PlantAlong

When to plant peppers in zone 4a

Transplant outdoors around May 26. That is 2 weeks after zone 4a's median last frost, May 121. Across most of zone 4a that date falls between May 19 and Jun 7.

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 Colorado State University Extension, written for CO and applied here to zone 4a's frost dates.

Last reviewed
Stations
276
ZIP codes
720
Photo: Yan Krukau / 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 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. Peppers 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 316 weeks before last frost (Oregon State, central OR)
Transplant outdoors
May 26range May 19 – Jun 7
First harvest, roughly
Aug 470 days from transplant1

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

Table 1. Peppers rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
SourceWhatRuleZone 4a median
Colorado State1Used aboveTransplant outdoors2 weeks after last frostMay 26
Utah State3Transplant outdoors2 weeks after last frostMay 26
Oregon State, central OR4Transplant outdoors2 weeks after last frostMay 26
Oregon State, central OR4Used aboveStart indoors6 weeks before last frostMar 31
Texas A&M (E-502)5Transplant outdoors1-8 weeks after last frostMay 19 – Jul 7
Texas A&M (E-502)5Plant for fall12-16 weeks before first frostJun 8 – Jul 6

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. Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 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. Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. Retrieved 2026-11-02.
  4. Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
  5. Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.