USDA Hardiness Zone 5a
Frost Dates for 59635 — East Helena, Montana
Averages from Helena Rgnl Ap, Mt US, the nearest reporting station, 4.3 miles away.
Plant tender crops after May 9, and expect the first autumn freeze around September 30. That leaves about 142 frost-free days.
- Last spring frost
- May 9
- First fall frost
- September 30
- Growing season
- 142 days
- Hardiness zone
- 5a
50% probability
50% probability
frost-free
-20 to -15°F annual low
Planting calendar for 59635
| When | What to do |
|---|---|
| February 28 – March 14 | Start peppers and aubergines indoors They need eight to ten weeks under lights before they can go out. |
| March 14 – March 28 | Start tomatoes indoors Six to eight weeks gives a transplant big enough to set fruit early. |
| April 11 – April 25 | Sow peas, spinach, lettuce, radish outdoors These tolerate light frost and prefer to germinate in cool soil. |
| May 9 onward | Transplant tomatoes, peppers, basil, squash, cucumbers The average last freeze has passed — still watch the actual forecast. |
| by July 8 | Sow the autumn crop of carrots, beets and brassicas They need about twelve weeks to size up before growth stops. |
| by September 16 | Harvest tender crops, or be ready to cover them A first freeze two weeks early is well within normal variation. |
| September 30 onward | Plant garlic, mulch beds, lift tender bulbs Garlic wants a cold spell; bare soil loses structure over winter. |
What these dates mean
Wait until after May 9 to put tender plants — tomatoes, peppers, basil, squash — in the ground. That is the date by which half of years in East Helena, Montana are past their last freeze. Harvest before September 30, which leaves roughly 142 frost-free days.
ZIP 59635 sits in USDA hardiness zone 5a, meaning the average coldest night of the year falls between -20 to -15°F. Zone decides what survives the winter; frost dates decide when to plant each spring. They are different questions, and a plant can pass one and fail the other.
These are thirty-year NOAA averages for 1991–2020, not a forecast. Any individual year can frost two to three weeks either side of these dates, so check the actual forecast before planting out.