Can You Compost Onions? (Yes, Here’s How)

Can You Compost Onions

Reading Time: 9 minutes 🍃

Many composting resources recommend avoiding composting pungent, acidic, or spicy plants. Why is this, and do onions fall into this category, or can you safely compost onions in your compost system?

Onions can be composted. Every part of the onion, from the leaves to the skins and the onion bulb, can be successfully composted, provided the right composting method is selected and the onion is properly prepared before composting. Hot composting is the best composting method for onions.

Onions are organic material, not animal by-products, so they should be compostable. Onions can be composted, but some care is required regarding the parts of the onion that can be composted and the composting methods you use.

Can You Put Onions In Your Compost?

Many people overlook the benefits of adding onions to compost because they assume this vegetable may cause problems for their composting system.

Pretty much any plant-based organic material is suitable for composting, but some items require specific composting methods or treatment before composting before they can be added you your compost pile.

If you grow onions in your garden, it is not only the onion root or bulb that you may be interested in composting. There is organic waste left over from the plant when the growing season is over, and onion skins are removed before onions are prepared for cooking.

The main reason onions are often put on the exclusion list for items that can be composted is their acid content. However, onions can be composted if the right methods are used and if the onion is prepared correctly.

An onion plant is made up of the root, which we eat, an outer papery skin layer, and leaves that appear above the ground as the onion grows.

What part of the onion can be composted, and what are the most suitable composting methods for these items?

Can You Compost Onion Greens?

Onion greens are the green leaves of the onion plant that grow above the ground in your garden. Many onion growers harvest some of these green leaves for use in salads and other dishes where the mild onion flavor adds new dimensions to the taste.

If you do not harvest these leaves, they are generally left on the plant till the end of the growing season, when they begin to brown and die. It is usually at this time that the onion root is harvested.

The leaves are usually trimmed from the onion root before the onions are stored or packaged for sale.

The acid content in the onion leaves is very low, and the onion flavor in the leaves is very mild. Green onion leaves can be composted in hot or cold composting methods, but they should be added to a vermicomposting system with caution.

The onion flavor in the green leaves may be too strong for the worms to find palatable, and they may not eat the leaves.

Some composting worms will eat green onion leaves, while others will not. You can perform a simple test to determine whether your composting works will eat the green leaves.

Add a small about of green onion leaves in a specific location in the worm bin. Check the worm bin after a day or so to see if the worms have eaten the leaves.

If the worms have eaten the surrounding organic material but have avoided the onion leaves, they do not like them. Remove the onion leaves from the worm composting bin and put them in your other composting systems.

If the leaves are old and have turned brown, much of the distasteful flavor has disappeared, and they can be offered to the worms in your vermicomposting system.

The worms will readily eat the dried-out, brown onion leaves and recycle them into valuable nutrient-dense compost!

Check out: Composting Rhubarb Leaves

Can You Compost Onion Skins?

Onions bulbs have a papery outer layer that is often removed before onions are packaged for sale to the public. However, if you purchase onions from a farmers market or grow your own onions, they will come with a brown, dry skin layer around the onion bulb.

This papery skin on the onion is an organic material and is perfect for adding to your composting systems. 

The skin has very little onion flavor and is not acidic enough to cause any problems with the pH of your compost pile.

You can add these skins to any composting system, including worm farms. The worms will not be adversely affected by the skins and will gladly consume them and turn them into compost.

Brown onion skins are also low in moisture, which will not affect the moisture balance in your compost pile. 

If you use the layering composting technique, layering green and brown organic material, these skins would be considered part of the brown or carbon layer for the pile. 

Can You Compost Onion Bulbs?

Onion bulbs, or onion roots, are the part of the onion that we most commonly eat. This part of the onion is also the most acidic and pungent part of the plant. Can the onion bulb be composted?

There are several reasons why many people recommend that you don’t put onions in your compost.

  • Onions are acidic. The concern here is that the onions’ acidity will affect the compost pile’s pH and make the environment too acidic. This can cause the good microbes and bacteria responsible for decomposition to die off. The result will be a slowing down in the decomposition rate in your composting system.
  • Onions smell when they decompose. If the onions take too long to decompose, they begin to stink as they rot. The smell of rotting onions may attract unwanted pests and pets to the compost pile. 
  • Certain composting methods are not suitable for composting onions. Some composting methods are more effective than others at decomposing onion waste. 
  • Whole onions may begin to grow in the compost. Compost piles offer a warm, moist environment that can encourage whole onions to begin to sprout and grow in the compost.

While these arguments have some validity, most can be overcome relatively easily, allowing onions to be composted safely in your normal compost system.

1. Add Onions To Your Compost In Moderation

Adding small quantities of onions to your compost pile is unlikely to dramatically affect the pH balance in your compost pile.

Normal household quantities of onion scraps from your kitchen are insufficient to add enough acidity to harm the microbes and slow down the composting process.

However, if an entire bag of onions has gone bad, it may not be a good idea to compost the entire bag in a single compost pile.

Large quantities of onions added simultaneously can adversely affect your compost pile, and you may see some of the problems we have mentioned begin to take effect.

