Big Arm, MT frost dates
Last spring frostMay 15median date
First fall frostSep 28median date
Growing season136 daysfrost-free
Hardiness zone6aZone 6a
How Big Arm compares
- In 8 years out of 10, the last spring frost in Big Arm falls between May 3 and June 1, a 29-day window. The first fall frost falls between September 13 and October 11.
- The median last frost is 4 days earlier than the Montana median of May 19.
- Its 136-day growing season is longer than 72% of the 325 Montana places with a regular frost season (state median 125 days).
- The last frost is 16 days later than the zone 6a median of Apr 29. Hardiness zones measure winter cold, not season length, so use these dates rather than a zone 6a chart.
- The weather station records an average of 153 nights a year at or below 32°F.
- Compared with Missoula, MT, 104 km away, the last frost here is 5 days later than there and the growing season is 8 days shorter.
Frost probability
| Chance of frost after | Date |
|---|---|
| 90% (early) | May 3 |
| 50% (median) | May 15 |
| 10% (safe for tender plants) | June 1 |
| Chance of frost before | Date |
|---|---|
| 10% (early) | September 13 |
| 50% (median) | September 28 |
| 90% (late) | October 11 |
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 Big Arm, MT
Big Arm, MT frost dates — last frost May 15, first frost Sep 28, zone 6a. whenisthelastfrost.com
Crop
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Start indoors: Feb 20 to Mar 6Transplant outside: Apr 17 to May 1
Start indoors: Mar 6 to Mar 20Transplant outside: May 29 to Jun 12
Start indoors: Mar 6 to Mar 20Transplant outside: May 29 to Jun 12
Start indoors: Mar 20 to Apr 3Transplant outside: May 22 to Jun 5
Start indoors: Mar 20 to Apr 3Transplant outside: Apr 24 to May 8Sow outdoors: Apr 17 to May 22Plant for fall: Aug 3 to Aug 31
Start indoors: Mar 20 to Apr 3Transplant outside: Apr 17 to May 1Sow outdoors: Apr 17 to May 1Plant for fall: Jul 20 to Aug 10
Start indoors: Mar 20 to Apr 3Transplant outside: May 1 to May 15Plant for fall: Jun 22 to Jul 13
Start indoors: Mar 20 to Apr 3Transplant outside: Apr 24 to May 8Plant for fall: Jun 22 to Jul 13
Start indoors: Mar 20 to Apr 3Transplant outside: May 1 to May 15Plant for fall: Jun 22 to Jul 13
Start indoors: Apr 3 to Apr 17Transplant outside: Jun 5 to Jun 19Sow outdoors: Jun 5 to Jun 19
Start indoors: Apr 3 to Apr 17Transplant outside: May 22 to Jun 5
Sow outdoors: Apr 3 to Apr 24Plant for fall: Jul 6 to Jul 27
Sow outdoors: Apr 3 to Apr 24Plant for fall: Aug 3 to Aug 24
Sow outdoors: Apr 10 to May 22Plant for fall: Aug 17 to Sep 7
Start indoors: Apr 17 to Apr 24Transplant outside: May 29 to Jun 5Sow outdoors: May 22 to Jun 5Plant for fall: Jul 13 to Jul 27
Start indoors: Apr 17 to Apr 24Sow outdoors: May 22 to Jun 5Plant for fall: Jul 20 to Aug 3
Start indoors: Apr 17 to Apr 24Sow outdoors: May 22 to Jun 5
Start indoors: Apr 17 to Apr 24Transplant outside: May 29 to Jun 12
Sow outdoors: Apr 24 to May 22Plant for fall: Jul 6 to Jul 20
Sow outdoors: Apr 24 to May 22Plant for fall: Jul 20 to Aug 3
Sow outdoors: Apr 24 to May 15
