Onions: Why They’re Stems, Not Roots

  • By: SFUAA
  • Date: May 5, 2022
  • Time to read: 5 min.

Onions are the second most popular type of vegetable in the world. Its ability to change from eye-wateringly intense when raw to mild and sweet when cooked makes it an invaluable ingredient in most kitchens. Because of its unique appearance, classifying this versatile and delicious plant as a root or stem vegetable category can be tricky.

Onions are shortened, underground stems that form bulbs. These modified stems have a flattened basal plate from which a mass of short roots grows. Bulbs are compressed stems that add new layers of fleshy leaf growth from the inside. They are covered with a papery exterior called a tunic. 

Most of us haven’t thought too much about what part of a plant the familiar, layered onion bulb is – all you know is that it is delicious. Let’s find out a little more about this everyday vegetable so the next time an onion brings tears to your eyes, you can correctly identify the offending section of this remarkable plant. 

Is An Onion A Stem, Or Is It A Root? 

There is quite a lot of confusion, even among plant experts, about whether an onion is a stem or root. Since it grows underground, many people naturally conclude that it is a root. However, if you carefully study a bulbous, juicy onion, you will notice tiny root structures protruding from the base. 

Even if you don’t have your own vegetable garden, you’ve probably encountered an onion plant’s long, tubular blue, green top leaves. In the case of spring onions, the leaves are used to add taste, color, and texture to dishes. So if leaves grow on top and roots grow out the bottom, the bridge between must be a stem. 

According to PennState Extension, like all true bulbs, onions are stems. They are similar in structure to flowering bulbs like daffodils and tulips. These plants all have a flattened lower surface on the base called a basal plate. Below this flattened surface, roots protrude, but above layer, upon layer of thickened fleshy leaf growth forms.

Onions do not have a regular long, skinny stem that conveys nutrients from the roots to the leaves but have taken on a more useful form to store nutrients for the plant. In the case of an onion, the bulb is a modified, short section of stem enclosed by layers of covering leaves. When cutting onions, the bottom stem section, which contains the attached roots, is usually cut off and discarded. 

At the end of each growing period, onion leaves above the soil surface die down, and the outside layers surrounding the fleshy onion become dry and brittle. This tunicate bulb formed from the plant’s stem and surrounded by fleshy leaves is what we refer to as an onion. 

What Are Plant Bulbs?

Not all plants that have underground storage sections are bulbs. There are plenty of examples of plants with swollen underground storage systems, which can fit into one of four distinct categories.

  1. Bulbs – Examples of true bulbs are onions and lilies
  1. Corms – Enlarged underground stems. Differ from bulbs in that the center of the globe storage section is solid tissue. Bulbs are layers of developing leaves. Examples are saffron and Chinese water chestnut.
  2. Tubers – Thick, underground nutrient storage organs. Often a tuber is a thickened section of a rhizome. Examples are potatoes and dahlias. 
  1. Rhizomes – The main stem runs horizontally underground and sends up new upward shoots at intervals. Examples are ginger, turmeric, and bamboo.

The term ‘bulbs’ has become an umbrella term used by gardeners when a true bulb has significant differences from other underground plant systems. 

Onion bulbs are considered true bulbs because they contain five main parts which are necessary for this classification:

  1. Basal plate – the lower section that roots grow from
  2. Tunic – thin papery covering to protect the fleshy inside
  3. Fleshy scales – Storage area
  4. Central shoot – Includes developing flower and leaf buds
  5. Lateral buds – develop into new bulb pups.

In short, a plant is a bulb if it stores its complete life cycle in a bulbous, underground structure. They are most often perennials that send up leaves and flowers during the growing season but die back completely to ground level. Every part of a living plant is stored in the bulb and ready to shoot up new growth in the next growing cycle. 

What’s The Difference Between A Seed And A Bulb? 

Since a bulb is an entire plant presented in a compact, dormant form, how is that different from a seed? In short, a bulb is already an all-inclusive plant and does not require germination to grow. There is already a stem, roots, and immature leaves inside the globe-like structure. 

Seeds are the embryonic state of plants and need to go through the process of germination and growth to reach maturity. They do not yet have any stems, roots, or leaves in place, and those will only form if growing conditions are favorable. 

Seeds can be biennial, perennial, or annual, whereas bulbs are almost always perennial and the top part of the plant dies back to ground level end the end of the growing season. Seeds can be stored and will remain viable for extended periods. They can be planted by scattering or covering lightly with soil.

Nutrients to sustain bulb plants are stored inside the fleshy underground sections, which are usually covered in a papery outer shell known as a tunic. To plant a bulb, the entire globe section must be inserted into a hole in the soil at the correct depth. 

Plants that are bulbs, like onions, also produce flowers and seeds during the growing season. When onion seed is planted in optimal conditions, the stem will become modified to form an underground bulb that will sustain the plant through all seasons and continually push up new growth during each growing season. 


An onion is a stem, but not the regular sort found on most plants. It is a modified, short, flattened structure that remains underground. Roots spring out from below a basal plate on the stem. Fleshy scalelike leaves form around the stem to form a storage bulb.


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