Onion with a green bit in the middle

What causes onions to sprout?

Onions are designed to eventually sprout. Their sprouts produce flowers, which produce seeds, thus continuing the onion’s life cycle. 

The onion pictured at the top of this post is in an early visible stages of sprouting (here’s an onion with a far more advanced sprout). 

Sprouts begin at the lower center of the onion, in what is technically the “stem,” but what you may recognize as the little nubby disk that bridges the onion’s hairy roots and layers of storage leaves that make up the bulb. As soon as cues, like temperature, hormones and the onion’s age, tell the vegetable it’s a good time to get growing, a sprout or sprouts will start to develop. (For more on how sprouting and stem quirks might cause onions to look different than expected, check out this onion that was filled with little sprouts; and this onion with a particularly large stem.)

Can you eat an onion that’s started to sprout?

You can eat a sprouted onion, sprout and all! If it looks like the onion at the top of this post, you probably won’t notice any flavor differences. An onion with a more advanced sprout will be sending moisture and nutrients into the green shoot, which may make the bulb taste more bitter and fibrous, but, honestly, I’m guessing you won’t notice. And the sprouts themselves are also edible! But again, no promises on whether their texture will be ideal. 

What happens if an onion sprout keeps growing?

The sprout will eventually develop a flower (flowers from alliums like onions look like delicate spheres), which would eventually lead to seed distribution, thus continuing the onion’s life cycle. The onions we purchase from markets have never flowered. Cultivated onions basically just complete the first portion of their pungent little lives: they grow leaves and fatten their storage bulbs so they can survive the elements and have plenty of fuel for sprouting and flowering when conditions are right. Farmers often deem onions ready to harvest when the leaves above the bulb have flopped over.

How should you store onions to prevent them from sprouting?

When storing onions you want to keep them relatively cool (to prevent sprouting), dry (to prevent mold and rot), away from bright lights or sunlight (to prevent greening) and well ventilated (to prevent moisture build-up). So, definitely don’t keep them in sealed plastic bags. 

But how cool should they be? The National Onion Association recommends that, on the commercial scale, they be kept at 45 to 55 degrees Fahrenheit. Such a range is obviously impractical for consumers, as it’s higher than refrigeration temperatures (which are pegged to food safety) and lower than room temperature. 

For me, a pantry usually suffices, even if it’s warmer than ideal. If you’re storing onions that are particularly high moisture, such as sweet onions, the National Onion Association suggests wrapping them in newspaper or paper towels (I suspect a reusable cloth or cotton bag would do the job too) and storing them in the fridge.