USDA hardiness zone 3b
In zone 3b, the average annual extreme minimum temperature is -35 to -30°F (-37.2 to -34.4°C), based on 1991–2020 data from the 2023 USDA Plant Hardiness Zone Map.
What zone 3b means for your garden
A plant labeled "hardy to zone 3b" should survive a typical winter low of -35°F. Only very cold-hardy perennials survive; look for plants rated to this zone or colder, and use season-extension for vegetables. Zone 3a is the colder half of zone 3.
ZIP codes in zone 3b: 204 in contiguous U.S., 26 in Alaska. New to zones? Read how to read your hardiness zone.
Frost dates in zone 3b
No major city on this site is in zone 3b. Typical 30%-risk frost dates (estimate from 58 NOAA stations): last spring frost around May 24, first fall frost around Sep 18. Treat these as rough estimates — local frost dates vary a lot with elevation.
Estimated planting calendar for zone 3b
Based on the estimated frost dates above.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 29 – Apr 12 | May 31 – Jun 14 | — | — |
| Pepper Short season (117 frost-free days): choose early varieties. | Mar 15 – Mar 29 | Jun 7 – Jun 21 | — | — |
| Lettuce | Apr 12 – Apr 26 | May 10 – May 31 | Apr 26 – Jun 7 | Jul 10 – Aug 7 |
| Green Beans | — | — | May 31 – Jul 5 | Jun 26 – Jul 10 |
| Sweet Corn | — | — | May 31 – Jun 28 | — |
| Zucchini & Summer Squash | Apr 26 – May 3 | May 31 – Jun 14 | May 31 – Jun 21 | Jun 26 – Jul 10 |
| Cucumber | Apr 26 – May 3 | Jun 7 – Jun 14 | May 31 – Jun 21 | Jun 26 – Jul 10 |
| Carrot | — | — | May 3 – Jun 7 | Jun 26 – Jul 10 |
| Onion | Mar 1 – Mar 15 | Apr 26 – May 10 | — | — |
| Potato | — | Apr 26 – May 17 | — | — |
| Basil Short season (117 frost-free days): choose early varieties. | Apr 12 – Apr 26 | Jun 7 – Jun 21 | Jun 7 – Jun 28 | — |
| Spinach | — | — | Apr 12 – May 10 | Jul 24 – Aug 21 |
| Kale | Mar 29 – Apr 12 | Apr 26 – May 17 | Apr 26 – May 17 | Jul 10 – Aug 7 |
| Broccoli | Mar 29 – Apr 12 | May 10 – May 31 | — | Jun 26 – Jul 10 |
| Peas | — | — | Apr 12 – May 3 | Jul 10 – Jul 24 |
| Eggplant | Mar 15 – Mar 29 | Jun 7 – Jun 21 | — | — |
| Cabbage | Mar 29 – Apr 12 | Apr 26 – May 10 | — | Jun 26 – Jul 10 |
| Radish | — | — | Apr 19 – Jun 7 | Aug 7 – Sep 4 |
| Strawberry | — | Apr 26 – May 17 | — | — |
| Beets | — | — | May 3 – Jun 14 | Jul 10 – Jul 24 |
Where zone 3b occurs
None of the 512 major cities on this site sits in zone 3b. It occurs mostly in thinly populated areas (ZIP codes: 204 in contiguous U.S., 26 in Alaska).
← Zone 3a · Zone 4a → · All zones
Zone 3b questions
How cold does it get in zone 3b?
Zone 3b is defined by an average annual extreme minimum temperature of -35 to -30°F (-37.2 to -34.4°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 3b carry some risk in a severe year.
When is the last frost in zone 3b?
Roughly May 24 for spring and Sep 18 for the first fall frost (estimate from 58 NOAA stations). Treat these as rough: frost dates swing widely with elevation.
Which states have zone 3b?
The 2023 listings put 230 ZIP codes in zone 3b, spread over 6 states and territories. The most are in Minnesota (123), North Dakota (54), Alaska (26), Montana (25), Maine (1).
Did zone 3b grow on the 2023 map?
No: 440 ZIP codes were in zone 3b on the 2012 map and 230 are on the 2023 map (-210). Of today's zone 3b ZIP codes that were also listed in 2012, 20% were in a colder zone then, 77% were already 3b, and 3% were in a warmer zone. What changed in 2023.