Are Compost Fumes Toxic To Plants? ( Expert Details Revealed)

  • By: composthq
  • Date: May 21, 2022
  • Time to read: 13 min.

Many people are getting keen on home composting these days.

It’s a great part of the green movement, and if you’ve got a little bit of space, it’s a much more satisfying way to deal with food and garden waste than sending it to a landfill site.

It also gives you compost for your garden, which is very valuable if you’ve got green thumbs and you’re keen on gardening. Even if you just keep a few flowerpots, it’s satisfying to use your own compost.

If you have veggies and other plants growing, this can make gardening a lot cheaper, and it makes total sense to recycle materials you’ve already got – like kitchen waste and garden cuttings – into something you can use.

For anyone looking to reduce their environmental footprint and go green, composting is a great option – but if you’ve heard about compost fumes potentially being toxic to plants, it might put you off.

Why Does Compost Make Fumes?

When you’re creating compost, you tend to be piling up layer after layer of materials which are going to get broken down, mostly by microorganisms.

You are trapping air in the center of the heap, and that means that any gasses released while your compost is breaking down will also be trapped.

If your compost is an enclosed container, this is much more of an issue. Any gasses produced are sealed in along with the compost, rather than being able to disperse freely into the atmosphere.

This is also true if your compost is very compacted and there is little air movement going on in the compost pile.

There are a number of different reasons that your compost might start to be smelling bad or producing nasty fumes.

A healthy compost heap should not really smell (at least, not of much besides damp earth) so it’s important to work out what’s going on and why.

A heap that smells bad is giving off fumes, and you don’t really want that in your garden – aside from any risks to you or your plants, it’s unpleasant and off-putting, especially if it’s bothering your neighbors or guests, or making it hard to enjoy sitting outside.

If your compost heap is giving off unpleasant fumes, something has gone wrong somewhere, so we’re going to explore some of the potential causes and how you can get your compost pile back to full health and efficiency.

Potential Causes

Your Compost Pile Has Gone Anaerobic

That sounds really worrying, but it’s actually fairly easy to solve. If your compost starts to smell rotten and unpleasant, it’s lacking in oxygen: it has gone “anaerobic.”

Basically, there isn’t enough oxygen in the heap for the aerobic microorganisms to survive, so the pile has been claimed by anaerobic ones.

These anaerobic microorganisms still break the compost down, but they produce hydrogen-sulfide, and this smells really unpleasant.

If you get a whiff of rotting eggs when you approach your compost heap, it’s time to deal with it.

You can do so by turning the pile and introducing oxygen to the center. A garden fork is a good way to aerate your compost.

If you are able to build your compost heap on a pallet, you will also increase the airflow.

You shouldn’t ignore a compost heap that has gone anaerobic in the hopes it will sort itself out or just not matter very much.

One of the benefits of aerobic composting at home is that it prevents methane from being produced in large quantities – which is what tends to happen if we put organic materials in landfills.

Methane-producing microbes are only present in non-oxygenated conditions.

If your compost heap is getting weighed down and doesn’t have any oxygen, they will start to form, and will start producing methane.

That’s not good for the planet, and defeats one of the purposes of composting!

It’s unlikely that methane fumes will do any direct harm to your plants in the open space of your garden, however.

It will just disperse into the atmosphere, adding a little to the issues of global warming, but not having a direct effect on your garden.

Purely from the perspective of whether methane will hurt your plants, it probably won’t – unless you’re looking at the very global, very long-term picture, in which case it might.

However, we should all be avoiding adding to methane production, even minimally, so if your compost heap has gone anaerobic, grab your garden fork and get aerating, for the sake of the planet if not your petunias.

You’ve Added Excessive Nitrogen

Compost piles that smell strongly of ammonia probably have too much nitrogen in them.

You increase nitrogen by adding green materials, so if you’ve just piled on the cut grass or lots of leaf clippings, you might notice this happening.

It’s not good for your pile to have too much nitrogen in it. It can make the heap too hot, and causes an unpleasant odor. If possible, you should spread the heap out more thinly to let some of the ammonia disperse.

You could also remove some of the greenery, or turn your compost heap more regularly to try and overcome the issue.

Next, you should be seeking to increase the balance in your heap. If you keep ending up with the scent of ammonia, you’re consistently adding too much greenery. You need more “browns” for your heap.

Shredded card, newspaper, nutshells, sticks, twigs, straw, sawdust, etc., all make good additions that will restore balance.

Again, nitrogen fumes will not directly harm your garden plants.

In fact, many garden plants depend on nitrogen, but before you leave your compost heap to do that work for you, remember that they can’t absorb it from the air.

