Frost dates for Heron, MT
When the last spring frost and first fall frost usually come, and when to plant, from 30 years of NOAA weather records.
Last spring frost
May 20
typical year
Safe to plant
June 5
frost is over in 9 of 10 years
First fall frost
September 20
typical year
Growing season
119 days
frost-free, typical year
Frost watch this week
Tonight's and the next six nights' lows from the National Weather Service forecast for the station's location. Frost: 32°F or colder. Possible frost: 33–36°F, mostly in low spots on clear, still nights.
When to plant
Counted from this page's frost dates. Warm-season crops wait for the safe-to-plant date.
| Crop | Start indoors | Plant outside | Fall planting |
|---|---|---|---|
| Cool-season crops | |||
| Peas | – | Apr 22 – May 6 | – |
| Spinach | – | Apr 8 – 29 | Jul 26 – Aug 9 |
| Lettuce | – | Apr 22 – May 13 | Jul 26 – Aug 9 |
| Radishes | – | Apr 22 – Jun 3 | Aug 9 – 23 |
| Onions | – | Apr 22 – May 6 | – |
| Potatoes | – | Apr 29 – May 13 | – |
| Carrots | – | Apr 29 – May 20 | Jun 28 – Jul 12 |
| Beets | – | Apr 22 – May 6 | Jul 12 – 26 |
| Kale | Mar 11 – 25 | Apr 22 – May 6 | Jul 12 – 26 |
| Broccoli | Mar 11 – 25 | May 6 – 20 | Jul 12 – 26 |
| Cabbage | Feb 25 – Mar 11 | Apr 22 – May 6 | Jul 12 – 26 |
| Garlic | – | – | Aug 9 – 30 |
| Warm-season crops | |||
| Tomatoes | Apr 10 – 24 | Jun 5 – 19 | – |
| Peppers | Apr 3 – 17 | Jun 12 – 26 | – |
| Cucumbers | – | Jun 5 – 26 | – |
| Zucchini | – | Jun 5 – Jul 3 | – |
| Pumpkins | – | Jun 5 – 26 | – |
| Green beans | – | Jun 5 – Jul 12 | – |
| Sweet corn | – | Jun 5 – Jul 3 | – |
| Basil | May 1 – 15 | Jun 12 – 19 | – |
Onions: from sets. Green beans: until 10 weeks before the first fall frost. Garlic: harvest next summer.
The year at a glance
Start indoors Plant outside Fall planting
The chance of frost, by date
| 1 year in 10 | Half of years | 9 years in 10 | |
|---|---|---|---|
| Spring frost after | June 5 | May 20 | May 4 |
| Fall frost before | September 1 | September 20 | October 5 |
Frost here means 32°F or colder. Read across: in 1 year in 10, spring frost carries on past the first date, which is why that date is the safe-to-plant date.
Hard freeze (28°F)
A hard freeze kills most garden plants, even hardy ones. The last one in spring usually comes around May 2 (May 16 in a late year), and the first one in fall around October 7.
Where these dates come from
NOAA weather station HERON 2 NW, MT US (USC00244084), 1991–2020 climate normals. It's the nearest station for 2 zip codes, all within 10 miles:
59844, 59853
Planting dates follow common university extension advice; your local extension service can fine-tune them for your soil and microclimate.