How to Save Onion Seeds

Onion seeds are not usually difficult to grow or to collect. The only thing to keep in mind is that onions are a biennial, meaning that they seed once every two years. By growing and saving your own onion seeds, you can develop a healthy reserve of seeds for next year's garden, for eating directly, or for sprouting.

Steps

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1
Plant onions, and leave them in the ground for two years. Watch for flowers and then seed heads to form during the late summer of the second season.^{[1]XResearch source}
- You may wish to plant extra plants if you want onions to eat during the first season.
Most of the flowers will be dry, and the seeds will begin to fall out on their own.^{[2]XResearch source}
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^{[3]XResearch source}
4
Separate the seeds from the stems and other matter that makes up the seed head. Many seeds will fall out on their own. For the rest, place them in a bag, pound the whole bag against a hard surface.^{[4]XResearch source} If you have many seeds, you may be able to use the wind to separate them from stems and other matter. Use a large bowl and toss them in the air, or pour them from one container to another in a light breeze. The wind should sift the lighter stems away and leave the heavier seeds to fall.
- There's no harm in having a little bit of stem or seed head in your seeds unless you are sprouting them. If you plant it along with the seeds, it will just decompose.
5
Store the seeds in a cool, dry place.^{[5]XResearch source}^{[6]XExpert SourceOlivia Choong
Plant & Gardening SpecialistExpert Interview. 3 September 2021.} Label the seeds with the year you saved them, or plant them immediately in a mild climate. Most seeds work best if they are used within one year of saving them^{[7]XExpert SourceOlivia Choong
Plant & Gardening SpecialistExpert Interview. 3 September 2021.} — you may get an acceptable germination rate the second year, though.
- To prolong the seeds viability, put them in a paper envelope to absorb the moisture and keep them in an airtight container in the fridge.^{[8]XExpert SourceOlivia Choong
  Plant & Gardening SpecialistExpert Interview. 3 September 2021.}

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Question
Do the flowers need to be pollinated?
Community Answer
Just like a woman cannot get pregnant without semen from a man, a flower normally cannot go to seed without some kind of pollination. However, this doesn't necessarily need to be done by insect pollinators. Onion flowers sit close to each other, so the wind can make some pollination happen. You can also bend the flowers toward each other when the pollen is ripe and pollinate each with the other like that. Don't worry if you don't regularly get bumblebees visiting your windowsill, though; other insects like flies and such do also pollinate plants. They're just less noticeable. If you see a flying insect on your onion flowers that isn't actively eating them, it's likely an undercover pollinator.
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Can I save and use the seed from onions that have bolted?
Community Answer
I tried this with an onion that had sprouted leaves in my cupboard, and my onion ended up sprouting two giant flower stalks and going to seed with no trouble. Do make sure you give it enough space - the roots don't go deep, but the whole thing gets larger than you'd expect. Get a pot with two or three inches of extra space around the onion. Leave the onion sitting up in shallow water to encourage root growth for a week or so first (I used a wide glass so the sides of the glass prevented it from falling over). You can just sit the onion on top of the soil with the bottom a bit buried.
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How many onion seeds are there in an average flower?
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There are 42 onion seeds in the average flower. Most of them are in the roots.
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Tips

Onions are biennials. For onions you will eat, you will harvest the same year that you plant. If you want seeds, you will have to wait a second year. If you want both seeds and onions to harvest, plant extra plants for two years running.
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Onions will cross-pollinate if they are grown close to other varieties of onions. That means that you may not get what you started with. If you're sprouting or growing the seeds as spring onions, or if you're willing to try growing onions from the genetic grab bag, that may not matter much to you. If you need the same variety as last year's onions, you must either take steps to prevent cross pollination or buy seed from a source that does.
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