Frost Dates for Fort Collins, CO 80521

Last spring frost May 5 · First fall frost September 28 · 146 frost-free days · USDA Zone 5b

Frost probability for 80521

Frost10%50%90%
Spring (last frost)May 18May 5April 23
Fall (first frost)September 16September 28October 10

Dates come from NOAA 1971–2000 freeze probabilities at the nearest reporting station (47.4 km away); the 50% date is the median year.

Month by month in Fort Collins

MonthWhat to do
JanuaryPlan your garden layout and order seeds. Research varieties suited to your frost dates.
FebruaryStart the slowest-growing crops indoors: onions, leeks, and celery (10–12 weeks before last frost).
MarchStart peppers and eggplant indoors 8–10 weeks before your last frost (around May 5). Start tomatoes about two weeks later, 6–8 weeks before last frost.
AprilStart herbs, basil, and squash indoors (4–6 weeks before frost). Direct sow peas, spinach, and lettuce if soil is workable.
MayLast frost around May 5. Transplant cold-hardy crops (broccoli, kale, cabbage). Harden off tomato and pepper seedlings.
JuneFrost has passed. Transplant tomatoes, peppers, and basil outdoors. Direct sow beans, cucumbers, squash, and zucchini.
JulySuccession plant lettuce and greens. Keep beds weeded and watered consistently.
AugustFirst fall frost approaching (around September 28). Transplant fall brassicas. Sow spinach, arugula, and radishes.
SeptemberFirst frost around September 28. Harvest tomatoes and peppers before frost. Cover cold-hardy crops with row covers to extend the season.
OctoberHarvest remaining cold-hardy crops: kale, carrots, and beets. Plant garlic bulbs for next year.
NovemberClean up spent plants. Add compost to beds. Rest, review, and plan next year's garden.
DecemberClean up spent plants. Add compost to beds. Rest, review, and plan next year's garden.

What grows in a 146-day season

Frequently Asked Questions

What is the last frost date for Fort Collins, CO (ZIP 80521)?

The median last spring frost date for Fort Collins, CO (ZIP code 80521) is around May 5. This is the 50th-percentile date — meaning half of all years see frost end before this date. Use the 10% and 90% dates to understand the full range of risk in your area.

What is the first fall frost date in Fort Collins, CO?

The median first fall frost date for Fort Collins, CO is around September 28. Plan to harvest or protect frost-sensitive crops like tomatoes and peppers at least a week before this date to avoid losses.

How long is the growing season in Fort Collins, CO?

The frost-free growing season in Fort Collins is about 146 days. This window — from your last spring frost to your first fall frost — is when warm-season vegetables thrive. Cold-hardy crops like kale, spinach, and peas can extend your season by weeks on either side.

What USDA hardiness zone is Fort Collins, CO?

Based on frost date patterns, Fort Collins falls approximately in 5b. USDA hardiness zones are determined by average annual minimum winter temperatures and guide both plant selection and planting timing for your location.

When should I start tomatoes indoors in Fort Collins, CO?

Start tomato seeds indoors 6–8 weeks before your last frost date (around May 5). For Fort Collins, that's roughly March 10 to March 24. Transplant outdoors 1–2 weeks after your last frost.

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