Habit (mature) - Colin / Wikimedia Commons - CC BY-SA 3.0
Limited coverage
Onion
Allium cepa
A biennial Allium grown as an annual for its bulb. Day-length sensitive — northern gardens need long-day cultivars; southern gardens need short-day cultivars; intermediate-day cultivars suit zones 6-7. Planted in spring (sets) or fall (over-wintering); harvested in summer when tops fall over. Edible-as-greens at any stage (green onions / scallions).
Edible
Light
Full sun
Water
Moderate water
Mature size
12-24" tall · 4" apart
Hardy in zones
Annual everywhere
AHS heat range
1-11
Plant range authored in AHS heat-zone terms.
Native in Illinois
No
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Edible bulb + greens (scallions).
Cold hardiness
Future
This plant is grown as an annual; hardiness zones don't apply.
Heat tolerance
Future
Heat tolerance values are location-based too: heat days today are observed at this site, and the 2050 value projects this same location under a future climate.
Loading AHS heat-zone data for this location...
Sources & citations
Cite this page
For lesson plans, articles, or research that uses this page. To cite a single upstream fact instead, use its specific source listed below.
Plotwright. (2026, May 17). Onion (Allium cepa). Retrieved 2026, June 14, from https://plotwright.garden/plants/allium-cepa
Sources for every fact
Every fact on this page traces to a source. 18 fields cited - 18 source-backed.
NC State Extension Gardener Plant Toolbox
University extension service
Backs 17 fields
Identity
Summary
Plant type
Light
Moisture
Hardiness
Heat zone
Size
Spacing
Habit
Design roles
Seasonal interest
Growth stages
Lifecycle
Regional guidance
Success tips
Designer notes