn coffee grounds indoors?
Yes, but with precautions: only in a well-ventilated room (with the window open), in a suitable container, and never in a bedroom. Keep it short (15-20 minutes maximum).
Why doesn’t my coffee grounds smell like coffee when they burn?
There are two possible reasons:
- It’s not dry enough → dry again for another 1-2 days
- It is too old (more than 2 weeks after extraction) → the marc has lost its volatile aromas
Does decaffeinated coffee grounds also work?
Surprisingly, yes! Studies have even shown that decaffeinated coffee is sometimes more effective against mosquito larvae than conventional coffee. The diterpenes (which remain present) play an important role in the repellent effect.
How long do you keep dry coffee grounds?
In an airtight container, in a dry place away from light: up to 2-3 months. After that, it gradually loses its fragrance and effectiveness.
Can coffee grounds ash be reused?
No, once burnt, the pomace has released all its volatile compounds. The remaining ashes are good as fertiliser, but no longer have any repellent properties.
My protocol for the summer of 2025
Here’s exactly what I plan to do this summer, after three years of experimentation:
For terrace parties (2-3 times a week) :
- 2 tins with holes and grounds + 50/50 ground coffee
- Add dried lavender for fragrance
- Switch on 30 minutes before the aperitif
- Refill every 40 minutes if the evening goes on longer
For indoor deodorisation (weekly) :
- Ceramic incense burner with 1 tablespoon of pure pomace
- Use after cooking fish or smelly meals
- 20 minutes window open
Plan B for important evenings:
- Compressed marc tablets (zero stress, zero hassle)
- Combination with citronella candles for extra protection
- Skin repellent for sensitive guests
Conclusion: Is it really worth it?
After everything I’ve just shared with you, you may be wondering whether it’s really worth taking the plunge into burning coffee grounds.
My answer: It’s up to you.
If you :
- Drink lots of coffee and naturally produce lots of grounds
- Love natural and ecological solutions
- Have a little patience for drying and lighting.
- Look for a unique and warm olfactory ambience
So go for it. It’s rewarding, environmentally friendly and surprisingly effective once you’ve got the hang of it.
If you :
- Want an ultra-powerful anti-mosquito solution with a guarantee
- Don’t want to deal with drying and preparation
- Prefer “plug and play” solutions
So opt for commercial pellets or a good classic repellent. There’s no shame in that.
My final piece of advice? Try it at least once. Even if you give up after a few attempts, you’ll have learned something about coffee, mosquitoes, and perhaps discovered a ritual you like.
After three years, I’m still doing it. Because there’s something deeply satisfying about transforming a waste product into a useful solution, while enjoying the bewitching smell of roasted coffee that wafts over my terrace on summer evenings.
Are you ready to light your first coffee grounds smoke?
