PlantAlong

When to plant beans in zone 4a

Transplant outdoors around May 19. That is 1 weeks after zone 4a's median last frost, May 121. Across most of zone 4a that date falls between May 12 and May 31.

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: Gobral / 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. Beans 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
Apr 213 weeks before last frost (Oregon State, central OR)
Transplant outdoors
May 19range May 12 – May 31

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

Table 1. Beans rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
SourceWhatRuleZone 4a median
Colorado State3Used aboveSow outdoorsOn the last frostMay 12
Utah State4Sow outdoorsOn the last frostMay 12
Oregon State, central OR1Used aboveTransplant outdoors1 weeks after last frostMay 19
Oregon State, central OR1Used aboveStart indoors3 weeks before last frostApr 21
Texas A&M (E-502)5Sow outdoors1-4 weeks after last frostMay 19 – Jun 9
Texas A&M (E-502)5Plant for fall8-10 weeks before first frostJul 20 – Aug 3
Texas A&M (E-502)5Plant for fall14-16 weeks before first frostJun 8 – Jun 22

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. Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. 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.