Boulder, MT frost dates
Last spring frostJun 7median date
First fall frostSep 8median date
Growing season93 daysfrost-free
Hardiness zone5aZone 5a
How Boulder compares
- In 8 years out of 10, the last spring frost in Boulder falls between May 24 and June 25, a 32-day window. The first fall frost falls between August 25 and September 21.
- The median last frost is 19 days later than the Montana median of May 19.
- Its 93-day growing season is longer than 7% of the 325 Montana places with a regular frost season (state median 125 days).
- The last frost is 35 days later than the zone 5a median of May 3. Hardiness zones measure winter cold, not season length, so use these dates rather than a zone 5a chart.
- The weather station records an average of 205 nights a year at or below 32°F.
Frost probability
| Chance of frost after | Date |
|---|---|
| 90% (early) | May 24 |
| 50% (median) | June 7 |
| 10% (safe for tender plants) | June 25 |
| Chance of frost before | Date |
|---|---|
| 10% (early) | August 25 |
| 50% (median) | September 8 |
| 90% (late) | September 21 |
Frost here means 32°F (0°C). A median date means half of years have their last frost earlier and half later.
Planting calendar for Boulder, MT
Boulder, MT frost dates — last frost Jun 7, first frost Sep 8, zone 5a. whenisthelastfrost.com
Crop
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Start indoors: Mar 15 to Mar 29Transplant outside: May 10 to May 24
Start indoors: Mar 29 to Apr 12Transplant outside: Jun 21 to Jul 5
Start indoors: Mar 29 to Apr 12Transplant outside: Jun 21 to Jul 5
Start indoors: Apr 12 to Apr 26Transplant outside: Jun 14 to Jun 28
Start indoors: Apr 12 to Apr 26Transplant outside: May 17 to May 31Sow outdoors: May 10 to Jun 14Plant for fall: Jul 14 to Aug 11
Start indoors: Apr 12 to Apr 26Transplant outside: May 10 to May 24Sow outdoors: May 10 to May 24Plant for fall: Jun 30 to Jul 21
Start indoors: Apr 12 to Apr 26Transplant outside: May 24 to Jun 7
Start indoors: Apr 12 to Apr 26Transplant outside: May 17 to May 31
Start indoors: Apr 12 to Apr 26Transplant outside: May 24 to Jun 7
Start indoors: Apr 26 to May 10Transplant outside: Jun 28 to Jul 12Sow outdoors: Jun 28 to Jul 12
Start indoors: Apr 26 to May 10Transplant outside: Jun 14 to Jun 28
Sow outdoors: Apr 26 to May 17
Sow outdoors: Apr 26 to May 17Plant for fall: Jul 14 to Aug 4
Sow outdoors: May 3 to Jun 14Plant for fall: Jul 28 to Aug 18
Start indoors: May 10 to May 17Transplant outside: Jun 21 to Jun 28Sow outdoors: Jun 14 to Jun 28
Start indoors: May 10 to May 17Sow outdoors: Jun 14 to Jun 28
Start indoors: May 10 to May 17Sow outdoors: Jun 14 to Jun 28
Start indoors: May 10 to May 17Transplant outside: Jun 21 to Jul 5
Sow outdoors: May 17 to Jun 14
Sow outdoors: May 17 to Jun 14Plant for fall: Jun 30 to Jul 14
Sow outdoors: May 17 to Jun 7
Sow outdoors: May 24 to Jun 21
Sow outdoors: May 24 to Jun 14Plant for fall: Jul 14 to Aug 4
Sow outdoors: Jun 14 to Jul 5
Sow outdoors: Jun 14 to Jun 28
Transplant outside: Jun 21 to Jul 12
Plant for fall: Aug 25 to Sep 29
Start indoorsTransplant outsideSow outdoorsPlant for fallMedian frost dates
When to plant vegetables in Boulder, MT
| Crop | Start indoors | Transplant | Sow outdoors | Fall crop |
|---|---|---|---|---|
| Onions | Mar 15 – Mar 29 | May 10 – May 24 | — | — |
