Zone 5a

The coldest winter night here averages -20 to -15°F1. Frost dates vary more than that suggests.

ZIP codes: 2,426
Weather stations: 655
Last reviewed: 2026-11-02

Your zone tells you how cold winter gets. It does not tell you when to plant.

Typical last frost: May 3most ZIPs Apr 26 – May 20
Typical first frost: Oct 5most ZIPs Sep 26 – Oct 13
Spread in spring: 24 daysbetween early and late ZIPs

Last spring frostmedian May 3Full range across the zone: Apr 13 to Jun 20Most of the zone (10th to 90th percentile): Apr 26 to May 20Zone median: May 3
First fall frostmedian Oct 5Full range across the zone: Sep 1 to Oct 28Most of the zone (10th to 90th percentile): Sep 26 to Oct 13Zone median: Oct 5

Frost-free season (median)

Figure 1. When frosts fall across zone 5a: the full range (pale), most ZIP codes (10th to 90th percentile, solid) and the median (marker). From 655 NOAA stations matched to 2,089 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.

When to plant in zone 5a

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

TomatoesStart indoors across most of the zone: Mar 1 to Mar 25Start indoors at the zone median: Mar 8 (Oregon State, central OR)Transplant outdoors across most of the zone: May 10 to Jun 3Transplant outdoors at the zone median: May 17 (Colorado State)
PeppersStart indoors across most of the zone: Mar 15 to Apr 8Start indoors at the zone median: Mar 22 (Oregon State, central OR)Transplant outdoors across most of the zone: May 10 to Jun 3Transplant outdoors at the zone median: May 17 (Colorado State)
CucumbersStart indoors across most of the zone: Apr 12 to May 6Start indoors at the zone median: Apr 19 (Oregon State, central OR)Transplant outdoors across most of the zone: May 3 to May 27Transplant outdoors at the zone median: May 10 (Oregon State, central OR)Sow outdoors across most of the zone: Apr 26 to May 20Sow outdoors at the zone median: May 3 (Colorado State)
ZucchiniStart indoors across most of the zone: Apr 19 to May 13Start indoors at the zone median: Apr 26 (Oregon State, central OR)Transplant outdoors across most of the zone: May 10 to Jun 3Transplant outdoors at the zone median: May 17 (Oregon State, central OR)Sow outdoors across most of the zone: Apr 26 to May 20Sow outdoors at the zone median: May 3 (Colorado State)
BeansStart indoors across most of the zone: Apr 5 to Apr 29Start indoors at the zone median: Apr 12 (Oregon State, central OR)Transplant outdoors across most of the zone: May 3 to May 27Transplant outdoors at the zone median: May 10 (Oregon State, central OR)Sow outdoors across most of the zone: Apr 26 to May 20Sow outdoors at the zone median: May 3 (Colorado State)
PeasStart indoors across most of the zone: Mar 1 to Mar 25Start indoors at the zone median: Mar 8 (Oregon State, central OR)Transplant outdoors across most of the zone: Mar 29 to Apr 22Transplant outdoors at the zone median: Apr 5 (Oregon State, central OR)Sow outdoors across most of the zone: Mar 29 to May 6Sow outdoors at the zone median: Apr 5 to Apr 19 (Colorado State)
LettuceStart indoors across most of the zone: Mar 8 to Apr 1Start indoors at the zone median: Mar 15 (Oregon State, central OR)Transplant outdoors across most of the zone: Apr 12 to May 6Transplant outdoors at the zone median: Apr 19 (Oregon State, central OR)Sow outdoors across most of the zone: Mar 29 to May 6Sow outdoors at the zone median: Apr 5 to Apr 19 (Colorado State)
CarrotsSow outdoors across most of the zone: Apr 12 to May 20Sow outdoors at the zone median: Apr 19 to May 3 (Colorado State)
OnionsStart indoors across most of the zone: Jan 5 to Jan 29Start indoors at the zone median: Jan 12 (Oregon State, central OR)Transplant outdoors across most of the zone: Mar 15 to Apr 8Transplant outdoors at the zone median: Mar 22 (Oregon State, central OR)Sow outdoors across most of the zone: Mar 29 to May 6Sow outdoors at the zone median: Apr 5 to Apr 19 (Colorado State)
GarlicPlant for fall across most of the zone: Oct 3 to Oct 27Plant for fall at the zone median: Oct 12 to Oct 19 (Wisconsin)
PotatoesSow outdoors across most of the zone: Apr 12 to May 20Sow outdoors at the zone median: Apr 19 to May 3 (Colorado State)
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 5a. 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 5a.
Crop	What	Zone median	Across most of the zone	Source
Tomatoes	Start indoors	Mar 8	Mar 1 – Mar 25	Oregon State, central OR3
Tomatoes	Transplant outdoors	May 17	May 10 – Jun 3	Colorado State4
Peppers	Start indoors	Mar 22	Mar 15 – Apr 8	Oregon State, central OR3
Peppers	Transplant outdoors	May 17	May 10 – Jun 3	Colorado State4
Cucumbers	Start indoors	Apr 19	Apr 12 – May 6	Oregon State, central OR3
	Transplant outdoors	May 10	May 3 – May 27	Oregon State, central OR3
	Sow outdoors	May 3	Apr 26 – May 20	Colorado State4
Zucchini	Start indoors	Apr 26	Apr 19 – May 13	Oregon State, central OR3
	Transplant outdoors	May 17	May 10 – Jun 3	Oregon State, central OR3
	Sow outdoors	May 3	Apr 26 – May 20	Colorado State4
Beans	Start indoors	Apr 12	Apr 5 – Apr 29	Oregon State, central OR3
	Transplant outdoors	May 10	May 3 – May 27	Oregon State, central OR3
	Sow outdoors	May 3	Apr 26 – May 20	Colorado State4
Peas	Start indoors	Mar 8	Mar 1 – Mar 25	Oregon State, central OR3
	Transplant outdoors	Apr 5	Mar 29 – Apr 22	Oregon State, central OR3
	Sow outdoors	Apr 5 – Apr 19	Mar 29 – May 6	Colorado State4
Lettuce	Start indoors	Mar 15	Mar 8 – Apr 1	Oregon State, central OR3
	Transplant outdoors	Apr 19	Apr 12 – May 6	Oregon State, central OR3
	Sow outdoors	Apr 5 – Apr 19	Mar 29 – May 6	Colorado State4
Carrots	Sow outdoors	Apr 19 – May 3	Apr 12 – May 20	Colorado State4
Onions	Start indoors	Jan 12	Jan 5 – Jan 29	Oregon State, central OR3
	Transplant outdoors	Mar 22	Mar 15 – Apr 8	Oregon State, central OR3
	Sow outdoors	Apr 5 – Apr 19	Mar 29 – May 6	Colorado State4
Garlic	Plant for fall	Oct 12 – Oct 19	Oct 3 – Oct 27	Wisconsin5
Potatoes	Sow outdoors	Apr 19 – May 3	Apr 12 – May 20	Colorado State4
Basil	No frost-relative rule is published; sources give conditions instead (see the crop page).

Sources

PRISM Group, Oregon State University, 2023 USDA Plant Hardiness Zone Map ZIP code data (PHZM 2023), 2023-11-15. Retrieved 2026-11-02.
NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
University of Wisconsin–Madison Division of Extension, Wisconsin Horticulture, Garlic, Allium sativum, Rev: 10/2023. Retrieved 2026-11-02.