An onion is a living flowering plant, the same kind of organism as an oak or a rose. The round part on your cutting board is not a root, as many people assume. It is mostly leaves.1GreenLand-food. Is an Onion a Vegetable? So when you buy an onion, you are holding a plant paused between its first and second years, packed with food meant for flowering.
What Part of the Plant the Onion Bulb Is
The edible onion is called a bulb. At its base sits a small stem, the tan, hard tissue just above the roots.2Biology LibreTexts. Is an Onion a Plant? Explaining Its Biology Thick, fleshy leaves wrap around that stem in layers.3Biology LibreTexts. Is an Onion a Plant? Explaining Its Biology Each ring you peel off is the swollen base of a leaf. Those leaf bases continue upward into the green leaves you see above ground on a growing onion.4Australian Journal of Plant Physiology. Translocation of Photosynthate in Bulbing Onions
The layers are not all the same thickness. The innermost rings are the youngest leaf bases, still forming around the growing center, which is why the middle looks small and tightly packed. The thickest, juiciest rings sit between the center and the skin, because those leaf bases have swollen the most with stored food. The dry outer layers have lost their moisture and food, and they now work as a wrapper that slows water loss in storage.
Even though it sits at or below the soil surface, the whole bulb is not a true root.5GreenLand-food. Is an Onion a Vegetable? It is not a botanical fruit either.6GreenLand-food. Is an Onion a Vegetable? A botanical fruit develops from a flower and holds seeds, so tomatoes and peppers count as fruits even though cooks treat them as vegetables. An onion bulb forms from leaf bases with no flower involved, and its seeds form later in the flower head. Cooks call it a vegetable, and botanists call it a bulb, a type of plant storage organ.
Mixing up underground foods is common in the kitchen. A carrot is a true swollen root, unlike an onion. A potato is a swollen underground stem with no fleshy leaf layers. Garlic is also a bulb, but it is split into separate cloves.
Where the Onion Fits in the Plant Family Tree
The onion’s scientific name is Allium cepa.7UC Agriculture and Natural Resources. Know Your Onions Part 1 | The Backyard Gardener Its genus, Allium, also includes garlic, leeks, chives, and shallots.8UC Agriculture and Natural Resources. Know Your Onions Part 1 | The Backyard Gardener The onion is also a monocot, the same broad group as grasses and lilies, plants that sprout with a single seed leaf.9PubMed Central. Onion (Allium cepa L.) and Drought: Current Situation and Perspectives Some botanists place it in the amaryllis family, others in a separate onion family.10UC Agriculture and Natural Resources. Know Your Onions Part 1 | The Backyard Gardener11PubMed Central. Onion (Allium cepa L.) and Drought: Current Situation and Perspectives
The family label differs between sources because family trees get redrawn. Older classification grouped plants mainly by how they looked, while newer work compares DNA, which can shift where a group is placed. Everyone agrees on the genus, Allium.
Why the Onion Stores Food in Its Leaves
Like other green plants, onion leaves use sunlight to make sugars. Much of that sugar stays in the green blade or moves down into that leaf’s swollen base.12Australian Journal of Plant Physiology. Translocation of Photosynthate in Bulbing Onions The bulb stores it mainly as fructan, a chain of fructose sugars.13Scientia Horticulturae. Sugar accumulation and activities of enzymes involved in fructan dynamics from seedling to bulb formation in onion (Allium cepa L.) Some varieties pack in lots of fructan, while others hold more simple sugars and sucrose.14Scientia Horticulturae. Sugar accumulation and activities of enzymes involved in fructan dynamics from seedling to bulb formation in onion (Allium cepa L.)
Fructan packs a lot of sugar into a compact form without making cells overly sweet or watery. When growth restarts, the plant breaks the chain back into simple sugars.
A healthy set of green leaves matters for bulb size: leaves lost early leave less food to send down. Toward the end of the season the tops fall over and dry, a sign that the bulb has finished filling.
In storage, an onion slowly uses up its stored sugar.15PubMed Central. Variation of Fructan and Its Metabolizing Enzymes in Onions with Different Storage Characteristics Varieties that sprout early tend to burn through their stored sugar faster than slow-sprouting ones.16PubMed Central. Variation of Fructan and Its Metabolizing Enzymes in Onions with Different Storage Characteristics A sprouting onion in your pantry is spending its savings to start growing again.
At home, store onions cool, dry, and dark to slow sprouting. A sprouted onion is still edible, though it may be softer.
How an Onion Grows, Flowers, and Makes Seeds
Onions are biennial, meaning they are two-year plants that need two growing seasons to complete their life.17PubMed Central. Induction of flowering with a photoperiod extension coupled with gibberellic acid treatment: physiological and molecular insights in Allium species In the first season the plant forms a bulb.18PubMed Central. Induction of flowering with a photoperiod extension coupled with gibberellic acid treatment: physiological and molecular insights in Allium species In the second season it uses that stored food to flower and produce seed.19PubMed Central. Induction of flowering with a photoperiod extension coupled with gibberellic acid treatment: physiological and molecular insights in Allium species That is why farmers harvest bulbs before the plant ever flowers.
