USDA hardiness zone 1b
In zone 1b, the average annual extreme minimum temperature is -55 to -50°F (-48.3 to -45.6°C), based on 1991–2020 data from the 2023 USDA Plant Hardiness Zone Map.
What zone 1b means for your garden
A plant labeled "hardy to zone 1b" should survive a typical winter low of -55°F. Only very cold-hardy perennials survive; look for plants rated to this zone or colder, and use season-extension for vegetables. Zone 1a is the colder half of zone 1.
ZIP codes in zone 1b: 11 in Alaska. New to zones? Read how to read your hardiness zone.
Frost dates in zone 1b
No major city on this site is in zone 1b. Typical 30%-risk frost dates (estimate from 2 NOAA stations): last spring frost around May 29, first fall frost around Aug 27. Treat these as rough estimates — local frost dates vary a lot with elevation.
Estimated planting calendar for zone 1b
Based on the estimated frost dates above.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato Short season (90 frost-free days): choose early varieties. | Apr 3 – Apr 17 | Jun 5 – Jun 19 | — | — |
| Pepper Short season (90 frost-free days): choose early varieties. | Mar 20 – Apr 3 | Jun 12 – Jun 26 | — | — |
| Lettuce | Apr 17 – May 1 | May 15 – Jun 5 | May 1 – Jun 12 | — |
| Green Beans | — | — | Jun 5 – Jul 10 | — |
| Sweet Corn Short season (90 frost-free days): choose early varieties. | — | — | Jun 5 – Jul 3 | — |
| Zucchini & Summer Squash | May 1 – May 8 | Jun 5 – Jun 19 | Jun 5 – Jun 26 | — |
| Cucumber Short season (90 frost-free days): choose early varieties. | May 1 – May 8 | Jun 12 – Jun 19 | Jun 5 – Jun 26 | — |
| Carrot | — | — | May 8 – Jun 12 | — |
| Onion | Mar 6 – Mar 20 | May 1 – May 15 | — | — |
| Potato | — | May 1 – May 22 | — | — |
| Basil Short season (90 frost-free days): choose early varieties. | Apr 17 – May 1 | Jun 12 – Jun 26 | Jun 12 – Jul 3 | — |
| Spinach | — | — | Apr 17 – May 15 | Jul 2 – Jul 30 |
| Kale | Apr 3 – Apr 17 | May 1 – May 22 | May 1 – May 22 | — |
| Broccoli | Apr 3 – Apr 17 | May 15 – Jun 5 | — | — |
| Peas | — | — | Apr 17 – May 8 | — |
| Eggplant Short season (90 frost-free days): choose early varieties. | Mar 20 – Apr 3 | Jun 12 – Jun 26 | — | — |
| Cabbage | Apr 3 – Apr 17 | May 1 – May 15 | — | — |
| Radish | — | — | Apr 24 – Jun 12 | Jul 16 – Aug 13 |
| Strawberry | — | May 1 – May 22 | — | — |
| Beets | — | — | May 8 – Jun 19 | — |
Where zone 1b occurs
None of the 512 major cities on this site sits in zone 1b. It occurs mostly in thinly populated areas (ZIP codes: 11 in Alaska).
← Zone 1a · Zone 2a → · All zones
Zone 1b questions
How cold does it get in zone 1b?
Zone 1b is defined by an average annual extreme minimum temperature of -55 to -50°F (-48.3 to -45.6°C) for 1991–2020 — the average of each year's single coldest night, not the record low. Individual winters can be colder, so plants rated exactly for zone 1b carry some risk in a severe year.
When is the last frost in zone 1b?
Roughly May 29 for spring and Aug 27 for the first fall frost (estimate from 2 NOAA stations). Treat these as rough: frost dates swing widely with elevation.
Which states have zone 1b?
The 2023 listings put 11 ZIP codes in zone 1b, spread over 1 states and territories. The most are in Alaska (11).
Did zone 1b grow on the 2023 map?
No: 23 ZIP codes were in zone 1b on the 2012 map and 11 are on the 2023 map (-12). Of today's zone 1b ZIP codes that were also listed in 2012, 36% were in a colder zone then, 55% were already 1b, and 9% were in a warmer zone. What changed in 2023.