You don’t want your compost heap producing high amounts of nitrogen, but it’s mostly because it means it isn’t functioning efficiently and it will smell awful.

You don’t need to worry about the effect the fumes will have on your plants – only yourself and your neighbors!

Your Compost Has Got Too Wet

Wet compost heaps also tend to smell unpleasant. If you’re getting too much moisture in your heap, it’s going to start rotting instead of being broken down by worms and microorganisms.

In the spring, you might notice your compost heap smells unpleasant because it’s got wet over the winter.

It might also look slimy or green, and some of the oxygen will have been pressed out of it; the water adds weight to the pile, squashing it down and driving out the air.

To help your compost heap dry out, turn it, and add material like paper, sticks, straw, sawdust, leaves, etc.

These will help to soak up the moisture and will create air pockets inside the compost heap to encourage the aerobic bacteria.

Again, this isn’t going to cause issues with your plants. It might not smell nice, but it’s not going to set the plants in your garden wilting or poison them or anything like that.

Sorting it out will mean your compost pile is operating efficiently again and will reduce the smell, but it doesn’t need to be done for the sake of your plants.

You Have Added The Wrong Things

Most things can go in a compost pile no problem, but you should keep your compost vegetarian at all times

Meat, dairy, fish, and other animal products shouldn’t be in a compost heap. Nor should high-fat foods or oils.

A tiny bit won’t hurt, but on the whole, these things don’t fare well in backyard compost piles. They won’t break down fast, and may introduce bacteria and other things into your heap.

They will also encourage vermin, or even neighborhood dogs or cats to start digging around in the pile.

You should also try and keep a balance and avoid adding huge amounts of any one food or plant type.

Your compost pile is full of little worms which help with the decomposition process, and if you overwhelm them with any one thing, they may not thrive.

They struggle to handle things like citrus and onion skins and eggshells, etc., so don’t add large amounts of these.

Don’t spend hours laboring over what quantities or which foods to add: keep it simple, and avoid animal or high-fat products, or huge amounts of any one food group.

Once more, adding the wrong things to your compost pile might cause unpleasant odors and stop it from functioning effectively, but it shouldn’t harm your plants.

Don’t worry too much about whether or not the fish skins you accidentally threw in there last week are going to cause your roses to die; they won’t!

How Might Compost Fumes Harm Plants?

Overall, then, it seems that compost fumes aren’t very likely to harm your plants while the compost is still safe in your compost pile.

The fumes aren’t very strong and even if they blow over your plants, they shouldn’t do them any harm.

Plants that are growing right beside the compost heap should survive perfectly well, even if they are subjected to the odd unusual scent, a dose of high nitrogen, or even a little bit of built-up methane.

Since plants get most of their nutrients from the soil and the sun, the air quality won’t affect them massively.

It is thought that very high levels of pollutants or strong gasses might hinder their growth or reduce the functioning of the leaves, but you probably couldn’t produce this result from a backyard compost heap even if you tried!

However, there are a few things that you should watch out for in terms of your compost routine and your plants, because your compost bin could potentially pose other dangers to your garden greenery.

Fungal Spores

This is probably the closest “fume” that might come from your compost heap to harm your plants, except that of course, it isn’t a fume at all.

Fungal spores have occasionally picked up media attention as a danger posed by compost heaps, albeit a very rare one.

A healthy compost heap shouldn’t be producing fungal spores, but it does happen sometimes – especially if you have put diseased plants in there. After all, the fungi do not magically disappear as a result of being moved to the compost bin.

Dealing with mold and fungi when it comes to composting is tricky, and there is a lot of different advice out there.

Some people would say you should never add any infected crop to a compost heap, because if the infection isn’t killed, you could massively spread it when you use the compost.

That’s true, so you need to know what you’re doing and do your research.

Anything which is moldy or has got a disease such as powdery mildew or black spot can cause problems if you add it to your compost pile.

Diseases can lie dormant for surprising amounts of time, and might impact on next year’s crop or plants.

However, surprisingly, that isn’t to say that you can’t put these things in your compost heap, but that you should do so with a good foundation of understanding.

Any gardener will probably have to deal with diseased plants or moldy leaves at some point, and knowing what to do can be hard. You have a couple of choices.

You can burn them. This will certainly kill off the disease and you can then add the cold ash to the compost heap, safe in the knowledge that it won’t be spreading infection to your other plants.

However, this takes more work and may not be legal in your area, or you may not have the space to safely do so.

Alternatively, you can compost them if you know that your compost pile reaches high enough temperatures to kill off the disease.

