PlantAlong

Hardiness zone

Zone 4b

The coldest winter night here averages -25 to -20°F1. Frost dates vary more than that suggests.

ZIP codes
1,284
Weather stations
443
Last reviewed
Photo: Alexander Aubin / Pexels
Your zone tells you how cold winter gets. It does not tell you when to plant.
Typical last frost
May 8most ZIPs May 1 – May 25
Typical first frost
Oct 1most ZIPs Sep 22 – Oct 8
Spread in spring
24 daysbetween early and late ZIPs
  1. Last spring frostmedian May 8
  2. First fall frostmedian Oct 1
  • Frost-free season (median)
Figure 1. When frosts fall across zone 4b: the full range (pale), most ZIP codes (10th to 90th percentile, solid) and the median (marker). From 443 NOAA stations matched to 1,127 ZIP codes.

Zone and frost date are different things. The zone comes from the coldest winter night; frost dates come from spring and fall temperatures at weather stations2. Your own dates can sit anywhere in the range above, so the calendar for your ZIP is the better guide.

Five digits. The calendar is computed on your device.

When to plant in zone 4b

Each crop uses its primary extension source, applied to this zone's frost spread. Rules written for another state are labelled with their source.

  1. Tomatoes
  2. Peppers
  3. Cucumbers
  4. Zucchini
  5. Beans
  6. Peas
  7. Lettuce
  8. Carrots
  9. Onions
  10. Garlic
  11. Potatoes
  12. BasilNo sourced window
  • 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 2. Planting windows across zone 4b. Solid and dashed bars: the window at the zone's median frost dates. Pale bars: where the same window falls across most of the zone's ZIP codes.
Table 1. Planting windows in zone 4b.
CropWhatZone medianAcross most of the zoneSource
TomatoesStart indoorsMar 13Mar 6 – Mar 30Oregon State, central OR3
Transplant outdoorsMay 22May 15 – Jun 8Colorado State4
PeppersStart indoorsMar 27Mar 20 – Apr 13Oregon State, central OR3
Transplant outdoorsMay 22May 15 – Jun 8Colorado State4
CucumbersStart indoorsApr 24Apr 17 – May 11Oregon State, central OR3
Transplant outdoorsMay 15May 8 – Jun 1Oregon State, central OR3
Sow outdoorsMay 8May 1 – May 25Colorado State4
ZucchiniStart indoorsMay 1Apr 24 – May 18Oregon State, central OR3
Transplant outdoorsMay 22May 15 – Jun 8Oregon State, central OR3
Sow outdoorsMay 8May 1 – May 25Colorado State4
BeansStart indoorsApr 17Apr 10 – May 4Oregon State, central OR3
Transplant outdoorsMay 15May 8 – Jun 1Oregon State, central OR3
Sow outdoorsMay 8May 1 – May 25Colorado State4
PeasStart indoorsMar 13Mar 6 – Mar 30Oregon State, central OR3
Transplant outdoorsApr 10Apr 3 – Apr 27Oregon State, central OR3
Sow outdoorsApr 10 – Apr 24Apr 3 – May 11Colorado State4
LettuceStart indoorsMar 20Mar 13 – Apr 6Oregon State, central OR3
Transplant outdoorsApr 24Apr 17 – May 11Oregon State, central OR3
Sow outdoorsApr 10 – Apr 24Apr 3 – May 11Colorado State4
CarrotsSow outdoorsApr 24 – May 8Apr 17 – May 25Colorado State4
OnionsStart indoorsJan 17Jan 10 – Feb 3Oregon State, central OR3
Transplant outdoorsMar 27Mar 20 – Apr 13Oregon State, central OR3
Sow outdoorsApr 10 – Apr 24Apr 3 – May 11Colorado State4
GarlicPlant for fallOct 8 – Oct 15Sep 29 – Oct 22Wisconsin5
PotatoesSow outdoorsApr 24 – May 8Apr 17 – May 25Colorado State4
BasilNo frost-relative rule is published; sources give conditions instead (see the crop page).

Sources

  1. PRISM Group, Oregon State University, 2023 USDA Plant Hardiness Zone Map ZIP code data (PHZM 2023), 2023-11-15. 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. Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
  4. Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
  5. University of Wisconsin–Madison Division of Extension, Wisconsin Horticulture, Garlic, Allium sativum, Rev: 10/2023. Retrieved 2026-11-02.