Aspen 1SW, CO Frost Dates & Growing Season

The two dates every Aspen 1SW garden is planned around, plus ready-computed planting windows for the most popular crops.

Last spring frost
Jun 2
~50% chance of frost after
First fall frost
Sep 23
~50% chance of frost before
Growing season
113 days
frost-free stretch
USDA zone
≈ 5b
estimated from nearby reference
Risky (1 year in 10)MedianSafe (9 years in 10)
Last spring frost May 17 Jun 2 Jun 17
First fall frost Oct 4 Sep 23 Sep 12
Hard freeze (28°F), median last May 18 · first Oct 1

Computed from NOAA 1991–2020 climate normals at the ASPEN 1SW 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 Aspen 1SW

Computed from Aspen 1SW's own frost dates. Click any crop for the full zone-5 growing guide — varieties, spacing, fall plantings, and more.

CropStart seeds indoorsTransplantDirect sow
🍅 Tomatoes Apr 7 – Apr 21 Jun 9 – Jun 16
🫑 Peppers Mar 24 – Apr 7 Jun 16 – Jun 23
🥬 Lettuce Apr 21 – May 5 May 5 – May 19 May 5 – Jun 2
🫛 Peas Apr 21 – May 5
🥕 Carrots May 12 – Jun 2
🥦 Broccoli Apr 14 – Apr 28 May 12 – May 26
🥬 Kale Apr 21 – May 5 May 5 – May 19 May 5 – May 19
🥒 Cucumbers May 5 – May 12 Jun 9 – Jun 16 Jun 9 – Jun 16
🥒 Zucchini May 5 – May 12 Jun 9 – Jun 16 Jun 9 – Jun 16
🫘 Green Beans (Bush) Jun 9 – Jun 16
🌽 Sweet Corn Jun 9 – Jun 16
🌿 Basil Apr 21 – May 5 Jun 9 – Jun 16 Jun 16 – Jun 23
🧄 Garlic Fall: Aug 12 – Sep 9

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 Aspen 1SW's dates automatically after a lookup.

Reading the season in Aspen 1SW

At about 113 frost-free days, Aspen 1SW 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 Aspen 1SW, CO?

The average last spring frost in Aspen 1SW is around Jun 2, 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 Aspen 1SW, CO?

The average first fall frost in Aspen 1SW arrives around Sep 23. 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 Aspen 1SW?

About 113 frost-free days on average, from Jun 2 to Sep 23. 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 Aspen 1SW in?

USDA zone 5b 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.