Harlem 20S, MT Frost Dates & Growing Season

The two dates every Harlem 20S garden is planned around, plus ready-computed planting windows for the most popular crops.

Last spring frost
May 24
~50% chance of frost after
First fall frost
Sep 22
~50% chance of frost before
Growing season
121 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 12 May 24 Jun 11
First fall frost Oct 3 Sep 22 Sep 8
Hard freeze (28°F), median last May 12 · first Oct 1

Computed from NOAA 1991–2020 climate normals at the HARLEM 20S 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 Harlem 20S

Computed from Harlem 20S'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 29 – Apr 12 May 31 – Jun 7
🫑 Peppers Mar 15 – Mar 29 Jun 7 – Jun 14
🥬 Lettuce Apr 12 – Apr 26 Apr 26 – May 10 Apr 26 – May 24
🫛 Peas Apr 12 – Apr 26
🥕 Carrots May 3 – May 24
🥦 Broccoli Apr 5 – Apr 19 May 3 – May 17
🥬 Kale Apr 12 – Apr 26 Apr 26 – May 10 Apr 26 – May 10
🥒 Cucumbers Apr 26 – May 3 May 31 – Jun 7 May 31 – Jun 7
🥒 Zucchini Apr 26 – May 3 May 31 – Jun 7 May 31 – Jun 7
🫘 Green Beans (Bush) May 31 – Jun 7
🌽 Sweet Corn May 31 – Jun 7
🌿 Basil Apr 12 – Apr 26 May 31 – Jun 7 Jun 7 – Jun 14
🧄 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 Harlem 20S's dates automatically after a lookup.

Reading the season in Harlem 20S

At about 121 frost-free days, Harlem 20S 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 Harlem 20S, MT?

The average last spring frost in Harlem 20S is around May 24, 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 Harlem 20S, MT?

The average first fall frost in Harlem 20S 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 Harlem 20S?

About 121 frost-free days on average, from May 24 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 Harlem 20S 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.