Can I Compost Onions: Composting Tips For Gardeners

Can I Compost Onions: Composting Tips For Gardeners

“Can I compost onions” is a question that is often asked by new gardeners or those who are just starting to learn about the composting process. The answer to this question is not straightforward for a number of different reasons which I’m going to explain in this article.

Essentially, as onions are organic, they should be fine for composting but there are certain considerations that you need to think about first. 

Why Is The Composting Of Onions Subjective?

pile of onion scraps

If you spent a little time researching whether you can compost onions, you’re likely to come across conflicting advice and here’s why.

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Onions contain a high level of sulfur compounds and these can slow down the composting process. Plus, some parts of the onion are better for composting than others.

Onions are also quite acidic and this could disrupt the pH balance of your compost and mean that the whole process slows down. Of course, there’s also that pungent odor that onions give off which is even worse as they start to decompose if you add them to a traditional compost pile.

In saying that, onions make up part of your daily food scraps and so the short answer is that you should be able to add them to your compost. You just need to choose the right composting method like using a compost bin with a tight-fitting lid or a bokashi bucket in your kitchen.

Let’s first look at the benefits of adding onions to your compost.

Interesting fact: Did you know that onions are one of the oldest vegetables in cultivation? In fact, evidence has been found of them being cultivated over 5000 years ago!

Benefits Of Adding Onions To Your Compost Heap

vegetable scraps in compost with hand holding some of the ready compost
  • Onions are a good source of nitrogen. As all gardeners know, nitrogen is an essential nutrient for all plant growth. Therefore, onions will amplify the nitrogen content in your compost.
  • Generally, onions will break down fairly quickly which is what we want in our compost piles or bins.
  • Being slightly acidic, onions can help to balance the pH level of your compost especially if you add lots of brown materials such as wood chips, straw and dry leaves.
  • Onions are excellent for repelling insect pests. This will keep your compost free from insects that you don’t really want in there.

What Are The Best Ways To Add Onions To Your Compost?

chopped red onions on a board

Now that you understand the benefits of adding onions to your compost and why their addition can sometimes be subjective, here are some tips for the best ways to compost onions.

  • Make sure your onions are cut into small pieces to speed up their decomposition.
  • Add your onion scraps as part of a balanced composting diet. This should include vegetable scraps, dry leaves, grass clippings, coffee grounds, eggshells and other plant material, I generally just throw my onion scraps into the bucket I have in the kitchen to collect kitchen scraps.
  • You can safely add moldy onions to your compost as they have already started the decomposition process.
  • Avoid adding diseased onions to your compost as backyard compost generally doesn’t get hot enough to kill off the pathogens that are responsible for the disease such as leaf blight or white rot. This means that you could be spreading the disease around your garden when you use the compost.
  • Avoid adding a lot of onions to your compost as this will increase the acidity to a high level. If you do have loads of onion scraps, you can balance this acidity by adding in some crushed eggshells.
  • It’s generally not a good idea to add cooked onions because the cooking process means that they’ve been cooked in oils or fats and these should not be added to your compost.

Should You Add Onions To Your Worm Farm?

hand in blue glove holding soil and worms

While it’s fine to add onions to a regular compost bin as long as you add a balanced mixture of other brown and green material, you should avoid putting them into a worm bin that you use for vermiculture.

You see, as you know, onions do have quite a strong smell and flavor and worms don’t really like this. Therefore, the worms in your bin may avoid the onion scraps and so these will take longer to break down.

Plus, the contents of a vermiculture bin should have a fairly neutral pH and it’s likely that the onions will add too much acidity.

In saying all of that, a very small amount of onion scraps that are cut into small pieces and added in combination with other food scraps won’t harm the worms in your worm bin.

The Best Way To Compost Onions Is To Use A Bokashi Bin

hands holding bowl full of food scraps ready to tip into a bokashi bin

By now you should be familiar with the bokashi method of composting. I wrote an entire article about it that you can read if you still don’t know what it is.

Putting onions along with other kitchen food scraps into your bokashi bucket is a great way to eliminate the strong odor that onions often give off. The bokashi composting method is anaerobic and uses fermenting to break down all food waste.

Make sure you cut your onions into small pieces before putting them into the bokashi bin in order to help speed up the composting process.

Use An Electric Composter Instead

Another alternative that you might like to consider is the use of an electric composter that is designed to live in your kitchen. These ingenious devices are designed primarily for people who live in apartments or similar small spaces and simply don’t have the space for a regular compost bin.

However, as we become more and more focused on sustainability and the need to reduce our waste, I believe these units will become more mainstream and a necessary addition to every kitchen.

You can read more about them in the article I wrote on composting in an apartment.

Frequently Asked Questions

Is it possible to throw whole onions into your compost?

In general, it’s not advisable to add an entire onion to your compost as it will take far too long to break down and might even resprout. It’s always a good idea to cut your onions into small pieces before throwing them into the compost.

Can you throw onion skins into the compost?

Onion skins or onion peelings will add the necessary carbon to your nutrient-rich compost as they are part of the brown materials that you should add to balance your compost. This is an added benefit of adding onion scraps to compost.

Can you add spring onions to your compost?

Yes, you can treat spring onions and red onions the same as other types of onions when adding them to your compost.

Interesting fact: You can make a fertilizer tea from onion peels by placing a handful into a litre of water and leaving this sit for three to four days. You can then dilute that onion water with clean water in equal parts and use it to water your plants. It will be rich in potassium, iron and calcium and provide an excellent tonic.

Final Thoughts

Now you know that you can compost onions as long as you follow a few simple tips. Always cut your onions into smaller pieces and mix them with a range of other materials before adding them to your compost.

Using a compost bin with a tight-fitting lid rather than a regular compost pile is ideal because it will eliminate any nasty odors as everything starts to break down. You could also consider getting a bokashi bin to compost your onions and other veggie scraps or one of those innovative electric composters that even look great on your kitchen bench.

If you’re into vermiculture, you should probably avoid feeding onions to your worms because they’re not really going to like it. However, a small amount of onion scraps mixed with other organic waste won’t harm the worms.

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