Saving onion seeds is a rewarding practice for home gardeners, allowing them to preserve favored varieties and complete the full agricultural cycle. Onion seeds are the small, black, angular structures produced after the plant flowers, and collecting them avoids the annual cost of purchasing new stock. This ensures a continuous supply of varieties known to perform well locally. Gardeners can also select for desirable traits like bulb size, storage quality, and disease resistance, gradually improving their personal strain over multiple seasons.
Understanding the Onion’s Two-Year Cycle
Onions (Allium cepa) are classified as biennial plants, meaning their natural life cycle spans two growing seasons to complete the seed-to-seed process. In the first year, the plant grows vegetatively, forming the edible bulb, which acts as a food storage organ. This bulb is harvested for consumption in a typical annual crop cycle, but for seed saving, the bulb must be allowed to enter dormancy.
The second year is dedicated to reproductive growth, triggered by a necessary cold period called vernalization. Vernalization requires exposure to low, non-freezing temperatures for an adequate duration, signaling the plant to switch from vegetative growth to flowering. This cold exposure is typically achieved with temperatures between 35 and 50 degrees Fahrenheit (2 to 10 degrees Celsius) for four to twelve weeks. Once this requirement is met, the plant uses the stored energy in the bulb to send up a tall flower stalk, known as bolting, culminating in seed production. Without this cold period, the bulb will remain dormant or continue vegetative growth, never initiating the seed head.
Preparing Onions for Seed Production (Bolting)
The success of onion seed saving begins with careful selection of the mother bulbs in the first year. Choose those that are healthiest, largest, and truest to the desired type for replanting. Exclude bulbs displaying signs of disease, premature bolting, or poor shape to maintain seed stock quality. Select a dozen or more bulbs to ensure genetic diversity and produce a robust seed crop.
After the first-year harvest, cure the selected bulbs in a warm, dry, and well-ventilated area until the outer skins are dry and papery. Store them through the winter in a cool, dark, and dry environment. Temperatures should be maintained near 40 degrees Fahrenheit (4 degrees Celsius) to keep the bulbs dormant without triggering a premature flower stalk. This period of cold storage satisfies the vernalization requirement.
In early spring of the second year, replant the vernalized bulbs into the garden, spacing them at least one foot apart to allow room for the flower stalks to develop. Plant the bulb at the same depth it grew the previous year. As the weather warms, the plant will emerge from dormancy and divert its energy into developing a thick, hollow flower stem, or scape. The end of this scape will swell to form the umbel, a spherical cluster of tiny white flowers that will eventually develop into the seed head.
Harvesting and Drying the Seed Heads
Timing the harvest correctly is essential to maximize the yield of viable seeds while preventing loss through shattering. The spherical flower heads, called umbels, transition from green to a dry, tan hue as the seeds mature inside the capsules. The ideal time to harvest is when a small number of black seeds become visible, indicating the capsules have started to split open.
This stage means the seeds are physiologically mature, even though the entire head is not yet fully dry. Waiting until the umbel is completely dry risks losing a significant portion of the seeds to shattering. The umbel should be clipped with a sharp tool, leaving a few inches of the stalk attached. This attached stalk provides a convenient handle for the next steps.
The collected seed heads must then undergo a final drying process in a sheltered location with good air circulation, away from direct sunlight. A common method involves placing the heads into a paper bag and hanging it upside down. This allows remaining moisture to dissipate slowly while released seeds fall directly into the bag. This drying period should last for several weeks until the stalks are brittle and the capsules are completely dry.
Extracting, Cleaning, and Storing Seeds
Once the seed heads are thoroughly dried, extract the small, black seeds from the surrounding plant material, or chaff. The simplest extraction method involves crushing the dried umbels by hand, rubbing them between the palms, or gently rolling a pin over them inside a sturdy bag. This action breaks open the remaining capsules and separates the seeds from the dried floral parts.
The resulting mixture of seeds and chaff requires essential cleaning. This is often accomplished through winnowing, which uses a gentle breeze or fan to separate the heavier seeds from the lighter debris. By slowly pouring the mixture from one container to another, the seeds drop straight down while the lighter chaff blows away. Alternatively, the mixture can be passed through a series of screens with different mesh sizes to sift out the largest and smallest particles.
The seeds must be completely dry before long-term storage. Spread them thinly on a plate or screen in a low-humidity environment for a few days to reach a moisture content of around 6 to 8 percent. The final storage container should be airtight, such as a glass jar or sealed plastic bag. Keep the seeds in a cool, dark place, like a refrigerator or freezer, to maintain viability for up to two years.