USDA hardiness zone 1a
In zone 1a, the average annual extreme minimum temperature is -60 to -55°F (-51.1 to -48.3°C), based on 1991–2020 data from the 2023 USDA Plant Hardiness Zone Map.
What zone 1a means for your garden
A plant labeled "hardy to zone 1a" should survive a typical winter low of -60°F. Only very cold-hardy perennials survive; look for plants rated to this zone or colder, and use season-extension for vegetables. Zone 1b is the warmer half of zone 1.
ZIP codes in zone 1a: 5 in Alaska. New to zones? Read how to read your hardiness zone.
Frost dates in zone 1a
No major city on this site is in zone 1a. Typical 30%-risk frost dates (estimate borrowed from zone 1b (no stations in 1a)): 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 1a
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 1a occurs
None of the 512 major cities on this site sits in zone 1a. It occurs mostly in thinly populated areas (ZIP codes: 5 in Alaska).
Zone 1a questions
How cold does it get in zone 1a?
Zone 1a is defined by an average annual extreme minimum temperature of -60 to -55°F (-51.1 to -48.3°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 1a carry some risk in a severe year.
When is the last frost in zone 1a?
Roughly May 29 for spring and Aug 27 for the first fall frost (estimate borrowed from zone 1b (no stations in 1a)). Treat these as rough: frost dates swing widely with elevation.
Which states have zone 1a?
The 2023 listings put 5 ZIP codes in zone 1a, spread over 1 states and territories. The most are in Alaska (5).
Did zone 1a grow on the 2023 map?
No: 8 ZIP codes were in zone 1a on the 2012 map and 5 are on the 2023 map (-3). Of today's zone 1a ZIP codes that were also listed in 2012, 0% were in a colder zone then, 20% were already 1a, and 80% were in a warmer zone. What changed in 2023.