PlantAlong

Hardiness zone

Zone 4a

The coldest winter night here averages -30 to -25°F1. Frost dates vary more than that suggests.

ZIP codes
814
Weather stations
276
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 12most ZIPs May 5 – May 24
Typical first frost
Sep 28most ZIPs Sep 20 – Oct 4
Spread in spring
19 daysbetween early and late ZIPs
  1. Last spring frostmedian May 12
  2. First fall frostmedian Sep 28
  • Frost-free season (median)
Figure 1. When frosts fall across zone 4a: the full range (pale), most ZIP codes (10th to 90th percentile, solid) and the median (marker). From 276 NOAA stations matched to 720 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 4a

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 4a. 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 4a.
CropWhatZone medianAcross most of the zoneSource
TomatoesStart indoorsMar 17Mar 10 – Mar 29Oregon State, central OR3
Transplant outdoorsMay 26May 19 – Jun 7Colorado State4
PeppersStart indoorsMar 31Mar 24 – Apr 12Oregon State, central OR3
Transplant outdoorsMay 26May 19 – Jun 7Colorado State4
CucumbersStart indoorsApr 28Apr 21 – May 10Oregon State, central OR3
Transplant outdoorsMay 19May 12 – May 31Oregon State, central OR3
Sow outdoorsMay 12May 5 – May 24Colorado State4
ZucchiniStart indoorsMay 5Apr 28 – May 17Oregon State, central OR3
Transplant outdoorsMay 26May 19 – Jun 7Oregon State, central OR3
Sow outdoorsMay 12May 5 – May 24Colorado State4
BeansStart indoorsApr 21Apr 14 – May 3Oregon State, central OR3
Transplant outdoorsMay 19May 12 – May 31Oregon State, central OR3
Sow outdoorsMay 12May 5 – May 24Colorado State4
PeasStart indoorsMar 17Mar 10 – Mar 29Oregon State, central OR3
Transplant outdoorsApr 14Apr 7 – Apr 26Oregon State, central OR3
Sow outdoorsApr 14 – Apr 28Apr 7 – May 10Colorado State4
LettuceStart indoorsMar 24Mar 17 – Apr 5Oregon State, central OR3
Transplant outdoorsApr 28Apr 21 – May 10Oregon State, central OR3
Sow outdoorsApr 14 – Apr 28Apr 7 – May 10Colorado State4
CarrotsSow outdoorsApr 28 – May 12Apr 21 – May 24Colorado State4
OnionsStart indoorsJan 21Jan 14 – Feb 2Oregon State, central OR3
Transplant outdoorsMar 31Mar 24 – Apr 12Oregon State, central OR3
Sow outdoorsApr 14 – Apr 28Apr 7 – May 10Colorado State4
GarlicPlant for fallOct 5 – Oct 12Sep 27 – Oct 18Wisconsin5
PotatoesSow outdoorsApr 28 – May 12Apr 21 – May 24Colorado 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.