The common onion (Allium cepa) is botanically a biennial plant, though it is typically cultivated and harvested as an annual crop within its first year. The edible portion is a specialized organ that develops beneath the soil surface. This familiar globe-shaped vegetable is not a root, but a subterranean structure designed to store energy for the plant’s second year of life, which we utilize for food.
The Anatomy of the Onion: Understanding the Bulb
The onion you consume is a tunicated bulb, a modified stem and leaf structure, not a root. The actual stem is a highly compressed, disc-like structure located at the base of the bulb, called the basal plate. This basal plate is the growing point from which the true roots, known as adventitious roots, sprout downward to anchor the plant and absorb water and nutrients.
The large, fleshy layers that make up the bulk of the onion are modified leaf bases, or scales. These scales grow concentrically around a central growing point and serve as the plant’s food storage organs, accumulating sugars and nutrients produced during photosynthesis. Each leaf that grows above ground corresponds to one of these fleshy layers, meaning more leaves generally lead to a larger final bulb size. The outer, papery skin, or tunic, is the dried remains of the oldest leaf bases, which function to protect the inner tissue.
The Onion Life Cycle: Stages of Underground Development
The journey of the onion from seed to bulb involves two phases: vegetative growth and the bulbing phase. The vegetative stage begins after planting, focusing on producing a robust system of leaves above the ground. During this time, the green leaves actively photosynthesize, generating the energy that will eventually be stored in the bulb.
The transition to the bulbing phase is triggered by environmental cues, primarily photoperiod, or day length. Each variety of onion has a minimum daylight requirement it must reach before it stops producing new leaves and begins to swell its underground base. Short-day onions, for instance, begin to bulb with 10 to 12 hours of daylight, while long-day varieties may require 14 to 16 hours.
Once the photoperiod trigger is met, the plant redirects energy from the above-ground leaves into the underground leaf bases. This rapid accumulation of stored nutrients causes the base of the plant to swell dramatically, forming the bulb. Temperature also plays a role, as warmer temperatures accelerate the bulbing process once the day-length threshold has been reached.
Varietal Differences in Growth: Bulb vs. Green Onions
The distinction between common bulb onions and green onions, or scallions, often comes down to genetics and timing of harvest. True bulb onions, like yellow, white, or red storage onions, remain in the ground until they fully complete the bulbing phase. They are grown specifically for their large, mature underground storage organ, which forms after the photoperiod trigger has been met.
Green onions are often varieties that naturally produce minimal bulbs, or they are bulb-forming varieties harvested prematurely. Scallions are pulled from the soil during the vegetative growth phase, before the photoperiod has triggered significant nutrient storage and bulb swelling. This early harvest yields the long, straight white shaft and green tops, which are both tender and mild in flavor.