Circle, MT Frost Dates & Growing Season

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

Last spring frost
May 17
~50% chance of frost after
First fall frost
Sep 22
~50% chance of frost before
Growing season
128 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 4 May 17 May 31
First fall frost Oct 6 Sep 22 Sep 7
Hard freeze (28°F), median last May 6 · first Oct 1

Computed from NOAA 1991–2020 climate normals at the CIRCLE 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 Circle

Computed from Circle'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 Mar 22 – Apr 5 May 24 – May 31
🫑 Peppers Mar 8 – Mar 22 May 31 – Jun 7
🥬 Lettuce Apr 5 – Apr 19 Apr 19 – May 3 Apr 19 – May 17
🫛 Peas Apr 5 – Apr 19
🥕 Carrots Apr 26 – May 17
🥦 Broccoli Mar 29 – Apr 12 Apr 26 – May 10
🥬 Kale Apr 5 – Apr 19 Apr 19 – May 3 Apr 19 – May 3
🥒 Cucumbers Apr 19 – Apr 26 May 24 – May 31 May 24 – May 31
🥒 Zucchini Apr 19 – Apr 26 May 24 – May 31 May 24 – May 31
🫘 Green Beans (Bush) May 24 – May 31
🌽 Sweet Corn May 24 – May 31
🌿 Basil Apr 5 – Apr 19 May 24 – May 31 May 31 – Jun 7
🧄 Garlic Fall: Aug 11 – Sep 8

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 Circle's dates automatically after a lookup.

Reading the season in Circle

At about 128 frost-free days, Circle 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 Circle, MT?

The average last spring frost in Circle is around May 17, 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 Circle, MT?

The average first fall frost in Circle arrives around Sep 22. 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 Circle?

About 128 frost-free days on average, from May 17 to Sep 22. 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 Circle 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.