Producing your own onion seeds is a rewarding, multi-year endeavor that allows gardeners to preserve the genetic traits of their preferred varieties. Onions (Allium cepa) are biennial, meaning they require two full growing seasons to complete their reproduction. During the first year, the plant forms the edible bulb, which is a storage organ. The subsequent year, after cold exposure, the stored energy produces a tall flower stalk that yields the seeds. This practice ensures a continuous supply of a cultivar adapted to local conditions and provides a cost-effective alternative to purchasing new packets annually.
Selecting the Right Onions for Seed Production
The quality of the final seed harvest depends entirely on the characteristics of the parent plants chosen for propagation. Select only the healthiest, most vigorous bulbs that display the traits you wish to maintain, such as ideal size, color, shape, and good storage capability. It is recommended to use only open-pollinated (OP) varieties for seed saving, since hybrid varieties (F1) will not reliably produce offspring true to the parent plant.
Onions are naturally cross-pollinating, relying heavily on insects to transfer pollen between plants. To maintain the purity of a specific variety, implement an isolation distance from other flowering onion types, including chives and shallots. Home gardeners should aim for the greatest distance possible, or use physical barriers like caging to prevent unwanted cross-pollination. Selecting multiple superior bulbs also helps maintain genetic diversity within your chosen seed line.
Overwintering and Encouraging the Flower Stalk
The transition from a bulb to a seed-producing plant is triggered by vernalization, which is the exposure of the mature bulb to a prolonged period of cold temperatures. The optimal chilling duration is approximately 12 to 14 weeks at temperatures between 5°C and 10°C to uniformly initiate flowering. Gardeners in mild climates can often leave their selected bulbs in the ground over the winter. In regions with harsher winters, bulbs must be dug up in the late fall and stored in a cool, dark location before being replanted the following spring.
Once the vernalization period is complete, replant the bulbs early in the second spring, spacing them 12 to 18 inches apart for proper airflow and development. Plant the bulbs just deep enough so the neck is level with the soil surface. As the weather warms, the plant produces a thick, hollow flower stalk, known as a scape, which can grow quite tall. Because these scapes can become top-heavy when the seed head (umbel) forms, providing support with sturdy stakes is necessary to prevent the stalk from snapping. Adequate water is also required during this growth stage to support the rapid development of the tall stalk and large flower head.
Harvesting and Curing the Seed Heads
The onion umbel must be allowed to fully mature on the plant before harvest to ensure seed viability. The first visual cue that the seed is nearing readiness occurs when the small, papery capsules within the umbel begin to split open, exposing the tiny, black seeds. As the seeds ripen, the flower stalk supporting the head will begin to dry out and turn a pale brown color.
Harvesting should be done when about 25 to 50 percent of the capsules have opened, as waiting longer risks losing seeds to wind or rain. Cut the flower stalk about 6 to 8 inches below the umbel, and then place the harvested heads into a paper bag or a container with good air circulation. Curing involves drying the seed heads indoors in a cool, dark, well-ventilated space for several weeks. This extended drying period allows the remaining seeds to fully mature and ensures the moisture content is low enough for long-term storage.
Cleaning, Testing, and Storing Onion Seeds
After the seed heads have fully dried and the stalks are brittle, the seeds must be separated from the surrounding plant material, known as chaff. This is accomplished by threshing, where you rub the dried heads vigorously to release the seeds from their capsules. The lighter chaff is then separated from the heavier black seeds using a technique called winnowing. Winnowing involves gently pouring the mixture from one container to another in front of a light breeze or fan, allowing the air to carry away the lighter debris while the seeds drop back into the container.
Because onion seeds have a relatively short shelf life, typically remaining highly viable for only one to two years, performing a simple germination test is recommended before planting. The test involves placing a small, measured number of seeds on a moist paper towel and monitoring how many sprout over a week or two. For long-term storage, the cleaned, fully dried seeds must be kept in an airtight container in a cool, dark, and dry environment. A consistent temperature between 4°C and 15°C and a low humidity level are ideal for maximizing the longevity of your saved onion seed supply.