St Mary, MT Frost Dates & Growing Season

The two dates every St Mary garden is planned around, plus ready-computed planting windows for the most popular crops.

Last spring frost
Jun 12
~50% chance of frost after
First fall frost
Sep 5
~50% chance of frost before
Growing season
85 days
frost-free stretch
USDA zone
≈ 4b
estimated from nearby reference
Risky (1 year in 10)MedianSafe (9 years in 10)
Last spring frost May 27 Jun 12 Jul 3
First fall frost Sep 21 Sep 5 Aug 18
Hard freeze (28°F), median last May 19 · first Sep 20

Computed from NOAA 1991–2020 climate normals at the ST MARY station. "Safe" means frost past that date happens only about one year in ten. Your exact garden can still run days either way with elevation and terrain — check your own ZIP with the frost date lookup.

Planting windows for St Mary

Computed from St Mary's own frost dates. Click any crop for the full zone-4 growing guide — varieties, spacing, fall plantings, and more.

CropStart seeds indoorsTransplantDirect sow
🍅 Tomatoes Apr 17 – May 1 Jun 19 – Jun 26
🫑 Peppers Apr 3 – Apr 17 Jun 26 – Jul 3
🥬 Lettuce May 1 – May 15 May 15 – May 29 May 15 – Jun 12
🫛 Peas May 1 – May 15
🥕 Carrots May 22 – Jun 12
🥦 Broccoli Apr 24 – May 8 May 22 – Jun 5
🥬 Kale May 1 – May 15 May 15 – May 29 May 15 – May 29
🥒 Cucumbers May 15 – May 22 Jun 19 – Jun 26 Jun 19 – Jun 26
🥒 Zucchini May 15 – May 22 Jun 19 – Jun 26 Jun 19 – Jun 26
🫘 Green Beans (Bush) Jun 19 – Jun 26
🌽 Sweet Corn Jun 19 – Jun 26
🌿 Basil May 1 – May 15 Jun 19 – Jun 26 Jun 26 – Jul 3
🧄 Garlic Fall: Jul 25 – Aug 22

Want this for all 50 crops, or as a printable seed-shelf schedule? Use the planting calendar and the seed starting scheduler — both can use St Mary's dates automatically after a lookup.

Reading the season in St Mary

At about 85 frost-free days, St Mary rewards efficiency: start warm-season crops indoors, favor short-season varieties, and lean on cold-tolerant crops — peas, kale, roots — that shrug off the shoulder-season frosts on either end.

Frequently asked questions

When is the last frost in St Mary, MT?

The average last spring frost in St Mary is around Jun 12, based on 1991–2020 climate normals. That's a median — about half of years see a later frost, so tender crops are safest going out a week or two after this date.

When is the first frost in St Mary, MT?

The average first fall frost in St Mary arrives around Sep 5. Crops still in the garden need to mature before then, and frost-tender plants need harvesting or protection as the date approaches.

How long is the growing season in St Mary?

About 85 frost-free days on average, from Jun 12 to Sep 5. That favors short-season varieties and transplants for warm-season crops, with cool-season vegetables doing the heavy lifting in spring and fall.

What USDA zone is St Mary in?

USDA zone 4b on the 2023 Plant Hardiness Zone Map. Note that zones describe winter cold, not frost timing — these frost dates are the numbers to plan plantings around.