The Onion Life Cycle: What Are the Stages?
Understand an onion's development beyond simple stages, covering the environmental triggers and biological processes that govern its complete two-year life cycle.
The onion is a common garden vegetable with a distinct life cycle. The plant progresses through several phases to reach the mature bulb that is harvested for culinary use. Each stage has specific environmental needs and developmental milestones that influence the final size and quality of the crop. The journey from seed to a full-sized onion is governed by temperature, moisture, and the duration of daylight.
The Seed and Germination Stage
The onion’s life cycle begins with a small, black, and irregularly shaped seed. For germination to occur, the soil temperature should be between 50°F and 75°F, with consistent moisture in well-drained soil to prevent rot. When planting, seeds should be placed about one-quarter to one-half inch deep, as deeper planting can prevent the sprout from reaching the surface. After several days to two weeks, a thin, green shoot known as the cotyledon emerges from the soil.
The Vegetative Growth Stage
Following germination, the onion plant enters its vegetative growth stage. During this phase, the plant produces hollow, tube-like green leaves and uses photosynthesis to convert sunlight into energy for later use. The health of the plant’s leaves is directly related to the eventual size of the onion bulb. Each leaf corresponds to a ring on the onion, so a larger number of healthy leaves results in a larger bulb with more layers. The plant continues to produce new leaves from the center until it receives the environmental signal to begin bulbing.
The Bulbing Stage
The bulbing stage begins when the base of the plant’s leaves starts to swell, forming the onion bulb just below the soil surface. This process is not triggered by the plant’s age or size but by photoperiodism, the plant’s response to the length of daylight. Different onion varieties are categorized based on the amount of daylight they require to initiate bulbing.
Long-day onions require 14 to 16 hours of daylight and are best suited for northern latitudes. Short-day onions need 10 to 12 hours of daylight and are grown in southern regions. Intermediate-day onions need 12 to 14 hours of light, making them suitable for central latitudes. Selecting the correct type for a specific location is necessary for a successful harvest, as planting the wrong type will result in very small or no bulbs.
The Flowering and Seed Production Stage
Onions are biennial plants, meaning their life cycle spans two growing seasons, with the mature bulb harvested in the first year. If a bulb is left in the ground over winter or replanted the following spring, it enters its second year, which is focused on reproduction. This exposure to a cold period followed by warmth and longer days signals the plant to complete its cycle.
In its second year, the plant will “bolt,” sending up a tall stalk from the center of the bulb. This stalk is topped by a spherical flower head called an umbel, which after pollination develops seed capsules that dry and split open, releasing the small black seeds.