Why are onions considered bulb vegetables rather than root vegetables?

Onions are classified as bulb vegetables rather than root vegetables because of their distinct botanical structure. While the term "root vegetable" is a culinary category that includes various underground plant parts, it is not a precise botanical classification.1

Botanically, an onion is a modified stem consisting of a short basal plate that supports fleshy, overlapping leaves. These leaves are compressed into tight layers, known as scales, which function as food reserves to help the plant survive periods of dormancy. In contrast, true root vegetables—such as carrots or parsnips—consist of the actual root of the plant, often developed as a taproot for storage.213

Because onions are composed of modified leaves and stems rather than root tissue, they are botanically defined as bulbs, even though they are grouped with root vegetables in culinary and nutritional contexts.21