Can You Compost Onions? Is it a Good Idea?

Starting your own compost pile is one of the best things you can do for your garden. You’ll always have need of compost as all-natural fertilizer or as a soil amendment, but learning how to properly start and manage a pile is something of a skill in itself.

Knowing the right mix of green and brown items to add, and when, is fundamental to keeping it going and growing. But not everything that comes out of our kitchen, even food scraps, is suitable for our compost pile.

How about onions? Is it a good idea to compost onions?

Yes, onions can be safely composted and can be good for your compost pile, but they should be chopped up and added with alkaline items to offset their acidity.

If you’ve ever heard seasoned gardeners warn about adding onions to a compost pile, there’s a good reason for that: if you do it carelessly or don’t prepare them properly it is indeed possible to mess up your compost pile.

However, assertions that they will completely wreck it are generally overblown. Done right, onions can make a fine addition to your pile. Keep reading and I’ll tell you all about it…

Onions Can Be Composted But with Some Prerequisites

To be perfectly clear: it’s totally safe to put onions in your compost pile. Onions of any color, be they red, white, yellow, or otherwise.

Onions are organic matter the same as any other vegetable, and will contribute to giving you that rich, super-nutritious compost that you’re going for. There isn’t anything about them that is overtly harmful to your compost.

But, just like most things, you need to know when to add onions and what to do to offset or balance any negative effects. That’s all there is to it.

I’ll tell you what you need to do in the following sections so you don’t accidentally impede the progress of your compost pile when adding onions to it…

Onions are Acidic, So Add Alkaline Material to Balance it Out

The first thing you need to know about adding onions to your pile is that they’re highly acidic.

This isn’t bad enough itself, but considering that our compost pile generally needs to be alkaline throughout, adding too many onions or adding them carelessly can it disrupt that pH balance, pushing us closer to neutral and then acidic.

When that happens, our compost pile will fail.

The good news is that all you need to do to offset this acidic addition in the form of your onions is to include other alkaline materials when you add them.

Two of the very best, and most readily available, in the form of eggshells and chalk. Tossing either of these in at the same time, or any other alkaline additives, will keep that acidity from throwing your pile out of whack.

Never Add Whole or Half Onions to Your Compost Pile

Any onions that you add to your compost pile must be absolutely crushed or chopped up. Never throw in whole onions, and don’t include half onions either. Why?

Those mostly intact onions will readily sprout in the nutrient-rich conditions found in your compost pile. They can even do this if they are resting mostly on the surface!

Is that such a problem? Yes, because this will slow down the process of your compost bin, and even more worryingly, if you leave them like that you could be scattering growing onions in other places throughout your garden and around your home.

Do you want onions growing in your flower bed? Of course not!

You can avoid this happening if you just chop up the onions or, lacking that, crushing them completely flat.

As always, I recommend reducing the onions down to the smallest pieces possible because this will help the decomposition process begin and complete quicker.

You’ll know you messed up if you see those bright green sprouts poking out of your compost pile. Don’t say I didn’t warn you!

Onions Can Attract Pests

As delicious and versatile as they can be in the kitchen, I think pretty much everyone agrees that onions, or at least raw onions, smell pretty rank.

Old and rotting onions smell even worse. It’s bad enough to do more than water your eyes; it can make you gag!

And as hideous as that odor can be to us, it seems to have a strange compulsion on many pests, particularly scavenging mammals.

Having raccoons, possums, dogs, and cats rooting through your compost pile is not sanitary, and it’s going to disrupt the process and make a huge mess at the same time.

Nobody wants that, and although there’s only so much you can do to conceal the stench of onions that are breaking down, if you keep your compost pile well turned and the onions covered you can dramatically cut down on the negative effect.

Not for nothing, you’ll be thankful too as will your family if you can’t smell them!

Onion Pieces are a Bad Idea for Vermicomposting

If you are harvesting worm castings through vermicomposting, think twice before adding onions at all. Unlike our traditional compost pile, worms prefer an environment that is pH neutral.

As we learned, onions are acidic, highly so compared to most foods, and adding them to your bin here is going to upset your worms.

Many worms will stop eating in the area around an onion, and most will flee from it, potentially slowing and disrupting the composting process before it is complete or before they have enriched the soil completely with their droppings.

It’s possible to add the outer husk of an onion to a vermicomposting bin without causing any issues, but I still recommend you avoid them entirely to be sure you aren’t introducing contamination.

Throw Out Any Onions You Suspect of Being Diseased

One thing you must never do is add onions that you know or suspect are diseased. Onions that have been compromised by any plant-based pathogen must be discarded in the trash, not added to your compost pile!

As you might expect, it’s too easy for these germs to make their way to other plants after you spread your compost around.

And on this subject, don’t be too quick to believe anyone that claims the heat from the composting process is sufficient to completely eradicate these various germs.

It might be, but then again it might not be, and the peak heat that is generated by a compost pile is hardly uniform throughout, meaning a complete elimination is dubious at best.

Trust me: you care about the other plants in your garden and elsewhere on your property, never add a diseased onion, or any other disease to plant matter to your compost pile.

How to make Onion Peel Compost

You Can Add Onion Husks and Skins without Worry

One thing you don’t have to worry about when adding onions to your pile is that papery husk, or skin.

This material is nowhere near as acidic as the rest of the onion, and can be added freely to your compost pile at pretty much any time so long as it is in balance already.

Since that tough, thick outer layer present on onions that protects the moist inner layers is similarly less acidic than other parts, keep that in mind if you’re really trying to get your pile back up into the alkaline range.

If you’re cooking with onions you might decide to keep both of those parts for your pile, and either use or discard the rest.

Moldy or Rotting Onions are Also Okay, Just Chop First

Diseased onions are one thing- never good! – but old, moldy or rotting onions are another. These are completely safe to add to a compost pile, because they’re going to get moldy and nasty before the process is done anyway!

If you had some onions long forgotten about under your counter or rolling around in a drawer in your fridge and you discover them too late, you don’t have to send them to the trash and completely waste them.

Just smash them flat or chop them up as described previously and they will be ready to go into your compost pile along with other alkaline components to offset their acidity.

Whatever you do, resist the temptation to just toss them into the pile whole, because it isn’t out of the question that they can still sprout and cause trouble like I said above!