Potato, sweet potato, carrot, turnip, ginger, and taro are all familiar examples of root vegetables in cooking. But are onions considered a root vegetable too?
In culinary terms, onions are considered root vegetables, even though the bulbs we eat are not true roots. An onion is a tunicate bulb with a short basal stem and fleshy storage leaves. So it fits the broad kitchen category, while its botanical structure is different from a carrot’s root.
Onions add flavor to soups, stir-fries, salads, and many other dishes. Understanding the bulb’s structure helps explain why they belong to this broad kitchen category.
Why Is Onion Considered A Root Vegetable?
In cooking, the term root vegetables generally groups plants grown for edible storage parts that develop below or near the soil surface.
When we think of root vegetables, the first ones that spring to mind are usually potato, carrot, beetroot, or sweet potato.
Why do onions seem different from these vegetables?
An onion’s bulb develops at the base of the plant, with roots below it. It fits the broad kitchen grouping even when part of the bulb is visible above the soil.
However, there are some key differences between “classic” root vegetables and onions.
Root Vegetables: True Roots vs. Non-Roots
The kitchen label covers several different plant organs. A useful way to understand them is to separate true roots from storage structures that are not roots:
- True roots include the storage taproot of a carrot and the enlarged storage roots of sweet potato or cassava.
- Non-root storage organs include bulbs, corms, rhizomes, and stem tubers. Onion is a bulb, ginger and turmeric are rhizomes, and potato is a stem tuber. A rhizome is a stem that usually grows horizontally below ground, while a corm is a swollen, solid stem base.
These different plant parts can serve a similar purpose: storing reserves that the plant uses for growth.
Those reserves are not always starch. Onion bulbs, for example, store sugars and fructans, which are another form of stored carbohydrate.
The stored reserves can help a plant survive periods of dormancy and support new growth.
In the kitchen, we can use the term root vegetables without separating true roots from these other storage organs.
But for interest’s sake, onions fall into the non-root category.
What Is An Onion Bulb Made Of?
While we may call onions root vegetables, they aren’t actually roots.
An onion is a tunicate bulb, which has a short stem at its base surrounded by fleshy leaves. Bulbs are often described as modified stems, but this shorthand does not mean that every layer of an onion is stem tissue.
The onion’s roots grow out of the flat bottom of the onion (the basal plate).
The layers that make up an onion are fleshy scale leaves.
In the middle of the bulb is the plant’s growing tip.
When onions sprout green leaves, they grow out of the center of the onion.
Recommended: Is a Potato Classified as a Legume? | Can You Microwave Sweet Potatoes? | Exploring the World of Sweet Onions | Is A Tomato A Berry?
Are Spring Onions Root Vegetables?
The parts we eat from spring onions include green leaf blades and the lower, pale leaf bases. They are not all stem tissue.
Some spring onions are common bulb onions (Allium cepa) harvested while they are young. However, names such as spring onion, green onion, and scallion vary by region, so the name alone does not tell you the species.
In the kitchen, spring onions may be grouped with root vegetables, but their edible green tops are leaves.
Are Green Onions Root Vegetables?
Green onions are often called scallions, and these names can overlap with spring onions. They may be young Allium cepa, bunching onions such as Allium fistulosum, or hybrids.
Bunching onions generally do not form the large, rounded bulbs that we associate with dry bulb onions.
Whether a recipe groups them with root or leafy vegetables, the parts used are the green leaves and pale lower leaf bases.
How Do Onions Grow?
The familiar dry bulb onion is a mature storage bulb that can remain dormant for a time. Fresh green onions are harvested in a different state, so the following growth account focuses on common bulb onions.
They look very different when they are growing in the soil.
The pointy side of an onion is where the green foliage used to grow from, and the bearded side is where the roots grew from.
Young onion plants have slender green leaves before they develop a large bulb.
Initially, onions don’t have a round bulb at their base.
