When to plant onions in zone 7a

Transplant outdoors around Mar 5. That is 6 weeks before zone 7a's median last frost, Apr 161. Across most of zone 7a that date falls between Feb 22 and Mar 17.

Why a range: zone 7a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 4 to Apr 282, 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: 865
ZIP codes: 3,807

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 14 to Jan 7Start indoors at the zone median: Dec 26 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: Feb 22 to Mar 17Transplant outdoors at the zone median: Mar 5 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: Mar 7 to Apr 14Sow outdoors at the zone median: Mar 19 to Apr 2 (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 7a. 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 2616 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 5range Feb 22 – Mar 17

What each source says, applied to zone 7a

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 7a's median frost dates.

Table 1. Onions rules from every source, at zone 7a's median last frost (Apr 16) and first frost (Oct 25).
Source	What	Rule	Zone 7a median
Colorado State3Used above	Sow outdoors	2-4 weeks before last frost	Mar 19 – Apr 2
Oregon State, central OR1Used above	Transplant outdoors	6 weeks before last frost	Mar 5
Oregon State, central OR1Used above	Start indoors	16 weeks before last frost	Dec 26
Texas A&M (E-502)4	Transplant outdoors	4-10 weeks before last frost	Feb 6 – Mar 19
Texas A&M (E-502)4	Sow outdoors	6-8 weeks before last frost	Feb 20 – Mar 5
Texas A&M (E-502)4	Plant for fall	8-10 weeks before first frost	Aug 16 – Aug 30

Frost across zone 7a

Last spring frostmedian Apr 16Full range across the zone: Mar 25 to Jun 11Most of the zone (10th to 90th percentile): Apr 4 to Apr 28Zone median: Apr 16
First fall frostmedian Oct 25Full range across the zone: Sep 10 to Nov 17Most of the zone (10th to 90th percentile): Oct 16 to Nov 2Zone median: Oct 25

Frost-free season (median)

Figure 2. Frost dates across zone 7a, from 865 NOAA stations matched to 3,807 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.