Can You Compost Onions? Be Careful

We’ve all seen it.  You buy that big bag of onions only to realize later that you haven’t finished them all before one of them starts to go bad.  Perhaps you have part of an onion leftover after cooking with it, and you don’t want to throw it out, adding more landfill.  So naturally, you wonder, can you compost onions? The answer is yes, but with a few notes of caution.

Onions are a good source of nutrients for your compost, but they also come with a few things to consider before you add onions to your compost.

Reasons to Be Cautious of Adding Onions to Compost

Let’s take a look at the things to be cautious of.  Once you look at these, you can decide whether you want to compost onions, how many onions you want to compost, and how to compost them.

Onions Grow in Compost

Normally, onions grow underground.  Like potatoes, they sprout really easily.  This means that if you throw a whole onion into the rich, fertile environment of a compost pile, you’re likely to grow onion plants in your compost or perhaps later in your garden.

Cutting up the onions or smashing them can help prevent this.

Rotting Onions Smell – Attracting Pests

Onions make us cry, and people often don’t like the smell of onions.  Rotting onions especially stink.  The problem is that what stinks for us attracts pests and critters to your compost pile.  Those pests come in, eat all the food in your compost and leave you with a pile of leaves before they move on to their next target; that’s not fun.

Worms Have Sensitive Stomachs – Composting Onions with Vermicomposting

If you’re using vermicomposting, where you use worms to help you compost, you want to avoid adding onions, garlic and other allium plants.  It turns out that worms have sensitive stomachs and, apparently, sensitive noses (not sure that they have noses at all, but you get the idea.) In general, worms don’t like foods that have strong odors.  Onions included. 

Worms won’t readily eat the onions, and that leaves the onions to rot.  Best just not to add onions to your worm composting bin.

Just to note – onions won’t actually hurt the worms; they just don’t eat.  So, the onions stay in the compost and rot. There’s a good chance that they will start to smell after a while.

Avoid Diseased Onions

This should go without saying and is true of anything you add to your compost pile, but if you think the onion has some sort of disease (usually only if it’s from a plant you are growing), don’t add it to your compost.  You don’t want that disease to circulate into your compost and then into your garden or lawn when you add the compost to your soil.

How to Compost Onions

Now that we know the things to be cautious of let’s look at how you can compost onions.

1. Don’t add whole onions to your compost.  Cut them up into smaller parts.
2. Bury the onions deep to avoid the smell attracting animals.  You want to bury the onions at least 10 inches deep in your compost.  This will cover up the odors. 
3. When you turn your compost, which you need to do every few days, make sure to keep the onions deep inside the compost pile.
4. Add onions in moderation.  Besides the smell, they also are somewhat acidic. Just like citrus fruit and tomatoes, you want to add onions in moderation.

With these warnings, if you use a traditional compost pile, you should be okay to compost onions.

Can I Compost Onions?

Like other vegetables, onions have a lot of nutrients that are good for your compost. Unfortunately, they also have a few things going for them that take more consideration.  You can compost onions if you aren’t using vermicomposting.  You may not want to, though.

Frequently Asked Questions About Adding Onions to Compost