The common onion, Allium cepa, is a widely cultivated vegetable whose growth habit often causes confusion about its location in the soil. The edible part is a modified structure known as a bulb, which develops at the base of the plant. A simple answer to whether it grows above or below ground is that it does both, requiring a closer look at the plant’s anatomy. The plant’s structure is specifically adapted to store nutrients underground while still using the sunlight above the surface to fuel its growth.
Where the Edible Bulb Forms
The process of bulb formation begins beneath the soil surface, as the plant channels energy created through photosynthesis downward. The edible bulb is initiated underground, but its final size and position mean it is often only partially subterranean upon maturity. As the plant approaches harvest, the bulb swells, pushing upward against the surrounding soil. This swelling causes the top half, or sometimes the upper two-thirds, to rise and rest directly at or just above the soil line. The soil resistance around the lower portion is necessary for the concentric leaf bases to compress and form the tight, firm storage structure, while thin, fibrous roots anchor the plant below the soil.
Anatomy Visible Above the Soil Line
The parts of the onion plant that are clearly visible above the soil are primarily the foliage, which are the engine of its growth. These leaves are hollow, cylindrical structures that emerge from the top of the bulb in a flattened, fan-shaped arrangement. The bluish-green color indicates chlorophyll, confirming their function in converting light energy into the sugars necessary for the plant’s survival. These photosynthetic leaves continuously transfer the sugars they produce down to the developing bulb below. If an onion plant is allowed to continue its lifecycle past the typical harvest time, it will enter its second year and produce a tall, stout flower stalk, or scape, topped by a cluster of small flowers that eventually produce seeds.
The Onion Bulb’s Biological Classification
The edible bulb is botanically classified as a tunicated bulb, a specialized underground storage organ, and is neither a true root nor a simple stem tuber. At the very base of the onion is a flattened, compressed structure called the basal plate, which is the plant’s actual, highly reduced stem. This plate is where the adventitious roots sprout downward and where the fleshy, concentric layers of the onion sprout upward. The rings that make up the bulk of the consumable onion are the fleshy, modified bases of the leaves, which swell with stored food reserves, primarily carbohydrates. The outermost layer is a papery, dry protective skin known as the tunic, which prevents moisture loss and shields the interior.