When to plant onions in zone 4b

Transplant outdoors around Mar 27. That is 6 weeks before zone 4b's median last frost, May 81. Across most of zone 4b that date falls between Mar 20 and Apr 13.

Why a range: zone 4b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 1 to May 252, 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: 443
ZIP codes: 1,127

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: Jan 10 to Feb 3Start indoors at the zone median: Jan 17 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Mar 20 to Apr 13Transplant outdoors at the zone median: Mar 27 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Apr 3 to May 11Sow outdoors at the zone median: Apr 10 to Apr 24 (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 4b. 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 1716 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 27range Mar 20 – Apr 13

What each source says, applied to zone 4b

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 4b's median frost dates.

Table 1. Onions rules from every source, at zone 4b's median last frost (May 8) and first frost (Oct 1).
Source	What	Rule	Zone 4b median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Apr 10 – Apr 24
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Mar 27
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Jan 17
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Feb 28 – Apr 10
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Mar 13 – Mar 27
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Jul 23 – Aug 6

Frost across zone 4b

Last spring frostmedian May 8Full range across the zone: Apr 22 to Jun 18Most of the zone (10th to 90th percentile): May 1 to May 25Zone median: May 8
First fall frostmedian Oct 1Full range across the zone: Sep 1 to Oct 20Most of the zone (10th to 90th percentile): Sep 22 to Oct 8Zone median: Oct 1

Frost-free season (median)

Figure 2. Frost dates across zone 4b, from 443 NOAA stations matched to 1,127 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.