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What Part of The Plant Is Onion? [Confusion Solved!]

Onions may look simple but have complex anatomy, hiding surprises within each layer.

Generally, the basal globular part of the Onion plant growing below the soil (the bulb) is the real Onion. But, in Spring Onions, the upper green leafy and the basal round bulb comprise a single Onion Plant.

To learn more about the morphology and habit of Onion plants, follow the article!

Overview of Onion Plant

Onion (Allium cepa) belongs to the family Amaryllidaceae, alongside Garlic, Leeks, Daffodils, and Shallots.

Hence, the basic structure of the family members is almost the same, with a few differences in their aroma and taste.

An Onion plant comprises a single bulb housing modified leaves called ‘cataphylls,’ stem, and fibrous roots.

To look further at Onions, go through the table below.

Naming & Growth Zones
Common Name
Common Onion & Bulb Onion

Botanical Name
Allium cepa

USDA Zones

Long Day Varieties: 1-4

Intermediate or Long Day Varieties: 5

Intermediate or Short Day Varieties: 6-7

Short Day Varieties: 8-11
EcologyHabit: Annuals, Biennials, or Perennials

Native Range: Central Asia
MorphologyStem: Bulb or Tunicate Bulb

Root: Fibrous

Leaves: Cataphylls

What Part of The Plant Is Onion?

Onion bulbs consist of a compressed central underground stem situated at the bottom.

Additionally, many layers of scale leaves or ‘cataphylls’ rise from the same stem giving it a bulb-like structure.

Image illustrates the structure of Onion bulb
Onion bulbs consist of modified leaves, reduced stems, and fibrous roots.

The lower part of the stem has fibrous roots, one of the characteristics of Onion plants.

Normally, the Onions we buy from the market are mature bulbs. Immature Onions with green leaves atop are the Spring Onions or Scallions.

Hence, harvesting the mature Onion bulbs may take 3-4 months if planted from the seeds.

Image illustrates structure of Spring Onion
Spring Onions are immature Onion plants.

But it only takes 20-50 days to harvest the Spring Onions. 

From Editorial Team


Spring Onions are young versions of Onion plants with a milder flavor and aroma.

Hence, if you want to incorporate a light and sweeter tang in your dishes, harvest Onions early while they develop.