When to plant onions in zone 5b

Transplant outdoors around Mar 20. That is 6 weeks before zone 5b's median last frost, May 11. Across most of zone 5b that date falls between Mar 10 and Apr 5.

Why a range: zone 5b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 21 to May 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: 687
ZIP codes: 2,445

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 31 to Jan 26Start indoors at the zone median: Jan 10 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Mar 10 to Apr 5Transplant outdoors at the zone median: Mar 20 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Mar 24 to May 3Sow outdoors at the zone median: Apr 3 to Apr 17 (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 5b. 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: Jan 1016 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 20range Mar 10 – Apr 5

What each source says, applied to zone 5b

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 5b's median frost dates.

Table 1. Onions rules from every source, at zone 5b's median last frost (May 1) and first frost (Oct 11).
Source	What	Rule	Zone 5b median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Apr 3 – Apr 17
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Mar 20
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Jan 10
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Feb 21 – Apr 3
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Mar 6 – Mar 20
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Aug 2 – Aug 16

Frost across zone 5b

Last spring frostmedian May 1Full range across the zone: Apr 8 to Jun 21Most of the zone (10th to 90th percentile): Apr 21 to May 17Zone median: May 1
First fall frostmedian Oct 11Full range across the zone: Sep 2 to Oct 29Most of the zone (10th to 90th percentile): Oct 1 to Oct 19Zone median: Oct 11

Frost-free season (median)

Figure 2. Frost dates across zone 5b, from 687 NOAA stations matched to 2,445 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.