The question of whether an onion grows underground is common because the familiar, rounded bulb is often completely hidden by soil. Onions are one of the world’s most widely grown and foundational vegetables, forming the base of countless cuisines. Their unique growth habit often leads to confusion about their botanical classification. Clarifying the onion’s structure and its physical location relative to the soil line can help explain why this widely consumed plant is often misunderstood.
The Anatomy of an Onion Bulb
The edible portion of the onion, the bulb, is not a root or a tuber like a potato, but a specialized underground structure known botanically as a tunicate bulb. This bulb is essentially a modified stem that is highly compressed into a disk-like shape, surrounded by fleshy layers. These layers are actually the bases of the plant’s leaves, adapted to store water, carbohydrates, and nutrients for the plant’s survival. The bulb forms at or just below the soil surface, not deep within the earth like a carrot or beet.
As the onion matures, the swelling bulb tends to push itself upward, often exposing the top half above the ground. The dry, papery outer layers, called the tunic, protect the inner storage tissues from moisture loss and external damage. This layered structure is what distinguishes it from true root vegetables. The bulb’s primary function is to store energy for the plant’s dormant period and subsequent regrowth.
Understanding the Root System
While the bulb itself is a modified stem and leaf tissue, the onion plant does possess a true root system anchored firmly in the soil. These are adventitious roots, meaning they grow from the flattened, basal plate—the compressed stem disk—at the bottom of the bulb. This collection of fibrous, string-like roots is responsible for absorbing water and dissolved minerals from the soil.
The onion’s root system is notably shallow and sparsely branched, with the majority of the roots concentrated in the top foot of soil. This limited root depth makes the plant particularly sensitive to nutrient availability and water stress. The onion is classified as a bulb, unlike true root vegetables such as carrots or radishes, where the edible part is the enlarged taproot itself. The roots serve to anchor the plant and facilitate nutrient uptake, while the bulb acts as the storage organ.
The Process of Onion Cultivation and Harvest
Onion cultivation begins by planting seeds, small bulbs called sets, or young transplants only an inch or two deep in the soil. This shallow planting depth is necessary because the bulb needs to expand laterally near the surface. This process, called bulbing, is triggered by specific day lengths and temperatures. As the plant focuses energy into the bulb, the top of the swelling bulb often becomes visible above the ground.
Farmers sometimes intentionally clear soil away from the top of the bulbs during the final stages of growth to encourage maximum sizing and faster drying of the outer layers. The harvest stage begins when the plant’s leafy tops naturally turn yellow, weaken, and fall over. This is a sign that the transfer of nutrients into the bulb is complete. Harvesting is done gently, often by lifting the bulbs with a fork to avoid bruising, which can lead to storage rot.
After being pulled from the soil, the onions undergo curing for long-term storage. Curing involves drying the bulbs for several weeks in a warm, dry, and well-ventilated area. This process allows the neck of the onion to fully seal and the outer skin to become dry and papery. This creates a protective layer that locks in moisture and prevents microbial entry. Curing produces the firm, dry onion bulb that is ultimately sold to consumers.