Can You Compost Onions?

Can You Compost Onions?

Onions are a kitchen staple!  In fact, onions are the 3rd largest fresh vegetable grown in the U.S.  No surprise, these antioxidant rich bulbs have been cultivated worldwide for more than a  thousand years. If you are stressing about tossing onion scraps into your compost bin, don’t!

Can You Compost Onions?

Yes! Every single part of an onion is totally compostable! You can even compost rotten and sprouting onions!  You may have been told otherwise, and yes, there are a few things to look out for, but no worries, onions can make their way to your compost bin, just like your other kitchen scraps! Onion’s strong smell, common for all Allium plants, even garlic, can even be a bonus for some compost systems!

Can You Compost Onion Peels?

Onion peels are the easiest and fastest part of garlic to break down. Onion peels are considered to be ‘brown matter’, which is an essential component in a compost system. The paper-like outer covering of an onion is so light and delicate that it will rapidly break down in any composting system, even a worm bin.  No need to worry about that oniony smell because onion peels rarely take on that pungent odor that onions are well known for.

Can You Compost Onion Skin?

As we work our way through the layers of an onion, the thin but juicy onion skin is quite often tossed before we get to the usable part. Onion skins come  with a strong smell once peeled away from the bulb.  No worries, you can even compost the onion skins in most systems. They contain much more moisture than the papery outer peels, but go ahead and toss them in with the rest of your kitchen scraps that are headed for your compost bin. The onion skins are ‘green matter’, adding nitrogen to your compost bin. 

Do you have a vermicomposter, aka worm bin?  Here is where it gets a bit tricky.  Our underground friends, who are hard at work breaking down our unwanted scraps, may be a bit offended by the strong smell of onions.  Best approach is to do a test run and simply toss a small amount into the corner of your bin. If they shy away from the onion skin, remove it, and avoid adding onions into your worm bin. No need to irritate your worms, toss scrap onion skins into your open compost bin, closed tumbler and as a last resort, into the trash.


Can You Compost a Whole Onion?

Whole onion, chopped onion, rotten onion, whatever form your onion is in, you can compost it. Only difference is the size of your kitchen scraps does affect how fast your compost will break down.

If you found a whole onion that is less than desirable, it may be wise to chop it up into smaller pieces before tossing it into your compost bin.  If not, tuck that whole onion as far into the composter as possible, where the heat and microbes, who are hard at work, will welcome a big old piece of organic matter.

Can You Compost Rotten Onion?

Onion in its many forms, even rotten onions,  can be composted! It happens. You know that smell of rotten onion from a mile away, and there it is, buried beneath other onions or tucked in with your potatoes.  Now what?  All good, it can be composted and come back as amazing fertilizer for your garden.  Rotten, stinky, fresh, or not, it is essential organic matter that can be added into the pile, and since it is already rotten, it has a head start on the process!  It is already teaming with microbes and breaking down before your eyes.  Toss it in there and watch nature do her magic.

As for your worm bin, most likely they will not be fans of a big stinky onion in their workspace.  Go ahead and avoid it!  No need to disrupt their efforts, plus if they don’t eat it, you will have to smell it for a few weeks.  Not worth it!

Hint:  you may want to either chop it up, if it isn’t too nasty and slimy, or tuck it way down inside of your compost bin.  Remember, the size of your scraps is one factor in how quickly things break down in your compost pile.

Can You Compost Cooked Onion?

Yes, you can generally compost any fruit or veggie that has been cooked, unless it is heavily soaked in oils or butter, or combined with animal products like dairy and meat. Poultry, fish, meat, dairy and all other animal products should never go into your compost bin. Due to the high protein and fat content of meat and dairy, they may break down at drastically different rates than other scraps in a compost bin.  This may throw off the natural balance, and could potentially attract unwanted scavengers and pests.

How To Compost Onions

The topic of whether or not to compost onions may come with varying opinions, but the fact is that onions are no different  than your other kitchen scraps.  Keep in mind, if using a worm bin, the strong sulfur rich smell of decaying onion can be offensive to your worms.  Give it a test run, see if your worms go for it, and if not, just remove the onion scraps and carry on!

On a positive note, onions can be a great pest deterrent in larger open and closed compost systems.  No need to worry about nighttime visitors snacking on your compost pile when you  toss some stinky onions in there.

