Rudyard 21 N, MT Frost Dates & Growing Season

The two dates every Rudyard 21 N garden is planned around, plus ready-computed planting windows for the most popular crops.

Last spring frost
May 30
~50% chance of frost after
First fall frost
Sep 11
~50% chance of frost before
Growing season
104 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 17 May 30 Jun 15
First fall frost Sep 24 Sep 11 Aug 27
Hard freeze (28°F), median last May 16 · first Sep 23

Computed from NOAA 1991–2020 climate normals at the RUDYARD 21 N 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 Rudyard 21 N

Computed from Rudyard 21 N'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 4 – Apr 18 Jun 6 – Jun 13
🫑 Peppers Mar 21 – Apr 4 Jun 13 – Jun 20
🥬 Lettuce Apr 18 – May 2 May 2 – May 16 May 2 – May 30
🫛 Peas Apr 18 – May 2
🥕 Carrots May 9 – May 30
🥦 Broccoli Apr 11 – Apr 25 May 9 – May 23
🥬 Kale Apr 18 – May 2 May 2 – May 16 May 2 – May 16
🥒 Cucumbers May 2 – May 9 Jun 6 – Jun 13 Jun 6 – Jun 13
🥒 Zucchini May 2 – May 9 Jun 6 – Jun 13 Jun 6 – Jun 13
🫘 Green Beans (Bush) Jun 6 – Jun 13
🌽 Sweet Corn Jun 6 – Jun 13
🌿 Basil Apr 18 – May 2 Jun 6 – Jun 13 Jun 13 – Jun 20
🧄 Garlic Fall: Jul 31 – Aug 28

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 Rudyard 21 N's dates automatically after a lookup.

Reading the season in Rudyard 21 N

At about 104 frost-free days, Rudyard 21 N 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 Rudyard 21 N, MT?

The average last spring frost in Rudyard 21 N is around May 30, 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 Rudyard 21 N, MT?

The average first fall frost in Rudyard 21 N arrives around Sep 11. 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 Rudyard 21 N?

About 104 frost-free days on average, from May 30 to Sep 11. 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 Rudyard 21 N 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.