When to plant onions in zone 7b

Transplant outdoors around Feb 23. That is 6 weeks before zone 7b's median last frost, Apr 51. Across most of zone 7b that date falls between Feb 17 and Mar 6.

Why a range: zone 7b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 30 to Apr 172, 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: 639
ZIP codes: 2,958

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: Dec 9 to Dec 27Start indoors at the zone median: Dec 15 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Feb 17 to Mar 6Transplant outdoors at the zone median: Feb 23 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Mar 2 to Apr 3Sow outdoors at the zone median: Mar 8 to Mar 22 (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 7b. 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 1516 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Feb 23range Feb 17 – Mar 6

What each source says, applied to zone 7b

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 7b's median frost dates.

Table 1. Onions rules from every source, at zone 7b's median last frost (Apr 5) and first frost (Nov 1).
Source	What	Rule	Zone 7b median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Mar 8 – Mar 22
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Feb 23
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Dec 15
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Jan 26 – Mar 8
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Feb 9 – Feb 23
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Aug 23 – Sep 6

Frost across zone 7b

Last spring frostmedian Apr 5Full range across the zone: Mar 21 to Jun 3Most of the zone (10th to 90th percentile): Mar 30 to Apr 17Zone median: Apr 5
First fall frostmedian Nov 1Full range across the zone: Sep 14 to Nov 26Most of the zone (10th to 90th percentile): Oct 24 to Nov 14Zone median: Nov 1

Frost-free season (median)

Figure 2. Frost dates across zone 7b, from 639 NOAA stations matched to 2,958 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.