Bismarck, North Dakota: planting zone & frost dates
Bismarck is in USDA plant hardiness zone 4a, where the average coldest winter night reaches -30 to -25°F (-34.4 to -31.7°C). The typical last spring frost is around May 14 and the first fall frost around Sep 25.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 23 | Sep 16 |
| Typical planning date (30% risk) | May 14 | Sep 25 |
| Median (50%) | May 10 | Sep 30 |
How to read this: after the spring "10% risk" date, a 32°F frost happens in only about 1 year in 10. In fall, the "10% risk" date is the early end — a frost before it happens about 1 year in 10.
Frost data comes from NOAA station BISMARCK 2.4 NNW, ND (USC00320817), 1.9 mi (3.0 km) from the city center and 79 ft higher than the city center (+24 m). Record used for the 1991–2020 normals: 12 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Bismarck
All 4 ZIP codes covering Bismarck are in zone 4a.
Vegetable planting calendar for Bismarck
Dates are counted from the 30%-risk frost dates above (last frost May 14, first frost Sep 25). Shift a week later in cold spots or low-lying ground.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 19 – Apr 2 | May 21 – Jun 4 | — | — |
| Pepper | Mar 5 – Mar 19 | May 28 – Jun 11 | — | — |
| Lettuce | Apr 2 – Apr 16 | Apr 30 – May 21 | Apr 16 – May 28 | Jul 17 – Aug 14 |
| Green Beans | — | — | May 21 – Jun 25 | Jul 3 – Jul 17 |
| Sweet Corn | — | — | May 21 – Jun 18 | — |
| Zucchini & Summer Squash | Apr 16 – Apr 23 | May 21 – Jun 4 | May 21 – Jun 11 | Jul 3 – Jul 17 |
| Cucumber | Apr 16 – Apr 23 | May 28 – Jun 4 | May 21 – Jun 11 | Jul 3 – Jul 17 |
| Carrot | — | — | Apr 23 – May 28 | Jul 3 – Jul 17 |
| Onion | Feb 19 – Mar 5 | Apr 16 – Apr 30 | — | — |
| Potato | — | Apr 16 – May 7 | — | — |
| Basil | Apr 2 – Apr 16 | May 28 – Jun 11 | May 28 – Jun 18 | — |
| Spinach | — | — | Apr 2 – Apr 30 | Jul 31 – Aug 28 |
| Kale | Mar 19 – Apr 2 | Apr 16 – May 7 | Apr 16 – May 7 | Jul 17 – Aug 14 |
| Broccoli | Mar 19 – Apr 2 | Apr 30 – May 21 | — | Jul 3 – Jul 17 |
| Peas | — | — | Apr 2 – Apr 23 | Jul 17 – Jul 31 |
| Eggplant | Mar 5 – Mar 19 | May 28 – Jun 11 | — | — |
| Cabbage | Mar 19 – Apr 2 | Apr 16 – Apr 30 | — | Jul 3 – Jul 17 |
| Radish | — | — | Apr 9 – May 28 | Aug 14 – Sep 11 |
| Strawberry | — | Apr 16 – May 7 | — | — |
| Beets | — | — | Apr 23 – Jun 4 | Jul 17 – Jul 31 |
Nearby cities in North Dakota
- Fargo — zone 4a, last frost May 11, first frost Sep 28
More: all North Dakota cities · other zone 4a cities: Fargo, ND
Bismarck gardening questions
When is the last frost in Bismarck, ND?
Plan around May 14. At NOAA's Bismarck 2.4 NNW, ND station, 1.9 mi from the city center, there is a 30% chance of a 32°F frost after that date. Half of springs have their last frost by May 10, and after May 23 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around May 1. The last frost is fairly predictable here: only 23 days separate the 90% and 10% dates (median for cities on this site: 36 days).
When should I plant tomatoes in Bismarck, ND?
Start seeds indoors Mar 19 – Apr 2 and set transplants outside May 21 – Jun 4, once nights stay above 50°F and the soil is near 60°F. That leaves 134 days between the 30% frost dates, enough for most varieties including 80–85-day slicers. More: tomato planting dates by zone.
How long is the growing season in Bismarck?
About 142 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 124 days, and in 1 year in 10 it runs 159 days or more. In fall there is a 10% chance of frost by Sep 16 and a 30% chance by Sep 25.
When should I plant a fall garden in Bismarck?
Count back from the first fall frost (Sep 25): transplant broccoli Jul 3 – Jul 17; sow kale Jul 17 – Aug 14; sow lettuce Jul 17 – Aug 14; sow radishes Aug 14 – Sep 11. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 9. How the math works: succession and fall planting.
Did Bismarck's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Bismarck is zone 4a on both the 2012 and the 2023 USDA maps (PRISM/OSU ZIP listings). About 42% of U.S. ZIP codes kept their half-zone in 2023. Why the 2023 map is warmer.