Onions

The onion (Allium cepa), also called the bulb onion or common onion, is a vegetable grown for its layered, strong-tasting bulb. It belongs to the Allium genus, along with garlic, leeks, chives and shallots, and it is one of the most widely eaten vegetables in the world.

Scientific Classification & Quick Facts

Common name	Onion
Scientific name	Allium cepa
Kingdom	Plantae
Phylum	Tracheophyta
Class	Liliopsida
Order	Asparagales
Family	Amaryllidaceae
Genus	Allium
Type	Bulb vegetable plant
Habitat	Loose, well-drained, nitrogen-rich soil; cool season
Colours	Blue-green leaves; yellow-white bulbs
Edible parts	Bulb
Special feature	Stores food at the base of its leaves

What an Onion Plant Looks Like

An onion plant has long, hollow, tube-shaped leaves that are blue-green and grow from the center of the plant in a fan. The leaves are widest near the bottom and taper to a blunt tip.

The part we eat is the bulb, and it is actually made of leaves! Each layer, or ring, of an onion is the swollen base of a leaf, packed with stored food. All the layers grow from a small, flat stem at the bottom called the basal plate. A bunch of thin, shallow roots grows down from the basal plate into the soil.

If an onion is left in the ground for a second year, it sends up a tall, hollow flower stalk topped with a round ball of small white or purple flowers that bees love.

Why Onions Make You Cry

When you cut an onion, you break open its cells and mix chemicals that were kept apart. They react to make a gas that floats up and stings your eyes, so your eyes make tears to wash it away. Chilling the onion first and using a sharp knife can help.

A Very Old Vegetable

Onions have been grown for at least 5,000 years, probably first in Central or Southwest Asia. The ancient Egyptians grew onions along with garlic and leeks. Onions appear in Egyptian tomb paintings, and the Greek historian Herodotus wrote that the workers who built the Great Pyramid were fed onions, radishes and garlic. Today onions are grown in almost every country.

Long-Day and Short-Day Onions

Onions are fussy about daylight. A bulb only starts to form once the days reach a certain length, so choosing the right type for where you live is important:

• Long-day onions need about 14 to 16 hours of daylight and are best in northern areas with long summer days. ‘Yellow Sweet Spanish’ is a popular large, round long-day onion.
• Short-day onions start bulbing with about 10 to 12 hours of daylight and suit southern areas with mild winters. The famous sweet Vidalia onions are short-day onions.
• Intermediate or “day-neutral” onions fall in between and grow well in many middle regions.

Onions are also sold as small bulbs called sets, which are easy for beginners. ‘Stuttgarter’, a flattened yellow onion, is a common variety sold as sets.

Growing Onions at Home

Soil

Onions like loose, fertile soil that drains well. In heavy, hard soil the bulbs stay small. Many gardeners grow onions in raised beds or rows raised about 4 inches above the ground. Mix compost or well-rotted manure into the soil before planting.

Planting

Plant onions in early spring, as soon as the soil can be dug. Young onion plants can handle cool weather and light frosts. Many gardeners also plant in autumn for an early summer harvest.

• Plant sets or young plants about 1 inch deep. Planting deeper can stop the bulbs from swelling properly.
• Space them about 4 to 5 inches apart, in rows 12 to 18 inches apart.
• Grow onions in a different spot each year (crop rotation) to help avoid pests and diseases.

Watering and Feeding

Onions have shallow roots, so they need steady moisture. About 1 inch of water a week, from rain or watering, is usually enough. A layer of mulch helps keep the soil damp and stops weeds, which onions do not compete well with.

Feed onions with a nitrogen-rich fertilizer every few weeks while the leaves are growing. Stop feeding once the bulbs begin to swell and push the soil aside. Do not pile soil over the swelling bulbs.

Care

Photo by: Rainer Haessner

Keep the bed weeded, gently, so you do not damage the shallow roots. If an onion sends up a flower stalk, it has “bolted”. Pull those onions and use them first, because they will not store well.

Pests

• Onion thrips are tiny, slender insects that scrape the leaves and leave silvery streaks. Sticky traps help you spot them, and insecticidal soap sprayed every few days can control them.
• Onion maggots are the young of a small fly that lays eggs at the base of the plants. Covering young onions with fine mesh or a row cover, sealed at the edges with soil, keeps the flies out. Crop rotation and clearing away rotting plant material also help.

Harvesting and Storing

Onions are ready when most of the tops turn yellow and flop over, usually in mid to late summer. Loosen the soil and pull the bulbs on a dry day, handling them gently to avoid bruises.

Next, the onions need to cure. Spread them out somewhere warm, dry and airy, out of strong sun, for two to three weeks, until the necks are dry and the skins are papery. Then trim the tops and store them in a cool, dry place with good air flow, such as in mesh bags. Keep them away from potatoes, which give off moisture and gases that make onions spoil faster.

A Note for Pet Owners

Onions are healthy food for people, but they are toxic to dogs and cats, even when cooked or powdered. Keep onions, and foods made with them, away from pets, and call a vet if your pet eats some.

Onion FAQs