| Peppers | Mar 29 – Apr 12 | Jun 21 – Jul 5 | — | — |
| Eggplant | Mar 29 – Apr 12 | Jun 21 – Jul 5 | — | — |
| Tomatoes | Apr 12 – Apr 26 | Jun 14 – Jun 28 | — | — |
| Lettuce | Apr 12 – Apr 26 | May 17 – May 31 | May 10 – Jun 14 | Jul 14 – Aug 11 |
| Kale | Apr 12 – Apr 26 | May 10 – May 24 | May 10 – May 24 | Jun 30 – Jul 21 |
| Broccoli | Apr 12 – Apr 26 | May 24 – Jun 7 | — | — |
| Cabbage | Apr 12 – Apr 26 | May 17 – May 31 | — | — |
| Cauliflower | Apr 12 – Apr 26 | May 24 – Jun 7 | — | — |
| Okra | Apr 26 – May 10 | Jun 28 – Jul 12 | Jun 28 – Jul 12 | — |
| Basil | Apr 26 – May 10 | Jun 14 – Jun 28 | — | — |
| Peas | — | — | Apr 26 – May 17 | — |
| Spinach | — | — | Apr 26 – May 17 | Jul 14 – Aug 4 |
| Radishes | — | — | May 3 – Jun 14 | Jul 28 – Aug 18 |
| Cucumbers | May 10 – May 17 | Jun 21 – Jun 28 | Jun 14 – Jun 28 | — |
| Zucchini & Summer Squash | May 10 – May 17 | — | Jun 14 – Jun 28 | — |
| Winter Squash & Pumpkins Warning: Your frost-free season is about 93 days, which is tight for winter squash & pumpkins (100+ days). Choose early varieties and use transplants or row covers. | May 10 – May 17 | — | Jun 14 – Jun 28 | — |
| Melons & Watermelon Warning: Your frost-free season is about 93 days, which is tight for melons & watermelon (85+ days). Choose early varieties and use transplants or row covers. | May 10 – May 17 | Jun 21 – Jul 5 | — | — |
| Carrots | — | — | May 17 – Jun 14 | — |
| Beets | — | — | May 17 – Jun 14 | Jun 30 – Jul 14 |
| Potatoes | — | — | May 17 – Jun 7 | — |
| Swiss Chard | — | — | May 24 – Jun 21 | — |
| Cilantro | — | — | May 24 – Jun 14 | Jul 14 – Aug 4 |
| Green Beans | — | — | Jun 14 – Jul 5 | — |
| Sweet Corn Warning: Your frost-free season is about 93 days, which is tight for sweet corn (80+ days). Choose early varieties and use transplants or row covers. | — | — | Jun 14 – Jun 28 | — |
| Sweet Potatoes Warning: Your frost-free season is about 93 days, which is tight for sweet potatoes (100+ days). Choose early varieties and use transplants or row covers. | — | Jun 21 – Jul 12 | — | — |
| Garlic | — | — | — | Aug 25 – Sep 29 |
Dates come from the median frost dates and common extension-service timing. Adjust for your soil temperature, microclimate and the varieties you grow.
About these dates
Based on 1991–2020 NOAA climate normals for the Boulder, MT weather station, about 1 km away (elevation 4,905 ft (1,495 m)).
Frost dates near Boulder
- Basin Jun 7 · Sep 8
- Jefferson City Jun 18 · Sep 5
- Clancy Jun 18 · Sep 5
- Radersburg May 19 · Sep 19
- Elliston Jun 9 · Sep 8
- Whitehall Jun 13 · Sep 5
- Toston May 19 · Sep 19
- Fort Harrison May 9 · Sep 30
- Winston May 19 · Sep 19
- East Helena May 9 · Sep 30
Frequently asked questions
When is the last frost in Boulder, MT?
The median last spring frost in Boulder, MT is June 7. There is still about a 10% chance of frost after June 25, so wait until then to plant frost-tender crops like tomatoes and peppers.
When is the first frost in Boulder, MT?
The median first fall frost is September 8, with about a 10% chance of frost before August 25.
How long is the growing season in Boulder, MT?
About 93 days between the median last spring frost and first fall frost.
What planting zone is Boulder, MT?
Boulder, MT is in USDA hardiness zone 5a.