How much sun do onions need?

Onions need full sun, at least 6 to 8 hours of direct sunlight per day, to grow best.

What kind of soil is best for onions?

They prefer well-drained soil rich in organic matter with a slightly acidic to neutral pH.

How often should I water onions?

Keep the soil consistently moist, especially during bulb formation, but avoid waterlogging.

When are onions ready to harvest?

Harvest when the tops begin to yellow and fall over, usually in mid to late summer.

Can I grow onions in containers?

Yes, onions can be grown in containers, provided they are deep enough to accommodate the bulbs.

Plant guide

Spring onion.

Allium cepa

Pronounced AL-ee-um SEE-pa

Also known as Garden Onion, Onion

Common Onion — Photo by petitcrabe (CC BY) via iNaturalist

Spring onions, also known as green onions or scallions, are a versatile allium with a mild onion flavor. They are typically harvested when young, before a large bulb forms, and are used in countless dishes worldwide.

At a glance

Care level: Easy
Plant type: Vegetable, Herb
Light: Full sun
Indoor light: direct
Water: Every 2 to 7 days
Mature size: 12-18 inches tall
Growth rate: medium
Hardiness: USDA 3 to 10
Bloom time: Late spring to summer

Care

Plant in well-draining soil and provide full sun. Keep the soil consistently moist, especially during dry spells. Harvest by snipping the greens or pulling the entire plant.

Watering: Keep soil consistently moist but not waterlogged; water more frequently in hot, dry weather.
Feeding: Fertilize lightly every 30 days with a balanced fertilizer, especially during active growth.
Pruning: Harvest outer leaves as needed, or cut the entire plant at the base. Pruning encourages new growth.
Repotting: Spring onions are often grown as annuals and may not require frequent repotting if planted directly in the ground. If container-grown, repot when root-bound.
Soil: Well-draining, fertile soil
Soil pH: 6.0-7.0
Temperature: 4 to 29°C
Humidity: 40 to 60%
Drought tolerance: moderate
Lifespan: Biennial, typically grown as an annual
Fragrance: strong
Deer and rabbits: moderate

Safety

People: Onions are generally safe for human consumption in typical amounts, though some individuals may experience digestive upset.
Pets: Non-toxic to cats and dogs.
Children: Small children might be sensitive to the strong smell or taste, and large pieces could pose a choking hazard if not properly prepared.
Contact allergens: The sap can cause mild skin irritation in sensitive individuals.
Airborne allergens: Pollen can be an airborne allergen for some people, especially during flowering.
Edibility: While onions are edible, this information is for general gardening purposes and not a guide to foraging or wild edibility.

How to identify it

Long, hollow green leaves
White, unswollen stem base
Mild, fresh onion aroma
Forms small bulbs if left to mature
Edible flowers in some varieties

Height: 0.3 to 0.45m
Spread: 0.06 to 0.15m
Leaves: Green, White, evergreen
Flowers: White, Pinkish, 0.5 to 1.5cm

Common problems

Onion Maggot — Small white maggots tunnel into bulbs and roots, causing wilting and decay.
Downy Mildew — Causes purplish-gray fuzzy growth on leaves, leading to yellowing and plant death.
Thrips — Tiny insects that feed on leaves, causing silvery streaks and distorted growth.
Basal Rot — A fungal disease that affects the roots and base of the bulb, causing rot and wilting.

Growing it

Propagation: Seed, Division
When to plant: Spring, Fall
Overwintering: Onions are typically grown as annuals and harvested before winter. If overwintering is intended for specific varieties in milder climates, ensure they are well-mulched to protect the bulbs from extreme cold and heaving.
In containers: Onions grow well in containers, especially smaller varieties. Use pots at least 8-10 inches deep to allow bulbs to form.

From Seed

Sow seeds indoors 6-10 weeks before the last frost date, or directly outdoors after the danger of frost has passed.
Plant seeds about 1/4 inch deep in well-draining soil.
Keep the soil consistently moist and provide adequate light.
Transplant seedlings outdoors when they are about pencil-thick, spacing them 4-6 inches apart.
Ensure they receive full sun and consistent moisture.

From Sets

Plant onion sets (small, immature bulbs) directly into the garden in early spring.
Ensure the soil is well-drained and has good fertility.
Plant sets about 1-2 inches deep and 4-6 inches apart.
Water regularly, especially during dry periods.
Harvest when the tops begin to yellow and fall over.

Plant onions near root vegetables like carrots and beets, but keep them away from legumes and asparagus.

Grows well with

Carrots — Onions can deter carrot rust flies, and carrots help break up the soil for onion roots.
Lettuce — Lettuce has a shallow root system and benefits from the shade provided by taller onion plants.
Chamomile — Chamomile is thought to improve the growth and flavor of onions.
Beets — These two crops do not compete for nutrients and can grow well together.

Keep away from

Asparagus — Onions can stunt the growth of asparagus due to chemical compounds they release.
Beans — Onions may inhibit the growth of certain types of beans.
Peas — Similar to beans, onions can negatively affect the growth of peas.

Similar species

Garlic — Garlic is closely related and shares similar growth habits and some companion planting benefits.
Leeks — Leeks are also members of the Allium family and have comparable cultivation needs.
Shallots — Shallots are a type of onion with a milder flavor and similar growing requirements.

Science

Family: Amaryllidaceae
Genus: Allium
Order: Asparagales
Class: Liliopsida
Native range: Central Asia
Conservation: Allium cepa is a widely cultivated species and not considered to be of conservation concern.

As a cultivated plant, Allium cepa relies on human intervention for propagation and survival, though feral populations can sometimes establish in disturbed areas.

Value to wildlife: nectar source
Pollinators: bees, butterflies, moths
Uses: culinary, medicinal

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 3/9
Soil reaction: 6.5/9
Moisture: 5/9
Nutrients: 7/9
Light: 8/9
Temperature: 7/9

Taxonomy resolved against GBIF.

Questions people ask

How much sun do onions need?: Onions need full sun, at least 6 to 8 hours of direct sunlight per day, to grow best.
What kind of soil is best for onions?: They prefer well-drained soil rich in organic matter with a slightly acidic to neutral pH.
How often should I water onions?: Keep the soil consistently moist, especially during bulb formation, but avoid waterlogging.
When are onions ready to harvest?: Harvest when the tops begin to yellow and fall over, usually in mid to late summer.
Can I grow onions in containers?: Yes, onions can be grown in containers, provided they are deep enough to accommodate the bulbs.

Where the name comes from

The scientific name 'Allium' is the ancient Latin name for garlic, and 'cepa' is derived from the Latin word for 'onion'.

Good to know

Onions contain sulfur compounds, which are responsible for their pungent smell and taste.
The tear-inducing chemical in onions is released when the cells are broken, like when chopping.
Different varieties of onions have distinct flavors, ranging from sweet to sharp.
Onions can be stored for long periods if kept in a cool, dry, and dark place.

Is your spring onion looking unwell?

This page is a general guide. Pocket Botanist diagnoses problems from a photo of your actual plant, works out watering for your light and conditions, and reminds you when care is due.

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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.