Can You Compost Onions – Is it Okay?

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In Brief: Onions in Compost – Is it Okay?

Onions are among the vegetables that man has known about the longest. They are a part of almost every meal we eat, but are they okay to add to your compost heap? The quick answer is yes, they can definitely be added to your compost pile! But you need to know the correct way to do so. This article explains how to integrate them into your compost heap to generate healthy, rich compost for your garden.

Given their tendency to regrow and acidic nature, some may wonder whether it would be appropriate to add onions to a compost.

Short answer, yes! Compost is alkaline when onions are added in moderation, and their acidic nature will be neutralized.

This article will provide factual information on this topic.

Benefits of Putting Onions in Your Compost

Onions are a fantastic addition to your compost.

You get to enjoy many benefits they have to offer. These benefits are as follows:

  1. They introduce organic matter necessary to feed the microorganisms responsible for the decomposition process
  2. Help balance the pH levels of your compost and, eventually, garden soil
  3. Introduces nutrients great for plant growth to you compost
  4. Improves the soils water retention, thus eliminating the need for constant watering
  5. Slowly releases nutrients to the ground avoiding leaching
  6. Enhance drainage and aeration in clay soil

Precautions while Putting Onions in Your Compost

Can You Compost Onions

Although we have stated it is okay to add onions to your compost, there are specific issues to consider.

Avoid cooked onions

While some videos and websites claim cooked onions rot faster, we strongly advise against adding them to your compost.

Given that you heat cooked onions with fat, they are not likely to decompose.

Compost heaps have a hard time decomposing fatty or greasy materials. These onions are also likely to spread the fat to other materials inhibiting them from composting.

Also, since cooked onions have a sweet aroma, you risk attracting pests and rodents to your compost. They cause quite a mess to your pile.

Do not add diseased onions

In general, any diseased plants should not be introduced to your compost.

In case your onion has been attacked by pathogens like leaf blight or white rot, you should consider other means of disposal.

When added to your compost, the pathogens in diseased plants may survive the decomposition and be introduced to your soil.

Alternatively, you can burn the onions and add the ash to your compost. High temperatures are known to denature pathogenic microorganisms.

It would help if you did not put too many onions together

Avoid concentrating onions in one area. Onions have a very pungent smell, which can be overwhelming when too many are focused on one place.

Chop the onion into smaller pieces

If you add a whole onion to your compost, the vegetable will sprout.

Therefore, you should ensure you chop the onion into smaller pieces. Slicing the onions into smaller pieces will also help them decompose faster.

How to Compost Onion

As stated earlier, it is possible to compost onions.

The first step is slicing the onion into smaller pieces. Chopping them increases their surface area and promotes faster decomposition.

When you add the onions to your compost, bury them at least 10 inches deep since they smell bad when rotting.

Due to their acidic nature, you are advised not to add many onions to your compost. For the case of onion skins, mix them with the other compost material.

By combining them with other materials in the compost adds moisture to them needed for decomposition.


Can I compost moldy onions?

You absolutely can. In case you have some onions in your kitchen that have grown moldy, perhaps due to insufficient storage, you will significantly benefit from adding it to your compost heap. Given mold is a fungus, adding moldy onions to your pile introduces fungi, a crucial microorganism for decay, which decomposes organic matter.

How long can I store onions?

Under the right storage conditions, onions may last up to 6 months. It is recommended to store them in a dry place away from sunlight. However, it is not unusual to find one or more of the onions trying to sprout or beginning to decay after a long period despite proper storage.

Do worms eat onions?

Worms do not eat fresh onions. When added to the compost heap, worms keep off the onions due to the smell. No need to worry since the odor fades away when the decaying process is complete. Once the onions reach a certain level of decomposition, the worms are ready to eat them. For this reason, slice your onion into smaller pieces for faster decomposition. In conclusion, worms do eat onions, though not when fresh.


Though several websites advise against adding onions to your compost, we have illustrated that it is okay to do so. Onions are acidic, which requires them to be used in moderation. It would help if you did not concentrate the onions in one are due to the pungent odor they emit when rotting.

For faster decomposition, slice the onions into smaller pieces before adding them to your compost.