When should I harvest?

Harvest when the tops begin to yellow and fall over.

Can I grow onions in pots?

Yes, they grow well in containers at least 20 centimeters deep.

Plant guide

Onion.

Allium cepa

Pronounced AL-ee-um SEE-pah

Also known as Garden onion, Onion

The onion is a popular biennial vegetable grown for its edible, pungent bulb. It is a staple in gardens worldwide because it is easy to grow from seeds or sets. You will enjoy harvesting these once the tops begin to yellow and fall over.

At a glance

Care level: Easy
Plant type: Vegetable
Light: Full sun
Indoor light: Direct sun
Water: Every 3 to 7 days
Mature size: 1 to 2 feet tall
Growth rate: Medium
Hardiness: USDA 3 to 9
Bloom time: Late spring to summer

Care

Plant in loose, well draining soil and ensure they get plenty of sun. Keep the soil consistently moist but not waterlogged to prevent bulb rot. Watch for yellowing foliage which signals that the bulb is ready for harvest.

Watering: Water deeply whenever the top inch of soil feels dry to the touch.
Feeding: Apply a balanced fertilizer once a month during the active growing season.
Pruning: Pruning is generally not required unless you are removing damaged or dead foliage.
Repotting: Onions are typically grown directly in the garden rather than in containers.
Soil: Well draining, fertile, loamy soil
Soil pH: 6.0 to 7.0
Temperature: 5 to 30°C
Humidity: 40 to 60%
Drought tolerance: Moderate
Lifespan: Biennial, usually grown as an annual.
Fragrance: Strong
Deer and rabbits: High

Signs it needs repotting

Roots growing out of drainage holes
Bulb becoming crowded in the container

Safety

People: Raw onions can cause mild digestive irritation in some individuals.
Pets: Onions are toxic to cats and dogs as they can cause damage to red blood cells.
Children: Onions are generally safe, but small bulbs can be a choking hazard for toddlers.
Contact allergens: Handling raw onions can cause skin irritation or dermatitis in sensitive individuals.
Airborne allergens: The volatile sulfur compounds released when cutting onions can irritate the eyes and respiratory tract.
Edibility: Onions are safe for humans but contain thiosulfate, which is toxic to dogs and cats.

How to identify it

Hollow, cylindrical blue green leaves
Underground fleshy bulb
Small white or purple flowers
Strong sulfurous aroma when cut

Height: 0.3 to 0.9m
Spread: 0.1 to 0.2m
Leaves: blue-green, deciduous
Flowers: white, lavender, 5 to 10cm
Fruit: brown, red, white, yellow, Late summer to autumn

Problems to watch for

Downy MildewCommon
Onion FlyCommon
Onion ThripsCommon
BoltingUncommon
Root RotUncommon
Soft RotUncommon
White RotRare

By symptom

Check your onion from a photo

Pocket Botanist looks at a photo of your plant and tells you what's wrong and what to try first.

Growing it

Propagation: seed, sets
When to plant: early spring, late summer
Overwintering: In cold climates, mulch heavily to protect the bulbs if they are left in the ground. Otherwise, store harvested bulbs in a cool, dry, and dark place.
In containers: Onions grow well in containers, provided the pot is at least 20 centimeters deep and has excellent drainage.

Seed Sowing

1. Sow seeds indoors 8 to 10 weeks before the last frost.
2. Keep soil moist and warm.
3. Transplant seedlings outdoors once they are pencil thin.

Plant onions near carrots to confuse pests that target both crops.

Grows well with

Carrot — Onions help repel carrot rust flies.
Lettuce — They make excellent neighbors due to their shallow root systems.
Tomato — Onions may help deter pests that bother tomato plants.

Keep away from

Pea — Onions can stunt the growth of peas.
Bean — Onions are known to inhibit the growth of beans.

Similar species

Shallot — They are closely related and share similar growth habits.
Chives — They are smaller relatives often grown for their edible leaves.

Science

Family: Amaryllidaceae
Genus: Allium
Order: Asparagales
Class: Liliopsida
Native range: Central Asia
Conservation: Not of conservation concern as it is a widely cultivated crop.

Onions survive by storing energy in a bulb to endure periods of drought or cold.

Value to wildlife: nectar source
Pollinators: bees, butterflies
Uses: culinary flavoring, companion planting, ornamental garden border

Through the year

JFMAMJJASOND

Flowering

Fruiting

Foliage

Ecological indicators

Moisture: 5/9
Nutrients: 7/9
Light: 8/9
Salinity: 2/9
Soil reaction: 6/9
Temperature: 6/9

Taxonomy resolved against GBIF.

Questions people ask

When should I harvest?: Harvest when the tops begin to yellow and fall over.
Can I grow onions in pots?: Yes, they grow well in containers at least 20 centimeters deep.

Where the name comes from

The name Allium is the Latin word for garlic, while cepa is the Latin word for onion.

Good to know

Onions are members of the Amaryllidaceae family.
The chemical that makes you cry is syn-propanethial-S-oxide.
Onions are composed of about 89 percent water.
They have been used as currency in some ancient cultures.

Also in

Keep your onion growing

This page is a general guide. Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked.

Get the app

This guide was generated with AI and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.