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Onions

When to start, transplant and harvest onions, as 5 university extension services put it, and what that means in each zone.

Family
Liliaceae5
Type
Bulb
Rules
9
Last reviewed
Garden experts do not always agree, so I show every rule side by side.

Five digits. The calendar is computed on your device.

Spring timing, source by source

Every rule counts from the last spring frost, so the same rule gives different dates in different gardens. Here are the rules themselves before any dates are worked out.

  1. Colorado StateSow outdoors
  2. Oregon State, central ORTransplant outdoors
  3. Oregon State, central ORStart indoors
  4. Texas A&M (E-502)Transplant outdoors
  5. Texas A&M (E-502)Sow outdoors
  • Plant outdoors
  • Start indoors
Figure 1. How each extension source times onions in spring, in weeks before or after the last frost. Sources disagree; each bar is one publication's rule, shown as written.

Fall timing, source by source

Fall rules count from the first fall frost.

  1. Texas A&M (E-502)Plant for fall
  • Plant outdoors
Figure 2. How each extension source times onions in fall, in weeks before or after the first frost. Rules written for warm states sit weeks earlier than those for cold ones.
Table 1. Planting rules for onions, as published.
SourceWritten forWhatWhen
Colorado State1COSow outdoors2-4 weeks before last frost
Oregon State, central OR8central OregonTransplant outdoors6 weeks before last frost
Oregon State, central OR8central OregonStart indoors16 weeks before last frost
Texas A&M (E-502)6TXTransplant outdoors4-10 weeks before last frost
Texas A&M (E-502)6TXSow outdoors6-8 weeks before last frost
Texas A&M (E-502)6TXPlant for fall8-10 weeks before first frost
Purdue (HO-66)3INPlant for fallCount back days to maturity from the first frost

Onions by hardiness zone

Applied to the frost records of every ZIP in each zone. The zone median is a starting point; your own station can be weeks earlier or later.

Table 2. Outdoor planting for onions by zone, using the Oregon State, central OR rule. Linked zones have their own page.
ZoneWhatZone medianAcross most of the zone
4aTransplant outdoorsMar 31Mar 24 – Apr 12
4bTransplant outdoorsMar 27Mar 20 – Apr 13
5aTransplant outdoorsMar 22Mar 15 – Apr 8
5bTransplant outdoorsMar 20Mar 10 – Apr 5
6aTransplant outdoorsMar 17Mar 5 – Mar 29
6bTransplant outdoorsMar 11Feb 28 – Mar 22
7aTransplant outdoorsMar 5Feb 22 – Mar 17
7bTransplant outdoorsFeb 23Feb 17 – Mar 6
8aTransplant outdoorsFeb 16Feb 7 – Feb 25
8bTransplant outdoorsFeb 4Jan 24 – Feb 19
9aTransplant outdoorsJan 20Jan 5 – Feb 5

Growing facts

Table 3. Each value as one source states it. Sources measure differently, so values are not averaged.
FactValueSource
Frost tolerancehardyColorado State1
Frost tolerancehardyUtah State2
Frost tolerancehardyPurdue (HO-66)3
Frost tolerancesemi hardyNebraska (G1343)4
Minimum soil temperature35°FColorado State1
Best soil temperature80°FColorado State1
Days to germinate7-12 daysColorado State1
Days to maturity110 days from seedColorado State1
Days to maturity100-130 days from seedUF/IFAS Florida5
Days to maturity90-120 days from seedTexas A&M (E-502)6
Days to maturity40 days from transplantPurdue (HO-66)3
Spacing4-6 in apart each wayColorado State1
Spacing4-6 in in the row, rows 14 in apartUF/IFAS Florida5
Spacing2-3 in in the row, rows 14-24 in apartTexas A&M (E-502)6
Spacing2 in in the rowPurdue (HO-66)3
Spacing3 in in the row, rows 12-24 in apartClemson7
Heat limit90°FColorado State1

Companions, and how sure anyone is

  • Carrots · Helps Tested in a study

    Intercropped onions reduced carrot fly (Psila rosae) attack on carrots, probably because onion volatiles disrupt host finding; onion thrips on the onions were also reduced.999101111

    Conditions (Uvah & Coaker 1984, abstract only): effect greatest with young onions and higher onion:carrot plant density ratios, and virtually ineffective once onions began to bulb. Matters only where carrot fly is a pest. WSU EM128E separately advises pairing taproot carrots with shallow-rooted onions to limit root competition (guidance, no study cited for this pair).

  • Beans · Avoid together Traditional pairing11

  • Peas · Avoid together Traditional pairing11

  • Tomatoes · Helps Traditional pairing11

    The chart says "onion family".

  • Lettuce · Helps Traditional pairing11

Traditional pairings come from a UC Master Gardener chart that traces to the UC Davis Edible Landscaping Handbook and ATTRA. The chart gives no reasons, and we found no trial of these pairs; treat them as garden lore.

All onions companions, with sources · The full companion chart

Sources

  1. Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
  2. Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. Retrieved 2026-11-02.
  3. Purdue Extension, The Fall Vegetable Garden (HO-66-W), reviewed September 2019. Retrieved 2026-11-02.
  4. University of Nebraska–Lincoln Extension, Fall Vegetable Gardening (NebGuide G1343), Revised March 2012. Retrieved 2026-11-02.
  5. University of Florida, IFAS Extension (Department of Horticultural Sciences), Florida Vegetable Gardening Guide (SP 103 / VH021), Original December 1999; revised October 2015, January 2016, May 2018, September 2020, September 2021, and June 2025. Retrieved 2026-11-02.
  6. Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
  7. Clemson Cooperative Extension, Home & Garden Information Center, Onion, Leek, Shallot, & Garlic (HGIC 1314), Updated: Jan 20, 2023. Retrieved 2026-11-02.
  8. Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
  9. Entomologia Experimentalis et Applicata 36(2):159-167 (John Wiley & Sons); abstract read via FAO AGRIS, Effect of mixed cropping on some insect pests of carrots and onions, 1984. Retrieved 2026-11-02.
  10. Washington State University Extension, Gardening with Companion Plants (Home Garden Series), EM128E, December 2023. Retrieved 2026-11-02.
  11. University of California Agriculture and Natural Resources (UCCE Master Gardener Program), Companion Planting Chart: Companion planting in the vegetable garden, File dated 2023-09 in URL path; no date printed. Retrieved 2026-11-02.