Biennial allium grown as an annual for storage bulbs. Variety choice depends as much on latitude as on hardiness zone.
Hardiness ratings
| System | Rating | Temperature range | How to read it |
|---|---|---|---|
| USDA hardiness zone | Zone 3–10 | −40 °C to 4.4 °C | Zones where it can be grown as an annual — not a frost-tolerance rating |
| RHS hardiness rating | H7 | down to −20 °C | Plant needs at least this level of cold tolerance |
| Canadian plant hardiness zone | Zone 3–9 | −34 °C and warmer | Zones where it can be grown as an annual — not a frost-tolerance rating |
| Australian (ANBG) zone | Zone 1–6 | −15 °C to 15 °C | Zones where it can be grown as an annual — not a frost-tolerance rating |
As a tender annual, Onion doesn't overwinter — the zone range shows where the growing season supports it. See the RHS rating for its actual cold tolerance.
About Onion
Onions are biennials grown as annuals, and when they bulb up is governed by day length rather than by temperature or age. Varieties are classified as long-day, intermediate or short-day, and planting a long-day variety in a short-day region produces plants that grow well and simply never form a decent bulb — one of the most common and least obvious causes of failure. The tear response comes from a volatile sulphur compound released when cells are cut, which reacts with moisture in the eye. Cured, dry-skinned bulbs store for months.
Growing notes
- Long-day cultivars (>14 hr daylight) bulb up north of ~35° latitude; short-day cultivars (10–12 hr) suit south of ~28°; intermediate-day for the band between
- Choosing the wrong day-length type is the most common reason home-grown onions fail to bulb
Pet caution: Onion is listed as potentially harmful to cats and/or dogs. Keep pets from grazing on it, and contact a vet if you suspect your animal has eaten some.
Categories
Related plants
Cross-check Onion against your zones