When to plant onions in zone 8a

Transplant outdoors around Feb 16. That is 6 weeks before zone 8a's median last frost, Mar 291. Across most of zone 8a that date falls between Feb 7 and Feb 25.

Why a range: zone 8a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 20 to Apr 72, 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: 650
ZIP codes: 2,528

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 29 to Dec 17Start indoors at the zone median: Dec 8 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Feb 7 to Feb 25Transplant outdoors at the zone median: Feb 16 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Feb 21 to Mar 24Sow outdoors at the zone median: Mar 1 to Mar 15 (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 8a. 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: Dec 816 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Feb 16range Feb 7 – Feb 25

What each source says, applied to zone 8a

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 8a's median frost dates.

Table 1. Onions rules from every source, at zone 8a's median last frost (Mar 29) and first frost (Nov 6).
Source	What	Rule	Zone 8a median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Mar 1 – Mar 15
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Feb 16
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Dec 8
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Jan 19 – Mar 1
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Feb 2 – Feb 16
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Aug 28 – Sep 11

Frost across zone 8a

Last spring frostmedian Mar 29Full range across the zone: Feb 28 to Jun 8Most of the zone (10th to 90th percentile): Mar 20 to Apr 7Zone median: Mar 29
First fall frostmedian Nov 6Full range across the zone: Sep 22 to Nov 26Most of the zone (10th to 90th percentile): Oct 31 to Nov 14Zone median: Nov 6

Frost-free season (median)

Figure 2. Frost dates across zone 8a, from 650 NOAA stations matched to 2,528 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.