When to plant onions in zone 8b

Transplant outdoors around Feb 4. That is 6 weeks before zone 8b's median last frost, Mar 171. Across most of zone 8b that date falls between Jan 24 and Feb 19.

Why a range: zone 8b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 6 to Apr 12, so a single zone date can be weeks off for you. The rule comes from Oregon State University Extension Service.

Last reviewed: 2026-11-02
Stations: 652
ZIP codes: 2,535

Photo: Halimatu Sa'diah - Koruch / Pexels

These dates use the typical last frost for the whole zone. Your ZIP code gives a closer answer.

Start indoorsOregon State, central ORStart indoors across most of the zone: Nov 15 to Dec 11Start indoors at the zone median: Nov 26 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Jan 24 to Feb 19Transplant outdoors at the zone median: Feb 4 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Feb 7 to Mar 18Sow outdoors at the zone median: Feb 18 to Mar 3 (Colorado State)

Start indoors
Plant outdoors
Range across the zone
Frost-free season (median)
Spring frost risk (90% to 10%)
Fall frost risk (10% to 90%)

Figure 1. Onions in zone 8b. Dark bars: at the zone's median frost dates. Pale bars: across most of the zone's ZIP codes. Hatched blue: where the zone's last and first frosts fall.

Start seeds indoors: Nov 2616 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Feb 4range Jan 24 – Feb 19

What each source says, applied to zone 8b

The same frost dates give different answers depending on whose rule you follow. These are all the published rules we hold for onions, worked out at zone 8b's median frost dates.

Table 1. Onions rules from every source, at zone 8b's median last frost (Mar 17) and first frost (Nov 13).
Source	What	Rule	Zone 8b median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Feb 18 – Mar 3
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Feb 4
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Nov 26
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Jan 7 – Feb 18
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Jan 21 – Feb 4
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Sep 4 – Sep 18

Frost across zone 8b

Last spring frostmedian Mar 17Full range across the zone: Feb 11 to Jun 3Most of the zone (10th to 90th percentile): Mar 6 to Apr 1Zone median: Mar 17
First fall frostmedian Nov 13Full range across the zone: Sep 14 to Dec 13Most of the zone (10th to 90th percentile): Nov 4 to Nov 23Zone median: Nov 13

Frost-free season (median)

Figure 2. Frost dates across zone 8b, from 652 NOAA stations matched to 2,535 ZIP codes.

Planting onions with others

Carrots 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.5 5 5 6 7 7

Sources

Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
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.
Washington State University Extension, Gardening with Companion Plants (Home Garden Series), EM128E, December 2023. Retrieved 2026-11-02.
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.