If you want to compost a diseased plant, research the disease and the specific conditions needed to kill it, and then make sure you are able to meet these conditions.

If you aren’t, you might end up spreading the disease all over your garden when you come to use the compost next year.

Even if you don’t plan to use the compost, if the spores thrive in the heap, they may spread themselves, and you could find that a strong wind is blowing them all over your garden and other plants.

This is probably your biggest risk when it comes to compost heaps and what they could spread to surrounding plants.

Fungal spores are also dangerous to people, so handle them with care, especially if your heap is contained and will result in concentrated spores.

If you know you’ve got fungal spores in your compost pile, do thorough research before you start dealing with them so you know how to approach each disease.

Choose a still day to minimize the wind spreading the spores around.

Wear a mask and protective clothing such as gloves when you’re dealing with diseased plants, and always wash your hands thoroughly when you’ve finished.

If you notice any ill-effects, seek medical attention promptly.

Fungal infections are manageable, but should be treated with extreme caution as they have occasionally resulted in fatalities in the past.

Take care of yourself and your plants, and avoid introducing diseased matter to your compost heap unless you know it is capable of managing it.

If you are unsure about your ability to control the disease, it’s safer to put the infected plants into landfill. This may not be ideal, but it’s better than the alternative risks.

Don’t Put Half-Composted Compost On Plants

This is not about fumes spreading from a compost heap, but maybe useful to understand anyway. If you’re new to composting, the idea of compost “maturity” might seem an odd one.

However, if you’re just getting going, how do you know when your compost is ready?

Don’t go adding compost that is only partially decomposed to your flowerbeds or vegetable garden.

It might seem like it will just carry on composting out in the open, but in fact, this sort of addition can cause problems for your plants – even though they will love fully matured compost!

Immature compost can cause chemical burns on your plants because it is still breaking down and reacting.

It has different microorganisms into mature compost, and is still “active” in ways that mature compost isn’t.

Immature compost may introduce bacteria that you don’t want near your flowers, and it might also leach nitrogen or other nutrients out of the soil as it continues to decompose – stealing resources which your plants need, rather than offering resources to help them grow.

Compost should be assisting your garden, not stealing from it, so you need to make sure that you don’t use it until it is mature.

How can you tell if compost is mature? There isn’t a given time period, as it will depend on how active your heap is and what you put in it.

When your compost starts to look and feel like earth, it’s getting there. It should smell of soil, not vegetables or plant-matter, and you shouldn’t be able to distinguish any leftover food or garden waste.

Think about the compost you get from the store.

If your compost doesn’t look like this, but still resembles a mix of all the things you put in, it needs more time and you should put it back in the compost pile.

In a pinch, you can actually use compost that is close to maturity near well-established plants, using a simple trick called side dressing.

Side dressing involves adding a ring of compost to the area around your plant, but not actually touching it to the plant itself.

This will avoid any issues of the compost burning the plant, and still make the nutrients available to the plant.

It can also help reduce the spread of weeds by providing a circle in which they won’t grow – at least until the compost has stopped “burning” plants.

Don’t Plant Seedlings In Fresh Compost

Even if your compost is fully decomposed and free from any spores or diseases, you shouldn’t plant baby plants in it.

They will struggle to survive and it’s very easy for homemade compost to accidentally introduce other problems.

You’re best to start seedlings off in a sterile compost mixture from a garden center

This will ensure there are no bugs or bad microbes which could hurt the plants, and will minimize the chance of an infection occurring.

If you can’t get sterilized compost from a garden center, you can sterilize your homemade compost by putting a small amount in the microwave and blasting it for a few minutes (with breaks to allow the heat to distribute itself).

However, this will kill good bacteria as well as bad, and isn’t an ideal solution.

Again, seedlings won’t struggle with the fumes produced by compost – fresh or otherwise – but may suffer from the bacteria and other organisms contained within it.

It’s best to give baby plants an easy start, and set them off using sterile conditions so they have the optimal growing conditions.

How Can I Contain Fumes From My Compost Pile?

If you are still concerned about your compost pile’s smell or fume production, you can buy compost bins with lids. This will contain any fumes or gasses which the compost releases as it decomposes.

However, bear in mind that those gasses will still exist – just under a lid. This means that when you take the lid off, you’ll face much more concentrated amounts, which may not be ideal.

It also makes it a bit harder to see what’s going on in the compost bin, and can make aerating it tricky.

On the other hand, a lidded compost bin can help to contain smells and reduce the amount of rain getting to your compost, so it has pros as well as cons, and if you really want to trap the gasses produced by your compost heap, this is the way forward.

Just remember that healthy compost shouldn’t smell bad and if it does, you need to take some action!

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