Onions are a kitchen staple, but their botanical classification can be confusing. Most cultivated onions (Allium cepa) are biennials, completing their life cycle over two growing seasons, yet are typically grown as annuals for culinary purposes. Some specific onion types, however, are true perennials, returning year after year from their rootstock.
Understanding Plant Life Cycles
Plants are categorized by the length of their life cycle, which dictates how long they live and when they reproduce.
Annual plants complete their entire life cycle, from seed germination to seed production, within one growing season, typically dying after flowering and setting seed. Examples of common annuals include marigolds, petunias, and zinnias.
Biennial plants require two growing seasons to complete their life cycle. During the first year, they focus on vegetative growth, developing roots, stems, and leaves, often storing energy. They then enter a dormant period, usually overwintering, before flowering, producing seeds, and dying in their second growing season. Carrots and parsley are well-known examples of biennials.
Perennial plants live for more than two years, with many returning reliably for several years or even decades. These plants establish deep root systems that allow them to survive dormant periods, such as winter, and regrow from the same rootstock each year. Hostas and peonies are common examples of herbaceous perennials, which die back to the ground in colder months but regrow in spring.
Onions: A Closer Look at Their Classification
Common bulb onions (Allium cepa) are biennials. In their first growing season, they develop the bulb, the swollen underground storage organ typically harvested for consumption. If left in the ground to overwinter, they resume growth in the second year, sending up a flower stalk and producing seeds before dying.
This progression to flowering and seeding in the second year is known as “bolting.” Bolting can also occur prematurely in the first year if the plant experiences stress, such as temperature fluctuations or insufficient water, causing it to focus energy on reproduction rather than bulb development.
Several onion varieties are true perennials, returning year after year. Multiplying onions (Allium cepa var. aggregatum), also known as potato onions or shallots, form clusters of bulbs at ground level and can be divided and replanted. Egyptian walking onions (Allium × proliferum) produce small bulbils (topsets) at the top of their stalks; these become heavy, causing the stalk to bend and root, effectively “walking” across the garden. Chives (Allium schoenoprasum), a popular culinary herb, are hardy perennials that grow in dense clumps, with hollow leaves that regrow after harvesting.
Practical Implications for Growing Onions
Understanding the life cycle of onions directly influences cultivation practices. For common bulb onions, gardeners replant them each year from seeds or small bulbs (sets) to ensure a consistent harvest of large, flavorful bulbs. Harvesting occurs when their tops begin to yellow, dry, and fall over, indicating bulb maturity. This prevents the plant from expending energy on flowering in a second year, which would diminish the bulb’s size and storage quality.
Perennial onion types offer the advantage of continuous harvests without annual replanting. Multiplying onions can be harvested by separating bulbs from established clumps, leaving some to continue multiplying. Chives can be harvested by clipping their leaves, which regrow throughout the season and return in subsequent years. Egyptian walking onions provide a unique harvest, as both the small underground bulbs and the topsets can be utilized, with topsets also serving as natural propagation. These perennial varieties reduce yearly planting effort while providing a long-term supply of onion flavor.