Onion
Allium cepa
Amaryllidaceae · onion family
Origin · Central Asia · Western Asia
- Matures
- 3–5 months
- Harvest
- Once · Annual
- Sun
- 6–8+ hrs
- Spacing
- 3–5 in
Planting window
When to sow, by microclimate — foggier blocks run a different calendar than sunnier ones.
| Jan | Feb | Mar | Apr | May | Jun | |
|---|---|---|---|---|---|---|
| Foggy | ● | ● | ● | ● | ||
| Sunny | ● | ● | ● | ● | ● |
| Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|
| Foggy | ● | ● | ● | |||
| Sunny | ● | ● | ● | ● |
Sunlight
How well it does at each level of direct sun per day.
Soil depth
Root room it needs — useful for sizing raised beds and containers.
Soil pH
The soil acidity it grows best in — most crops sit just below neutral, so this mostly matters for heavily amended or long-reused beds.
- Preferred range
- pH 6.0–6.5
Spacing
Room each plant needs — along the row and between rows — for sizing beds.
- In-row
- 3–5 in
- Between rows
- 18 in
Growth habit
The shape it makes in the bed and how tall it gets — what tells you whether to give it a trellis, a stake, or just room.
- Habit
- Upright
- Mature height
- 12–18 in
Stands on its own; the tallest types appreciate a stake in wind.
Growing in containers
Whether it does well in a pot, planter, or window box — the SF balcony-and-deck question, weighing its footprint, form, and staying power, not just root depth.
Happy in a pot, planter, or window box — no garden bed required.
How to start it
Sow it where it grows, or raise it under cover and set it out — and how far ahead to start the seedlings if it's the latter.
- Usually
- Direct-sow
- Start indoors
- 8 weeks ahead
Sown straight into the bed it will grow in.
Getting started
How it goes in the ground — how deep to sow or set it, and the soil temperature it needs to wake up. The cold-soil floor is the real limiter for direct-sowing in a foggy SF spring.
- Started from
- From seed
- Ideal soil temp
- 65–85°F
- Sow depth
- 0.25 in
- Time to emerge
- 4–7 days
Germinates between 32–95°F; happiest around 65–85°F.
Watering & feeding
How thirsty and how hungry it is — the water it wants and how much feeding the bed needs to keep it producing. In the fog belt even a thirsty crop drinks less than it would inland.
- Water need
- Moderate
- Feeding
- Heavy feeder
Wants steady, even moisture — roughly an inch a week, a little more in a heat spell.
Hungry — enrich the bed well before planting and feed through the season to keep it going.
Shallow-rooted and drought-sensitive — keep evenly moist for good bulbs, then stop once the tops fall over so they cure. A heavy feeder.
Pests & diseases
What tends to trouble this crop — what it is, what you'll see, and what to do about it, least-toxic first.
Pests2
Thrips
- What it is
- Most adult thrips are elongate, slender, minute (less than 1/20 inch long), and have long fringes on the margins of both pairs of their long, narrow wings. Immatures (called larvae or nymphs) are oblong or slender and elongate and lack wings.
- What you'll see
- Thrips feeding on plants can damage fruit, leaves, and shoots and very noticeably affect plants’ cosmetic appearance. However, thrips rarely kill or threaten the survival of trees and shrubs.
- What to do
- Thrips species that feed on many different plant species often move into gardens and landscapes when plants in weedy areas or grasslands begin to dry in spring or summer. Avoid planting susceptible plants next to these areas, and control nearby weeds that are alternate hosts of pest thrips.
Onion Maggot
- What you'll see
- White worm that feeds on seedlings, roots or bulbs.
- What to do
- Use crop rotation, avoid excessive amounts of organic matter, and apply insecticides at planting if maggots have been a problem in the past. Destroy infected plants.
Diseases3
Neck Rot
- What you'll see
- Fungal disease that occurs during storage. Watery decay inside the bulb.
- What to do
- Avoid excess moisture near harvest. Cure bulbs well before storing.
Basal Rot
- What you'll see
- Fungal disease that attacks the area where roots and leaves join. The area rots and plant growth is slow and weak.
- What to do
- Use long crop rotations, solarize the soil where onions are planted, and avoid over-watering.
Pink Root
- What you'll see
- Fungal disease that changes roots to a pink color. Roots eventually die and yields are severely reduced
- What to do
- Use long crop rotation and solarize the soil where onions are planted.
Harvest & storage
How to tell it's ready to pick, and how to keep it once it's in.
- Keeps
- MonthsA long keeper — stores for months.
- Ready to harvest
- Harvest after the tops yellow and fall over, about six months from planting.
- Storing
- Cure the bulbs a few weeks in a warm, airy spot until the necks are dry, then store cool, dark and well-ventilated for months.
Culinary use & nutrition
What it's for in the kitchen, and the nutrients a 100 g raw serving is a notable source of.
In the kitchen
- Raw
- Boiled / steamed
- Roasted
- Soups & stews
- Seasoning
Nutrition highlights
Not a standout source of any nutrient we track, per 100 g.
Share of the Daily Value in a 100 g raw serving (USDA FoodData Central).
Pollinators & wildlife
What this crop offers the garden's wildlife once it flowers — often the payoff for letting a bolting plant stand.
- Bees
- Honeybees and natives — bumble, mason, and sweat bees — working the flowers for pollen and nectar.
- Beneficial insects
- Hoverflies, lady beetles, and parasitoid wasps that keep aphids and caterpillars down.
Bulbs are lifted long before bloom, but an onion left to flower carries a papery white umbel that native sweat bees favor and hoverflies feed at.
Companion planting
What grows well nearby, and what to keep apart.
No research-backed companion pairings for this crop — folk pairings without evidence are left out on purpose.
Interplanted among cole crops, onions helped cut cabbage looper and imported cabbageworm damage in broccoli and cabbage trials.
Sources
Growing data on this page is drawn from Xerces Society for Invertebrate Conservation, Utah State University Extension, and the other references on our sources page.