USDA hardiness zone 6a
In zone 6a, the coldest night of the winter averages −10 to −5°F (−23.3 to −20.6°C). That is the lowest temperature of each winter, averaged over 1991–2020, on the 2023 USDA Plant Hardiness Zone Map.
−10 to −5
°F, average coldest winter night
−23.3 to −20.6°C.
5,187
ZIP codes in zone 6a
12.9% of the 40,326 ZIP codes on the map.
Cities in zone 6a
Same zone, different springs: the typical last frost is Apr 10 in Chicago and May 4 in Denver, about 3 weeks apart.
| City | Last frost · typical | First frost · typical | Zone from ZIP |
|---|---|---|---|
| Chicago, IL | Apr 10 | Nov 5 | 60608 |
| Denver, CO | May 4 | Oct 8 | 80238 |
| Milwaukee, WI | Apr 26 | Oct 18 | 53210 |
| Colorado Springs, CO | May 3 | Oct 8 | 80909 |
| Omaha, NE | Apr 16 | Oct 18 | 68114 |
Other larger places in this zone, by the ZIP nearest their center: Aurora, CO; Lincoln, NE; Fort Wayne, IN; Worcester, MA; Grand Rapids, MI; Lakewood, CO; Springfield, MA.
Frost dates are the typical dates of a 32°F freeze from NOAA’s 1991–2020 climate normals; each city page also gives a safer date. Zones can vary within a city.
What zones do and don’t tell you
What a zone tells you
How cold winter nights usually get: the lowest temperature of each winter, averaged over 1991–2020. Gardeners and nurseries use it to judge whether a perennial, shrub or tree is likely to survive the winter. It is an average, so some winters still get colder.
What it doesn’t
When to plant. Two gardens in zone 6a can have their last spring frost weeks apart, because frost dates depend on elevation, nearby water and how fast spring warms up, not on the coldest night of winter. The zone says nothing about summer heat either.
For planting dates
Vegetables, herbs and annual flowers are timed from frost dates. We give them by ZIP code, from the nearest NOAA weather station. Find your frost dates
Hardiness zones: 2023 USDA Plant Hardiness Zone Map, PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed 9 Nov 2026.