USDA hardiness zone 4a
In zone 4a, the average annual extreme minimum temperature is -30 to -25°F (-34.4 to -31.7°C), based on 1991–2020 data from the 2023 USDA Plant Hardiness Zone Map.
What zone 4a means for your garden
A plant labeled "hardy to zone 4a" should survive a typical winter low of -30°F. Most perennials sold at garden centers are rated for this zone or colder; marginal plants rated one zone warmer can survive in sheltered spots. Zone 4b is the warmer half of zone 4.
ZIP codes in zone 4a: 783 in contiguous U.S., 31 in Alaska. New to zones? Read how to read your hardiness zone.
Frost dates in zone 4a
Across 2 major cities in zone 4a, the 30%-risk last spring frost ranges from May 11 to May 14 (median May 12), and the first fall frost has a median of Sep 27. Frost timing depends on elevation, latitude and distance from water — a zone does not set a frost date, so check your city page for local dates.
Typical planting calendar for zone 4a
Based on the median frost dates above. For your exact dates, use your city page.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 17 – Mar 31 | May 19 – Jun 2 | — | — |
| Pepper | Mar 3 – Mar 17 | May 26 – Jun 9 | — | — |
| Lettuce | Mar 31 – Apr 14 | Apr 28 – May 19 | Apr 14 – May 26 | Jul 19 – Aug 16 |
| Green Beans | — | — | May 19 – Jun 23 | Jul 5 – Jul 19 |
| Sweet Corn | — | — | May 19 – Jun 16 | — |
| Zucchini & Summer Squash | Apr 14 – Apr 21 | May 19 – Jun 2 | May 19 – Jun 9 | Jul 5 – Jul 19 |
| Cucumber | Apr 14 – Apr 21 | May 26 – Jun 2 | May 19 – Jun 9 | Jul 5 – Jul 19 |
| Carrot | — | — | Apr 21 – May 26 | Jul 5 – Jul 19 |
| Onion | Feb 17 – Mar 3 | Apr 14 – Apr 28 | — | — |
| Potato | — | Apr 14 – May 5 | — | — |
| Basil | Mar 31 – Apr 14 | May 26 – Jun 9 | May 26 – Jun 16 | — |
| Spinach | — | — | Mar 31 – Apr 28 | Aug 2 – Aug 30 |
| Kale | Mar 17 – Mar 31 | Apr 14 – May 5 | Apr 14 – May 5 | Jul 19 – Aug 16 |
| Broccoli | Mar 17 – Mar 31 | Apr 28 – May 19 | — | Jul 5 – Jul 19 |
| Peas | — | — | Mar 31 – Apr 21 | Jul 19 – Aug 2 |
| Eggplant | Mar 3 – Mar 17 | May 26 – Jun 9 | — | — |
| Cabbage | Mar 17 – Mar 31 | Apr 14 – Apr 28 | — | Jul 5 – Jul 19 |
| Radish | — | — | Apr 7 – May 26 | Aug 16 – Sep 13 |
| Strawberry | — | Apr 14 – May 5 | — | — |
| Beets | — | — | Apr 21 – Jun 2 | Jul 19 – Aug 2 |
Cities in zone 4a (2)
Major cities whose center falls in zone 4a:
- Fargo, ND — last frost May 11, first frost Sep 28
- Bismarck, ND — last frost May 14, first frost Sep 25
← Zone 3b · Zone 4b → · All zones
Zone 4a questions
How cold does it get in zone 4a?
Zone 4a is defined by an average annual extreme minimum temperature of -30 to -25°F (-34.4 to -31.7°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 4a carry some risk in a severe year.
When is the last frost in zone 4a?
It depends far more on the city than on the zone. Among 2 zone 4a cities on this site, the 30%-risk last spring frost runs from May 11 in Fargo, ND to May 14 in Bismarck, ND — 3 days apart (median May 12).
Which states have zone 4a?
The 2023 listings put 814 ZIP codes in zone 4a, spread over 15 states and territories. The most are in North Dakota (316), Minnesota (200), Wisconsin (73), Montana (71), Alaska (31).
Did zone 4a grow on the 2023 map?
No: 1,117 ZIP codes were in zone 4a on the 2012 map and 814 are on the 2023 map (-303). Of today's zone 4a ZIP codes that were also listed in 2012, 30% were in a colder zone then, 68% were already 4a, and 2% were in a warmer zone. What changed in 2023.
Which big cities are in zone 4a?
The largest cities on this site in zone 4a are Fargo, ND, Bismarck, ND; 2 cities in all are listed below.