Onion
Allium cepa
Amaryllidaceae · onion family
- Matures
- 90–150 days
- 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 | ● | ● | ● | ● |
Well-suitedMarginalNot suited
Sunlight
How well it does at each level of direct sun per day.
<2 hrsNot suited
2–4 hrsNot suited
4–6 hrsMarginal
6–8 hrsWell-suited
8+ hrsWell-suited
Well-suitedMarginalNot suited
Soil depth
Root room it needs — useful for sizing raised beds and containers.
6–10"Well-suited
10–15"Well-suited
15–18"Well-suited
18–24"Well-suited
Well-suitedMarginalNot suited
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.
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
Direct-sowWell-suited
TransplantWell-suited
Sown straight into the bed it will grow in.
Well-suitedMarginalNot suited
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.
30°F110°F
IdealWorkable
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.
Sources
Every figure above is backed by these.
- UC Master Gardeners, San Mateo–SF Countiesbacks plantingWindow, startMethod · Foggy & Sunny calendars, Onion row (bulb, from seed/sets/transplant): foggy Jan-Apr & Sep-Nov; sunny Jan-May & Aug-Nov. Start method: Foggy & Sunny calendars, Onion row: sets, (S) sow seed and (T) transplant all listed.
- UC Master Gardeners, Sonoma County (Planting Summary)backs daysToMaturity, inRowSpacing, rowSpacing, startMethod · Vegetable Planting Summary, Onions, bulb row: 90-150 days, 4-5 in in-row spacing. Between-row spacing 18 in (Onions, bulb row). Start method: Vegetable Planting Summary, Onions, bulb row: method D/T — either, direct-sowing usual.
- Utah State University Extensionbacks soilDepth, inRowSpacing, daysToMaturity · 'How to Grow Onions in Your Garden' (USU Extension): 'rooting depth in onions is shallow' (shallow-rooted); '3-4 inches apart in the row' (corroborates spacing); 'Bulb onions require 100-120 days to mature' (corroborates days-to-maturity).
- Golden Gate Gardening (Pam Peirce)backs sunlight, lifecycle, harvestStyle · Onion is a full-sun allium; bulbing needs strong light. Habit and harvest: annual, once — from this source's entry for this crop.
- UC Master Gardeners, Sacramento County (Germination)backs germination · GN 154 soil-temp table, onions row: 32/65–85/95°F; germinationDays 4–7 derived from Table 2 across the optimum band.
- USDA GRIN Taxonomybacks botany
- Colorado State University Extension (Master Gardener)backs plantingDepth · Table 1 Cool Season, Onion dry (seed) row: 1/4 in.
- NC State Extension — Plant Toolboxbacks growthHabit · Allium cepa (Onion) entry: Height '1 ft. 0 in. - 1 ft. 6 in.' (12-18 in); Form 'Erect' and 'Arching' — hollow round upright leaves in a basal (rosulate) cluster over the bulb, so classified upright.
- UC Master Gardeners, Santa Clara Countybacks startMethod · Start method: Planting chart: Onions carry a Transplant row; From seed to transplant 8 weeks.