Collecting onion seeds is more challenging than harvesting other seeds, such as tomatoes, peppers or beans. The seed stalk will not develop until the second year. Because vegetables grown from seeds taken from hybrid plants tend to be inferior to the parent plant, avoid harvesting onion seeds from hybrids. Growing onions from seeds will require about five months for the plants to mature, which is why some gardeners start their onions by planting small bulbs (called sets) or small plants.
Watch for the seed heads to form on unharvested onion plants. This will happen after the spring growth. Seed heads are a round cluster of tiny flowers atop a tall stalk.
Allow the seed head to mature and dry on the plant.
Remove the dried seed head from the plant and place it immediately into a paper envelope, to prevent the seeds from dropping to the ground should the seed head break open.