When to plant onions in zone 9a

Transplant outdoors around Jan 20. That is 6 weeks before zone 9a's median last frost, Mar 21. Across most of zone 9a that date falls between Jan 5 and Feb 5.

Why a range: zone 9a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Feb 16 to Mar 182, 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: 432
ZIP codes: 1,671

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: Oct 27 to Nov 27Start indoors at the zone median: Nov 11 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Jan 5 to Feb 5Transplant outdoors at the zone median: Jan 20 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Jan 19 to Mar 4Sow outdoors at the zone median: Feb 3 to Feb 17 (Colorado State)
Plant for fallTexas A&M (E-502)Plant for fall across most of the zone: Sep 7 to Oct 14Plant for fall at the zone median: Sep 17 to Oct 1 (Texas A&M (E-502))

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 9a. 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 1116 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Jan 20range Jan 5 – Feb 5
Fall planting: Sep 178-10 weeks before first frost

What each source says, applied to zone 9a

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 9a's median frost dates.

Table 1. Onions rules from every source, at zone 9a's median last frost (Mar 2) and first frost (Nov 26).
Source	What	Rule	Zone 9a median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Feb 3 – Feb 17
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Jan 20
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Nov 11
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Dec 23 – Feb 3
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Jan 6 – Jan 20
Texas A&M (E-502)4Used above	Plant for fall	8-10 weeks before first frost	Sep 17 – Oct 1

Frost across zone 9a

Last spring frostmedian Mar 2Full range across the zone: Jan 6 to Jun 3Most of the zone (10th to 90th percentile): Feb 16 to Mar 18Zone median: Mar 2
First fall frostmedian Nov 26Full range across the zone: Sep 14 to Feb 1Most of the zone (10th to 90th percentile): Nov 16 to Dec 9Zone median: Nov 26

Frost-free season (median)

Figure 2. Frost dates across zone 9a, from 432 NOAA stations matched to 1,671 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.