The flower grows on a tall hollow stalk. The flower head is a round ball of many small white or purple flowers. Bees visit them, and they produce small black seeds. A bulb that sends up a stalk early usually gets woody inside.
An onion starts forming a bulb when days reach a certain length.20The Journal of Horticultural Science and Biotechnology. Changes in the expression of photoperiodic bulbing genes in response to increasing daylength in long-day and short-day onion varieties A gene switches on under long days and tells the plant to bulb, while another under short days holds it back.21Scientia Horticulturae. The role of organ – and daylength – specific gene expression in bulb development and resource management in onion (ALLIUM CEPA L.) So short-day onions suit southern gardens and long-day onions suit northern ones. Seed packets label them short-day, intermediate, or long-day, and planting the wrong type can give tiny bulbs or none. A short-day onion grown in the far north may start bulbing while plants are still small, giving small bulbs.
Farmers pull onions once the tops fall over and dry, when the bulb has finished filling and is ready to store. Leaving them in the ground much longer risks rot and regrowth, since the plant is heading toward its second year.
Why Cutting an Onion Makes You Cry
Onion cells hold sulfur compounds and an enzyme, kept apart until the cells are damaged.22PubMed Central. Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa The blade ruptures cells, the enzyme meets the compounds, and a quick chain of reactions produces reactive sulfur compounds.23PubMed Central. Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa A second enzyme then turns some of them into a small gas that irritates your eyes.24PubMed Central. Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa The gas drifts up from the board and dissolves in the moisture on the surface of your eye, and your tear glands respond to wash it away. The effect is short-lived, does not injure the eye in normal kitchen use, and fades as the gas disperses.
To cry less, use a sharp knife, which crushes fewer cells and releases less gas. Chilling the onion slows the reaction. Cut the root end last, since it holds many of the compounds. Cooking breaks these compounds down, which is why cooked onions taste mild.
For the onion, this is defense. The same sulfur compounds help fight off microbes.25PubMed. Thiosulfinate Tolerance Is a Virulence Strategy of an Atypical Bacterial Pathogen of Onion Compounds taken from onion waste can kill or repel aphids and slow fungi and bacteria.26Chemical and Biological Technologies in Agriculture. PTS and PTSO, two organosulfur compounds from onion by-products as a novel solution for plant disease and pest management
How Onions Feed With Such Small Roots
Onions have shallow, sparsely branched roots, mostly in the top foot of soil.27Onion Patch. Root Development of Onions The roots grow from the stem and rarely branch.28Plant Research International / Wageningen University and Research Centre. The root systems of onion and Allium fistulosum in the context of organic farming: a breeding approach Root hairs are missing or very short.29Plant Research International / Wageningen University and Research Centre. The root systems of onion and Allium fistulosum in the context of organic farming: a breeding approach Without them, the roots touch much less soil. Nutrients that barely move through soil, such as phosphorus, are the hardest for such a system to reach. So onions need far more fertilizer than they actually take up.30Plant Research International / Wageningen University and Research Centre. The root systems of onion and Allium fistulosum in the context of organic farming: a breeding approach Mature plants can send some roots out about 32 inches and down about 39 inches, though.31Onion Patch. Root Development of Onions
Onions depend heavily on mycorrhizal fungi, which live in partnership with them.32Center for Sustaining Agriculture and Natural Resources, Washington State University. Arbuscular Mycorrhizal Fungi in Columbia Basin Onion Cropping Systems These root fungi make up for the sparse roots by gathering phosphorus.33Center for Sustaining Agriculture and Natural Resources, Washington State University. Arbuscular Mycorrhizal Fungi in Columbia Basin Onion Cropping Systems Their fine threads grow out into the soil, reach areas the plant’s own roots cannot, and pass nutrients back. Fungal threads can draw phosphorus from at least 7 centimeters beyond the root.34New Phytologist. PHOSPHATE UPTAKE ZONES OF MYCORRHIZAL AND NON‐MYCORRHIZAL ONIONS Plants with these fungi held more phosphorus in roots, leaves, and bulbs.35PubMed Central. Use of mycorrhizal fungi and phosphorus fertilization to improve the yield of onion (Allium cepa L.) plant
This shapes how to grow onions. Water often and lightly, because roots stay shallow. Weed carefully, since onions compete poorly. Avoid heavy tilling that breaks up beneficial soil fungi. Keep fertilizer close to the plants, where the roots reach.
Common Onion Diseases Gardeners See
Botrytis fungus causes small yellow-white sunken spots on green leaves, usually late in the season.36University of Vermont Extension. Onion Diseases in the Northeast Heavy infections brown and kill the tops quickly, reducing bulb size.37University of Vermont Extension. Onion Diseases in the Northeast The fungus also spreads spores to other plants and fields.38University of Vermont Extension. Onion Diseases in the Northeast
Moisture drives the disease. Fungal spores need a film of water on the leaf to germinate, so long wet, cool periods favor infection. Dense plantings hold humidity among the leaves and keep them wet longer.
At home, space plants for airflow so leaves dry fast, and water at soil level rather than over the leaves. After harvest, remove infected plant debris.