Onion — Growing reference
| Scientific name | Allium cepa |
|---|---|
| Seed germination (days) | 10–14 |
| Soil temperature for germination (°C) | 15.6–23.9 |
| First harvest or flowering (days) | 100–120 |
| Measured from | Transplant |
| Plant spacing (cm) | 10.2–15.2 |
| Mature height (cm) | 45.7–61 |
| Mature spread (cm) | 10.2–15.2 |
| USDA hardiness zones | 2–11 |
| Light | Full sun |
| Water | Moderate |
| Sowing method | indoor-start |
Growing directions
Start seeds indoors 10–12 weeks before the outdoor transplant date in late winter, keeping them in bright light to prevent leggy growth; transplant pencil-thick seedlings outdoors as early as the ground can be worked and plant 4–6 inches apart in well-drained, fertile, slightly acidic soil. Onion bulb size is determined by day length: long-day varieties (zone 3–6) bulb in summer when days exceed 14–16 hours; short-day varieties (zone 7–10) bulb in spring when days are 10–12 hours; intermediate-day varieties work in zone 5–7. Keep well-weeded as onions compete poorly with weeds, and water evenly throughout bulb development to prevent cracking. Stop watering when tops begin to fall over naturally; cure bulbs in a warm, dry, well-ventilated location for 2–4 weeks before storage.
Source and local conditions
Typical planning ranges from OpenPlantDB (CC0). Cultivar, climate, season and growing method can change these values. Confirm the seed label and local guidance before planting.
Source & verification
OpenPlantDB · CC0