As a bulb develops, the leaf bases and storage scales thicken around the short basal stem.
When this bulbing process starts depends on the onion variety, daylength, and temperature.
As the bulb matures and dries, its outer scales become the thin, papery covering called the tunic.
The tunics are what we peel off of an onion.
For onions that will be dried and stored, yellowing, drying tops that fall over are useful signs of maturity. Onions for fresh use can be harvested earlier.
Lift the bulbs carefully to avoid damage. A garden fork can help loosen the soil, but it is not the only way to harvest them.
To prepare mature bulbs for storage, spread them out to cure in a warm, dry, well-ventilated place. This often takes about 2 or 3 weeks, but the necks and outer scales should be dry before the onions go into storage.
After curing, keep sound, whole storage onions in a cool, dry, dark place with good airflow. Some varieties can keep for around 3 months under suitable conditions, while others keep for a shorter time. Fresh green onions need refrigerated storage instead.
Ways To Use Onions In The Kitchen
Onions can be eaten raw or cooked, depending on the dish.
Here are a few ways to use them:
- Use sautéed onions as a base for a soup, stew, or stock.
- Include sliced onions in stir-fries.
- Add finely chopped red onions to your guacamole.
- Eat a tasty salsa of onion, tomato, and fresh coriander.
- Enjoy a salad of chickpeas, finely diced onion, and peppers.
- Add some finely chopped onion to homemade salad dressings.
- Sprinkle sliced red onion on top of salads, burgers, tacos, and fajitas.
- Add sliced onion when making an omelet, quiche, or frittata.
Related: Can Saucepans Be Recycled? | Can Saucepans Go In The Oven? | How Does A Griddle Work?
Types Of Root Vegetables
There are loads of different root vegetables out there!
The ones we are familiar with (onion, potato, carrot, beetroot) are just a handful of all the different types.
This list includes culinary root crops and other storage parts used in food. The organ notes explain what each example actually is, with kohlrabi included as an above-ground contrast. It is a classification guide, not advice on eating unfamiliar plants or eating these foods raw; preparation requirements differ.
- Arrowroot (Maranta arundinacea): a rhizome that supplies culinary starch.
- Beetroot: a familiar root crop.
- Carrot: a storage taproot.
- Cassava: enlarged storage roots.
- Celeriac: a swollen base that includes both the lower stem (hypocotyl) and upper taproot.
- Chinese artichoke (Stachys affinis): tubers that form at the ends of underground stems called stolons.
- Chinese water chestnut (Eleocharis dulcis): corms at the ends of underground stolons.
- Daikon: a radish whose swollen edible part includes both the main root and the hypocotyl.
- Dandelion root: the taproot of common dandelion (Taraxacum officinale).
- Ginger: a rhizome.
- Ginseng: American ginseng (Panax quinquefolius) has a thickened root with a short rhizome, or neck, above it.
- Jerusalem artichoke, or sunchoke (Helianthus tuberosus): tubers that form at the tips of underground rhizomes.
- Kohlrabi: the swollen stem above the ground. It is included here for comparison, rather than as an underground storage organ.
- Lotus root (Nelumbo nucifera): a rhizome, despite the word root in its culinary name.
- Onion: a bulb with a short basal stem and fleshy storage leaves.
- Parsnip: a root crop.
- Potato: a stem tuber.
- Radish: a root crop.
- Rutabaga: a root crop.
- Sweet potato: an enlarged storage root.
- Taro (Colocasia esculenta): a corm. The name taro is also used for other crops in some regions.
- Tiger nut, or cultivated chufa (Cyperus esculentus): tubers that form at the tips of rhizomes, rather than botanical nuts.
- Turmeric (Curcuma longa): a rhizome principally used as a spice.
- Turnip: a root crop.
- Yams, including ube: tuber crops from cultivated edible Dioscorea species. In the Philippines, ube refers to purple-fleshed forms of Dioscorea alata.
