Colstrip, MT frost dates
Last spring frostMay 15median date
First fall frostSep 30median date
Growing season138 daysfrost-free
Hardiness zone5aZone 5a
How Colstrip compares
- In 8 years out of 10, the last spring frost in Colstrip falls between May 2 and May 28, a 26-day window. The first fall frost falls between September 15 and October 14.
- The median last frost is 4 days earlier than the Montana median of May 19.
- Its 138-day growing season is longer than 79% of the 325 Montana places with a regular frost season (state median 125 days).
- The last frost is 12 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 162 nights a year at or below 32°F.
Frost probability
| Chance of frost after | Date |
|---|---|
| 90% (early) | May 2 |
| 50% (median) | May 15 |
| 10% (safe for tender plants) | May 28 |
| Chance of frost before | Date |
|---|---|
| 10% (early) | September 15 |
| 50% (median) | September 30 |
| 90% (late) | October 14 |
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 Colstrip, MT
Colstrip, MT frost dates — last frost May 15, first frost Sep 30, zone 5a. 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 5 to Sep 2
Start indoors: Mar 20 to Apr 3Transplant outside: Apr 17 to May 1Sow outdoors: Apr 17 to May 1Plant for fall: Jul 22 to Aug 12
Start indoors: Mar 20 to Apr 3Transplant outside: May 1 to May 15Plant for fall: Jun 24 to Jul 15
Start indoors: Mar 20 to Apr 3Transplant outside: Apr 24 to May 8Plant for fall: Jun 24 to Jul 15
Start indoors: Mar 20 to Apr 3Transplant outside: May 1 to May 15Plant for fall: Jun 24 to Jul 15
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 8 to Jul 29
Sow outdoors: Apr 3 to Apr 24Plant for fall: Aug 5 to Aug 26
Sow outdoors: Apr 10 to May 22Plant for fall: Aug 19 to Sep 9
Start indoors: Apr 17 to Apr 24Transplant outside: May 29 to Jun 5Sow outdoors: May 22 to Jun 5Plant for fall: Jul 15 to Jul 29
Start indoors: Apr 17 to Apr 24Sow outdoors: May 22 to Jun 5Plant for fall: Jul 22 to Aug 5
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 8 to Jul 22
Sow outdoors: Apr 24 to May 22Plant for fall: Jul 22 to Aug 5
Sow outdoors: Apr 24 to May 15
Sow outdoors: May 1 to May 29Plant for fall: Jul 22 to Aug 5
Sow outdoors: May 1 to May 22Plant for fall: Aug 5 to Aug 26
Sow outdoors: May 22 to Jun 12Plant for fall: Jul 15 to Jul 29
Sow outdoors: May 22 to Jun 5
Transplant outside: May 29 to Jun 19
Plant for fall: Sep 16 to Oct 21
Start indoorsTransplant outsideSow outdoorsPlant for fallMedian frost dates
When to plant vegetables in Colstrip, 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 5 – Sep 2 |
| Kale | Mar 20 – Apr 3 | Apr 17 – May 1 | Apr 17 – May 1 | Jul 22 – Aug 12 |
| Broccoli | Mar 20 – Apr 3 | May 1 – May 15 | — | Jun 24 – Jul 15 |
| Cabbage | Mar 20 – Apr 3 | Apr 24 – May 8 | — | Jun 24 – Jul 15 |
| Cauliflower | Mar 20 – Apr 3 | May 1 – May 15 | — | Jun 24 – Jul 15 |
| 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 8 – Jul 29 |
| Spinach | — | — | Apr 3 – Apr 24 | Aug 5 – Aug 26 |
| Radishes | — | — | Apr 10 – May 22 | Aug 19 – Sep 9 |
| Cucumbers | Apr 17 – Apr 24 | May 29 – Jun 5 | May 22 – Jun 5 | Jul 15 – Jul 29 |
| Zucchini & Summer Squash | Apr 17 – Apr 24 | — | May 22 – Jun 5 | Jul 22 – Aug 5 |
| 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 8 – Jul 22 |
| Beets | — | — | Apr 24 – May 22 | Jul 22 – Aug 5 |
| Potatoes | — | — | Apr 24 – May 15 | — |
| Swiss Chard | — | — | May 1 – May 29 | Jul 22 – Aug 5 |
| Cilantro | — | — | May 1 – May 22 | Aug 5 – Aug 26 |
| Green Beans | — | — | May 22 – Jun 12 | Jul 15 – Jul 29 |
| Sweet Corn | — | — | May 22 – Jun 5 | — |
| Sweet Potatoes | — | May 29 – Jun 19 | — | — |
| Garlic | — | — | — | Sep 16 – Oct 21 |
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 Colstrip, MT weather station, 4 km away (elevation 3,219 ft (981 m)).
Frost dates near Colstrip
- Lame Deer May 23 · Sep 15
- Forsyth May 9 · Sep 29
- Busby May 23 · Sep 15
- Ashland May 18 · Sep 22
- Hathaway May 8 · Sep 30
- Bighorn May 26 · Sep 16
- Sanders May 10 · Sep 29
- Rosebud May 9 · Sep 29
- Hysham May 10 · Sep 29
- Birney May 23 · Sep 15
Frequently asked questions
When is the last frost in Colstrip, MT?
The median last spring frost in Colstrip, MT is May 15. There is still about a 10% chance of frost after May 28, so wait until then to plant frost-tender crops like tomatoes and peppers.
When is the first frost in Colstrip, MT?
The median first fall frost is September 30, with about a 10% chance of frost before September 15.
How long is the growing season in Colstrip, MT?
About 138 days between the median last spring frost and first fall frost.
What planting zone is Colstrip, MT?
Colstrip, MT is in USDA hardiness zone 5a.