Sow outdoors: May 1 to May 29Plant for fall: Jul 20 to Aug 3
Sow outdoors: May 1 to May 22Plant for fall: Aug 3 to Aug 24
Sow outdoors: May 22 to Jun 12Plant for fall: Jul 13 to Jul 27
Sow outdoors: May 22 to Jun 5
Transplant outside: May 29 to Jun 19
Plant for fall: Sep 14 to Oct 19
Start indoorsTransplant outsideSow outdoorsPlant for fallMedian frost dates
When to plant vegetables in Big Arm, MT
| Crop | Start indoors | Transplant | Sow outdoors | Fall crop |
|---|---|---|---|---|
| Onions | Feb 20 – Mar 6 | Apr 17 – May 1 | — | — |
| Peppers | Mar 6 – Mar 20 | May 29 – Jun 12 | — | — |
| Eggplant | Mar 6 – Mar 20 | May 29 – Jun 12 | — | — |
| Tomatoes | Mar 20 – Apr 3 | May 22 – Jun 5 | — | — |
| Lettuce | Mar 20 – Apr 3 | Apr 24 – May 8 | Apr 17 – May 22 | Aug 3 – Aug 31 |
| Kale | Mar 20 – Apr 3 | Apr 17 – May 1 | Apr 17 – May 1 | Jul 20 – Aug 10 |
| Broccoli | Mar 20 – Apr 3 | May 1 – May 15 | — | Jun 22 – Jul 13 |
| Cabbage | Mar 20 – Apr 3 | Apr 24 – May 8 | — | Jun 22 – Jul 13 |
| Cauliflower | Mar 20 – Apr 3 | May 1 – May 15 | — | Jun 22 – Jul 13 |
| Okra | Apr 3 – Apr 17 | Jun 5 – Jun 19 | Jun 5 – Jun 19 | — |
| Basil | Apr 3 – Apr 17 | May 22 – Jun 5 | — | — |
| Peas | — | — | Apr 3 – Apr 24 | Jul 6 – Jul 27 |
| Spinach | — | — | Apr 3 – Apr 24 | Aug 3 – Aug 24 |
| Radishes | — | — | Apr 10 – May 22 | Aug 17 – Sep 7 |
| Cucumbers | Apr 17 – Apr 24 | May 29 – Jun 5 | May 22 – Jun 5 | Jul 13 – Jul 27 |
| Zucchini & Summer Squash | Apr 17 – Apr 24 | — | May 22 – Jun 5 | Jul 20 – Aug 3 |
| Winter Squash & Pumpkins | Apr 17 – Apr 24 | — | May 22 – Jun 5 | — |
| Melons & Watermelon | Apr 17 – Apr 24 | May 29 – Jun 12 | — | — |
| Carrots | — | — | Apr 24 – May 22 | Jul 6 – Jul 20 |
| Beets | — | — | Apr 24 – May 22 | Jul 20 – Aug 3 |
| Potatoes | — | — | Apr 24 – May 15 | — |
| Swiss Chard | — | — | May 1 – May 29 | Jul 20 – Aug 3 |
| Cilantro | — | — | May 1 – May 22 | Aug 3 – Aug 24 |
| Green Beans | — | — | May 22 – Jun 12 | Jul 13 – Jul 27 |
| Sweet Corn | — | — | May 22 – Jun 5 | — |
| Sweet Potatoes | — | May 29 – Jun 19 | — | — |
| Garlic | — | — | — | Sep 14 – Oct 19 |
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 Skq Dam, MT weather station, 14 km away (elevation 2,730 ft (832 m)).
Frost dates near Big Arm
- Elmo May 15 · Sep 28
- Dayton May 1 · Oct 15
- Rollins May 1 · Oct 15
- Polson May 10 · Oct 5
- Proctor May 1 · Oct 15
- Pablo May 10 · Oct 5
- Lakeside May 12 · Sep 25
- Lonepine May 15 · Sep 28
- Ronan May 10 · Oct 5
- Somers May 12 · Sep 25
Frequently asked questions
When is the last frost in Big Arm, MT?
The median last spring frost in Big Arm, MT is May 15. There is still about a 10% chance of frost after June 1, so wait until then to plant frost-tender crops like tomatoes and peppers.
When is the first frost in Big Arm, MT?
The median first fall frost is September 28, with about a 10% chance of frost before September 13.
How long is the growing season in Big Arm, MT?
About 136 days between the median last spring frost and first fall frost.
What planting zone is Big Arm, MT?
Big Arm, MT is in USDA hardiness zone 6a.