2. Bury the Onion In The Compost Pile

Burying the onion scraps in the compost pile rather than throwing them on top of the pile will help to mask the smell as the onion decomposes.

This strategy will limit the attraction of pests, pets, and wildlife to investigate the aroma emanating from your compost pile.

Burying the onions will also place the onion bits in warmer parts of the compost pile, accelerating the decomposition process.

3. Use The Right Composting Method

Certain composting methods are more effective at breaking onions down faster than others. Some composting methods are more susceptible to the problems associated with composting onions than others.

We will discuss the best composting methods for onions and which methods are less suitable for these pungent vegetables below.

4. Chop The Onions Up Before Composting

Onion bulbs tend to grow when placed whole in a compost pile. The warm, moist conditions in the compost stimulate the bub to sprout leaves and produce roots.

If you put whole onions in your compost, you may find the onions have started to grow in the compost rather than decompose.

The only method to avoid this is to chop the onions before adding them to your compost system. One of the easiest methods to do this if you do not have a chipper is to put the onions in a kitchen blender and blitz them a few times to chop them up into smaller pieces.

A similar problem can arise if you cut the root end off the onion and throw it in the compost. The roots will begin to grow in the compost medium, and the part of the onion flesh connected to the roots will sprout green leaves.

You can prevent this by blitzing in the blender, but cutting the root section into quarters is easier. The root section will not sprout if enough onion flesh is not connected to it to provide energy for sprouting.

Chopping the onions up into smaller chunks also speeds up the onion composting process and reduces the chances of the onion starting to rot and attracting unwanted pests.

When Shouldn’t You Compost Onions?

There are certain situations where you should not add onions to compost, but many of these exclusions apply to other organic vegetable matter as well, not only onions.

Never compost diseased onions in your compost system. Many parasitic infestations, bacterial infections, and fungi that can attack onion plants will not be killed or eliminated by composting methods.

If the disease is not neutralized in the composting process, it can contaminate the resulting compost, and you can spread the disease or infestations to other plants in your garden.

The only way to effectively neutralize these disease-causing issues is to use hot composting methods. 

The process must be carefully monitored to ensure the temperatures in the compost pile reach the levels necessary to kill these microbes. The minimum required temperature is 135°F or 57°C.

Don’t compost cooked onions. Onions absorb oils and fats during the cooking process, especially if you are frying the onions with oil or with meat.

Oil and grease are very slow to break down in compost and will significantly increase the time taken for the onion to decompose. The oil on the onions may also spread to other materials in the compost pile and slow their decomposition, slowing the overall process.

Should You Compost Sprouted Onions?

If you have onions stored in your kitchen, you may find from time to time that one or two of the onions have started sprouting green leaves.

Can you compost these onions? It is possible to compost onions that have started sprouting, but you must chop them up to prevent them from growing in the compost.

There is also nothing wrong with using sprouted onions in your cooking. Cut off the sprouting end and use the rest of the onion as you normally would.

Best Methods For Composting Onions

There are many different composting methods, each with its own strong points and limitations. All composting methods can be used to compost onions, but there are limitations and considerations for each type when including onions.

The following composting methods are the best methods to use when composting onions.

  • Hot composting. Hot composting is one of the fastest composting methods and can be sued to decompose a wider range of items that are challenging for other composting techniques.
  • In-vessel composting. This composting method uses sealed containers, which gives ultimate control over airflow, moisture, temperature, and pest access to the compost. The container is mechanically turned to aerate the compost. This composting method can compost any organic waste, including meat and onions. This is not the same as tumbler composting.
  • Bokashi composting. Compared to traditional composting methods, Bokashi composting uses different enzymes and bacteria to decompose organic material. The process is more fermentation than decomposition and can be used to compost onions effectively.
  • Compost appliances. These machines are a relatively new creation and can compost kitchen waste in a matter of hours. Compost appliances are countertop machines driven by electricity, the grind and compost kitchen waste, including onions, with ease.

Composting Methods Less Suitable For Onions

The composting methods less suited for composting onions will work, but the process is slower and more prone to problems when onions, particularly the onion bulb, are added to the mix.

  • Cold composting. Cols composting relies solely on bacteria and micro-organisms and less on heat. Adding too much acidic material can adversely affect the ecosystem in these compost systems.
  • Vermicomposting. Worms will eat onion waste if they have no other option. If there is other plant material in the worm bin, they will eat it first and leave the onion until last. This increases the potential for the onion to rot and causes issues in the system.
  • Compost tumblers. Compost tumblers are designed to be used with certain organic vegetation that will decompose quickly. Adding onion waste to this system can slow the process and attract flies and other pests to the compost tumbler. 

Final Thoughts

Onions can be composted, but the method by which the onions are composted should be carefully considered to optimize the composting process.

Almost any compost method can be sued to compost onions, but certain methods will work more efficiently than others, and some processes may be compromised when onions are added.


Was this article helpful?

Team Leafy

Hi! Thanks for reading our article, we hope you enjoyed it and helps make your garden grow greener. If you found this article helpful, please share it with a friend and spread the joy. Plant small. Grow big!

Recent Posts