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Counties with the Coldest Hardiness Zones

Published May 2, 2026

The coldest hardiness zones — 3a, 3b, 4a, and 4b — cover Alaska, northern Minnesota, and the highest mountain counties in the continental United States. In these counties, winter temperatures can plunge to -30°F or colder, defining every plant choice a gardener makes.

We identified the 25 counties with the coldest hardiness zones, where only the hardiest plants survive and soil remains frozen for months each year.

Counties with the Coldest Hardiness Zones

Beltrami County, MN is in zone 3b with an average extreme minimum of -35°F. Gardening here requires exceptional cold hardiness and soil management.

RankCountyStateZoneMin Temp (°F)
1Beltrami CountyMN3b-35
2Clearwater CountyMN3b-35
3Cook CountyMN3b-35
4Hubbard CountyMN3b-35
5Itasca CountyMN3b-35
6Kittson CountyMN3b-35
7Koochiching CountyMN3b-35
8Lake CountyMN3b-35
9Lake of the Woods CountyMN3b-35
10Marshall CountyMN3b-35
11Pennington CountyMN3b-35
12Red Lake CountyMN3b-35
13Roseau CountyMN3b-35
14St. Louis CountyMN3b-35
15Daniels CountyMT3b-35
16Hill CountyMT3b-35
17Phillips CountyMT3b-35
18Sheridan CountyMT3b-35
19Bottineau CountyND3b-35
20Cavalier CountyND3b-35
21Pierce CountyND3b-35
22Rolette CountyND3b-35
23Towner CountyND3b-35
24Jackson CountyCO4b-25
25Howard CountyIA4b-25

Soil Challenges in Cold Zones

  • Frozen soil limits root growth for 5-7 months of the year, compressing the establishment window.
  • Permafrost or seasonally frozen ground can cause frost heave, damaging root systems and garden structures.
  • Short decomposition seasons mean organic matter breaks down slowly, requiring active composting.
  • Spring thaw brings waterlogged soils that delay planting and increase disease pressure.

Methodology

Counties ranked by coldest USDA Plant Hardiness Zone (lowest zone number). Data from USDA PHZM 2023.

Data sourced from USDA Plant Hardiness Zone Map (2023) and USDA SSURGO Soil Survey. Hardiness zone data represents average annual extreme minimum temperatures and should be used as a general guide. Local microclimates may vary significantly from county averages.

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