Frost Dates for Castle Rock, CO 80104

Last spring frost May 17 · First fall frost September 27 · 133 frost-free days · USDA Zone 5b

Frost probability for 80104

Frost10%50%90%
Spring (last frost)June 2May 17April 30
Fall (first frost)September 15September 27October 9

Dates come from NOAA 1971–2000 freeze probabilities at the nearest reporting station (36.0 km away); the 50% date is the median year.

Frost dates by ZIP code in Castle Rock, CO

ZIPLast spring frost (50%)First fall frost (50%)Zone
80104May 17September 275b
80108May 17September 275b
80109May 17September 275b

Month by month in Castle Rock

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 17). 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 17. 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 27). Transplant fall brassicas. Sow spinach, arugula, and radishes.
SeptemberFirst frost around September 27. 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 133-day season

Frequently Asked Questions

What is the last frost date for Castle Rock, CO (ZIP 80104)?

The median last spring frost date for Castle Rock, CO (ZIP code 80104) is around May 17. 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 Castle Rock, CO?

The median first fall frost date for Castle Rock, CO is around September 27. 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 Castle Rock, CO?

The frost-free growing season in Castle Rock is about 133 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 Castle Rock, CO?

Based on frost date patterns, Castle Rock 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 Castle Rock, CO?

Start tomato seeds indoors 6–8 weeks before your last frost date (around May 17). For Castle Rock, that's roughly March 22 to April 5. Transplant outdoors 1–2 weeks after your last frost.

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