When to plant onions in zone 4a

Transplant outdoors around Mar 31. That is 6 weeks before zone 4a's median last frost, May 121. Across most of zone 4a that date falls between Mar 24 and Apr 12.

Why a range: zone 4a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 5 to May 242, 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: 276
ZIP codes: 720

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 14 to Feb 2Start indoors at the zone median: Jan 21 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Mar 24 to Apr 12Transplant outdoors at the zone median: Mar 31 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Apr 7 to May 10Sow outdoors at the zone median: Apr 14 to Apr 28 (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 4a. 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 2116 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 31range Mar 24 – Apr 12

What each source says, applied to zone 4a

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 4a's median frost dates.

Table 1. Onions rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
Source	What	Rule	Zone 4a median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Apr 14 – Apr 28
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Mar 31
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Jan 21
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Mar 3 – Apr 14
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Mar 17 – Mar 31
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Jul 20 – Aug 3

Frost across zone 4a

Last spring frostmedian May 12Full range across the zone: May 2 to Jun 14Most of the zone (10th to 90th percentile): May 5 to May 24Zone median: May 12
First fall frostmedian Sep 28Full range across the zone: Sep 1 to Oct 9Most of the zone (10th to 90th percentile): Sep 20 to Oct 4Zone median: Sep 28

Frost-free season (median)

Figure 2. Frost dates across zone 4a, from 276 NOAA stations matched to 720 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.