PlantAlong

When to plant peppers in zone 4b

Transplant outdoors around May 22. That is 2 weeks after zone 4b's median last frost, May 81. Across most of zone 4b that date falls between May 15 and Jun 8.

Why a range: zone 4b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 1 to May 252, 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 4b's frost dates.

Last reviewed
Stations
443
ZIP codes
1,127
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 4b. 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 276 weeks before last frost (Oregon State, central OR)
Transplant outdoors
May 22range May 15 – Jun 8
First harvest, roughly
Jul 3170 days from transplant1

Five digits. The calendar is computed on your device.

What each source says, applied to zone 4b

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 4b's median frost dates.

Table 1. Peppers rules from every source, at zone 4b's median last frost (May 8) and first frost (Oct 1).
SourceWhatRuleZone 4b median
Colorado State1Used aboveTransplant outdoors2 weeks after last frostMay 22
Utah State3Transplant outdoors2 weeks after last frostMay 22
Oregon State, central OR4Transplant outdoors2 weeks after last frostMay 22
Oregon State, central OR4Used aboveStart indoors6 weeks before last frostMar 27
Texas A&M (E-502)5Transplant outdoors1-8 weeks after last frostMay 15 – Jul 3
Texas A&M (E-502)5Plant for fall12-16 weeks before first frostJun 11 – Jul 9

Frost across zone 4b

  1. Last spring frostmedian May 8
  2. First fall frostmedian Oct 1
  • Frost-free season (median)
Figure 2. Frost dates across zone 4b, from 443 NOAA stations matched to 1,127 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.