It really is simple. Toss unwanted onion peels, skins, raw onion,  and even whole rotten  and sprouting bulbs into your compost with the rest of your kitchen scraps. If you are regularly turning your compost pile, you should notice what is breaking down and what isn’t. If you notice whole bulbs sprouting or onion scraps not breaking down, push them way down inside of your pile and let the heat and microbes take care of business.  As always, the size of your scraps will factor into how long it takes your waste to break down. Chopping up larger pieces may speed up the process and avoid having to pick out pieces that didn’t fully decompose, when finishing up your compost.

How Long Does It Take For Onions To Decompose In A Compost Pile?

This depends on a few factors…what form were your onion scraps in, what kind of compost system are you using, what is the core temperature of your composter, what season is it, what ratio of brown to green matter is in your system, how often do you turn your compost, and where is it, indoors or outdoors?  Onions are not fibrous, they have high water content and they may bring balance to your dry matter, otherwise known as ‘brown matter. With a healthy balance of 4 parts brown matter to 1 part green matter, your compost bin will break down in optimal time.

Typically, compost can take anywhere from 3 weeks to 1 year to be ready to use.  If you use your compost system correctly, add well chopped kitchen scraps and plant matter, and tend to it regularly, you can speed up your composts’  natural rate by turning it, adding water and even insulating it in the cooler months.

How To Use Onion Compost

Once your compost is ready to use, there should be no signs of any of the original plants, even those stinky onion scraps that went into making it. Aged compost should have a sweet earthy smell, and be dark and crumbly to the touch. There should be no identifiable pungent smell, even from the onions that you tossed in there months ago.

As long as your compost is well aged and ready to go, it is safe to apply directly to your veggies, fruits, herbs, flowers, trees, shrubs, and even houseplants. You can store your aged compost outside, under a tarp, or even in a closed bin.  Make sure that you keep it slightly moist and use it within 3-4 months.  Compost loses nutrients over time, but no matter what, it is always a welcomed addition to your garden.

Can Onion Compost Be Used As A Fertilizer?

Absolutely! Finished compost, even compost that has had onions in it,  can be added to almost any plant.  Incorporating compost into your garden soil before planting is essential to growing a spectacular  organic garden. You can also add a handful of compost into planting holes, top off plants with a handful, mix it into potting soil for your houseplants, and even make a compost tea! As long as your compost is fully finished, it is safe to use in almost any application!

Other Onion Recycling Tips

While it is nice to add in fresh green and even brown matter from your own kitchen, there are ways to reuse those fresh veggie scraps, especially onions. Onions provide amazing flavor, and are a key ingredient in veggie broths.  Why not save those onion scraps and make amazing homemade veggie broth?  It is easy, cheap and super healthy!

Simply keep a bag or container in your freezer and toss in leftover onion skins, onion bits, green onion tops, and other nutrient dense scraps, like broccoli stalks, garlic, parsley stems, celery tops, bits of carrots, and cabbage cores.  Once you have about 4 cups of scraps, place them in a large pot, cover with water, add in a teaspoon of salt, and bring to a boil.  Turn heat down to low and simmer for 1 hour, adding water as needed to keep the scraps covered.  Cool and flavor to your liking.  You just made homemade veggie broth, well done!

Now you have a pot of mushy, cooked veggie scraps and they are more than ready to head to the compost bin.  Go ahead and toss them right in there!

Worm Composting With Onions?

Worms are an essential component of soil health.As worms make their way through the soil, they break down organic matter, and leave behind ‘worm castings’, nutrient rich food for our plants and soil. If you are a worm bin parent, you probably know that your worms are not fans of everything you toss into the bin.

Most Alliums, like onions and their cousin, garlic, may not be welcomed additions to a worm bin. The strong sulfur emission from onions could potentially deter your worms from enjoying their job, and thus leaving you with rotting kitchen scraps and not the plant food that you were aiming for. The best way to know is to test your worms’ reaction to onions by only adding small amounts of onion into your worm bin.  If they don’t seem to mind, then go ahead and keep adding it.  If it goes untouched, then remove it and simply toss your onion scraps into another compost bin!  Remember, red worms feed at the soil surface.  If they are not happy, they will retreat deeper into the bin, avoiding the smell and taste of your kitchen scraps.  Paying attention to your worms’ feeding habits is the perfect indicator to what they like, and what you should avoid adding.

Short Conclusion

So, there you have it, yes you can compost onions!  If there is a way to keep fresh green matter out of our landfills, composting at home is definitely one of the best!  Homemade compost is an amazing addition to your garden, cheaper and safer than buying compost, and you know what went into making it, even stinky onions!

Composting can be confusing, but it doesn't have to be.  Want to learn how to compost more than just onions, check out our post... 11 Things In The Kitchen That You Didn't Know You Could Compost