USDA Hardiness Zone 4b
Frost Dates for 59457 — Lewistown, Montana
Averages from Lewistown Muni Ap, Mt US, the nearest reporting station, 1.1 miles away.
Plant tender crops after May 23, and expect the first autumn freeze around September 21. That leaves about 119 frost-free days.
- Last spring frost
- May 23
- First fall frost
- September 21
- Growing season
- 119 days
- Hardiness zone
- 4b
50% probability
50% probability
frost-free
-25 to -20°F annual low
Planting calendar for 59457
| When | What to do |
|---|---|
| March 14 – March 28 | Start peppers and aubergines indoors They need eight to ten weeks under lights before they can go out. |
| March 28 – April 11 | Start tomatoes indoors Six to eight weeks gives a transplant big enough to set fruit early. |
| April 25 – May 9 | Sow peas, spinach, lettuce, radish outdoors These tolerate light frost and prefer to germinate in cool soil. |
| May 23 onward | Transplant tomatoes, peppers, basil, squash, cucumbers The average last freeze has passed — still watch the actual forecast. |
| by June 29 | Sow the autumn crop of carrots, beets and brassicas They need about twelve weeks to size up before growth stops. |
| by September 7 | Harvest tender crops, or be ready to cover them A first freeze two weeks early is well within normal variation. |
| September 21 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 23 to put tender plants — tomatoes, peppers, basil, squash — in the ground. That is the date by which half of years in Lewistown, Montana are past their last freeze. Harvest before September 21, which leaves roughly 119 frost-free days.
ZIP 59457 sits in USDA hardiness zone 4b, meaning the average coldest night of the year falls between -25 to -20°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.