When to plant onions in zone 6a

Transplant outdoors around Mar 17. That is 6 weeks before zone 6a's median last frost, Apr 281. Across most of zone 6a that date falls between Mar 5 and Mar 29.

Why a range: zone 6a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 16 to May 102, 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: 924
ZIP codes: 4,375

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 26 to Jan 19Start indoors at the zone median: Jan 7 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Mar 5 to Mar 29Transplant outdoors at the zone median: Mar 17 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Mar 19 to Apr 26Sow outdoors at the zone median: Mar 31 to Apr 14 (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 6a. 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 716 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 17range Mar 5 – Mar 29

What each source says, applied to zone 6a

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 6a's median frost dates.

Table 1. Onions rules from every source, at zone 6a's median last frost (Apr 28) and first frost (Oct 17).
Source	What	Rule	Zone 6a median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Mar 31 – Apr 14
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Mar 17
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Jan 7
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Feb 18 – Mar 31
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Mar 3 – Mar 17
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Aug 8 – Aug 22

Frost across zone 6a

Last spring frostmedian Apr 28Full range across the zone: Apr 4 to Jun 21Most of the zone (10th to 90th percentile): Apr 16 to May 10Zone median: Apr 28
First fall frostmedian Oct 17Full range across the zone: Sep 6 to Nov 12Most of the zone (10th to 90th percentile): Oct 5 to Oct 24Zone median: Oct 17

Frost-free season (median)

Figure 2. Frost dates across zone 6a, from 924 NOAA stations matched to 4,375 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.