PlantAlong

When to plant peas in zone 4a

Transplant outdoors around Apr 14. That is 4 weeks before zone 4a's median last frost, May 121. Across most of zone 4a that date falls between Apr 7 and Apr 26.

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
Stations
276
ZIP codes
720
Photo: Mateusz Feliksik / Pexels
These dates use the typical last frost for the whole zone. Your ZIP code gives a closer answer.
  1. Start indoorsOregon State, central OR
  2. Transplant outdoorsOregon State, central OR
  3. Sow outdoorsColorado 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. Peas 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
Mar 178 weeks before last frost (Oregon State, central OR)
Transplant outdoors
Apr 14range Apr 7 – Apr 26

Five digits. The calendar is computed on your device.

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 peas, worked out at zone 4a's median frost dates.

Table 1. Peas rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
SourceWhatRuleZone 4a median
Colorado State3Used aboveSow outdoors2-4 weeks before last frostApr 14 – Apr 28
Oregon State, central OR1Used aboveTransplant outdoors4 weeks before last frostApr 14
Oregon State, central OR1Used aboveStart indoors8 weeks before last frostMar 17
Texas A&M (E-502)4Sow outdoors2-8 weeks before last frostMar 17 – Apr 28
Texas A&M (E-502)4Plant for fall2-12 weeks before first frostJul 6 – Sep 14

Frost across zone 4a

  1. Last spring frostmedian May 12
  2. First fall frostmedian Sep 28
  • Frost-free season (median)
Figure 2. Frost dates across zone 4a, from 276 NOAA stations matched to 720 ZIP codes.

Sources

  1. Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
  2. NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
  3. Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
  4. Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.