Onion seeds are the reproductive material of the Allium cepa plant, which allows the species to perpetuate itself. The seeds are small, black, and angular, forming within a structure that rises high above the part of the plant humans typically consume. Producing these seeds is a multi-step biological process that requires a specific sequence of environmental cues and two growing seasons.
The Biennial Life Cycle Requirement
The onion is naturally a biennial plant, meaning it requires two full growing seasons to complete its entire life cycle. During the first season, the plant focuses solely on vegetative growth, culminating in the familiar bulb we eat. This bulb is the plant’s food reserve, which it uses to fuel the reproductive process in the following year.
To trigger seed production, the mature bulb must undergo a period of chilling, known as vernalization, often requiring temperatures between 45 and 55 degrees Fahrenheit for a month or more. This cold period signals the plant to move into its second, reproductive phase. The bulb is either left in the ground to overwinter or is harvested, stored, and then replanted the following spring.
Once the chilling requirement is met and temperatures rise, the plant begins to “bolt,” which is the term for sending up a tall, rigid flower stalk called a scape. This process utilizes the sugars and nutrients stored in the bulb, which is why a bolted onion bulb becomes undesirable for eating, often developing a tough center. The bulb must have reached a certain size, typically with leaf bases greater than 3/8 inch in diameter, before the initiation of the seed stalk can occur.
Seed Head Formation and Pollination
The flower stalk, or scape, emerges from the center of the overwintered bulb and elongates, ultimately bearing the seed head at its tip. This seed head is anatomically known as an umbel, a dense, spherical cluster of many small, individual flowers. A single umbel can contain hundreds of small flowers.
Each tiny onion flower is perfect, containing both male and female reproductive parts, but they exhibit protandry. This means the male parts (stamens) shed their pollen before the female part (stigma) becomes receptive, which promotes cross-pollination. The umbel takes about two weeks for all the individual florets to open completely.
The transfer of pollen is carried out almost entirely by insects, as the pollen is heavy and not easily carried by the wind. Honey bees are the most common commercial pollinators used in large-scale onion seed production fields. Once successfully pollinated, the ovary in the base of the flower develops into a small capsule containing the black, angular seeds.
Harvesting and Processing Onion Seeds
The process of collecting onion seeds begins once the seed heads have fully matured and dried on the plant. Maturation is indicated when the umbels start to turn brown or black and the capsules begin to split open. If not harvested promptly at this stage, the mature seeds will fall from the inflorescence, a process known as shattering.
Harvesters typically cut the seed heads and transfer the umbels out of the field. The cut heads are then laid out to dry further in a protected, well-ventilated area for two to three weeks. This drying process is important to reduce the moisture content of the seeds and make separation easier.
After drying, the umbels are threshed, which is the physical action of separating the seeds from the surrounding plant material, or chaff. This can be done by hand or with specialized machinery. The raw seed is then cleaned to remove debris and is often further dried to a low moisture content to ensure long-term viability before packaging.