To collect seeds from onions you need to leave a few (2-3) onions in the ground so that in the 2nd year they will develop heads that contain the seeds for planting in future years.
Onions with Seed Heads
How to Collect Seeds to Grow Onions
These onion seed heads start developing in late Spring on the onions from the previous year and contain hundreds of seeds.
To collect the seeds you need to wait for the heads to increase in size and began to open up which will be mid Summer.
Do not harvest the heads until you see the heads open and start to turn brown to allow the seeds to develop completely.
Inside each head you will see little black seeds that are ready to pop out of the heads, this is your signal to harvest the seed heads and layout on paper to dry out completely.
If you wait too long the heads will start to drop seeds in your garden.
You can pull the onions after harvesting the seed heads since they will not produce a good onion bulb for eating.
Dry the seed heads in a warm area in a cardboard box, this allows good air circulation and drying. Some of the seeds will pop out from the heads as they dry.
To collect the seeds once they are completely dry I suggest you place the dry head in a paper sack to shake and separate the hull from the seeds as much as possible.
Once you have them separated you can now store the seeds for planting.
Since each seed head contains hundreds of seeds I recommend you only need to allow 2-3 onions to grow the 2nd year to develop seeds. You will have plenty of seeds to give to family and friends.
Onion seeds can be used for 1-3 years after harvesting so you do not need to let seed heads develop every year.
Happy Onion Seed collecting from the Urban Gardener.
I hope this post was helpful on How to Collect Seeds to Grow Onions and you will try this simple process.