When is the last frost in Castle Rock, CO?

The average last spring frost (50% probability of 32°F) in Castle Rock is around May 9, based on NOAA/NCEI 1991–2020 normals at the Castle Rock, CO station (8.8 mi away). Half of years it falls outside that date, so wait 1–2 weeks past it before setting out frost-tender plants.

When is the first fall frost in Castle Rock?

The average first fall frost in Castle Rock, CO is around October 2, giving roughly a 143-day frost-free growing season.

What planting zone is Castle Rock, CO?

Castle Rock is in USDA plant hardiness zone 5b (average annual extreme minimum temperature -15 to -10°F), from the 2023 USDA Plant Hardiness Zone Map.

What is the weather like in Castle Rock?

In Castle Rock, CO the warmest month is July (avg high 85.7°F) and the coldest is January (avg low 18°F). It averages about 17.6″ of precipitation a year and 57.5″ of snow.

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Castle Rock, CO Planting Calendar & Frost Dates

USDA hardiness zone 5b · ZIP 80104 · elevation 1885′ · NOAA & USDA data

Castle Rock is in USDA hardiness zone 5b (winters can dip to -15 to -10°F), with a growing season of about 143 days between the average last spring frost (May 9) and first fall frost (October 2). The warmest month is July (avg high 85.7°F); the coldest is January (avg low 18°F). It averages 17.6″ of precipitation a year and 57.5″ of snow, mostly mid-winter. Start cool-season crops several weeks before the last frost; wait until after it for tomatoes, peppers and other tender crops.

Hardiness zone

min temp -15 to -10°F

Last spring frost

May 9

avg (50% prob, 32°F)

First fall frost

October 2

avg (50% prob, 32°F)

Growing season

143 days

frost-free window

Warmest month

July

avg high 85.7°F

Month-by-month typical weather in Castle Rock

Average conditions from the NOAA/NCEI 1991–2020 U.S. Climate Normals at Castle Rock, CO (8.8 mi away). Temperatures °F, precipitation & snow in inches.

Month	Avg high	Avg low	Mean	Precip (in)	Snow (in)
January	45.6	18	31.8	0.6	8
February	46.7	19.9	33.3	0.7	10.7
March	53.8	26.5	40.2	1.5	10.7
April	59.1	32.5	45.8	2.1	8.3
May	68.4	41.6	55	2	1.1
June	80.2	50.5	65.3	2.2	0
July	85.7	56.4	71.1	2.3	0
August	83.6	54.7	69.1	2.5	0
September	76.7	46.6	61.7	1.4	0.4
October	64.8	34.6	49.7	0.9	2.7
November	53.9	25.7	39.8	0.8	6.7
December	45.7	18	31.8	0.7	8.9

What to plant when in Castle Rock

Crop	Season	What to do & when
Tomatoes	warm	Start seeds indoors: March 28 · Transplant out: May 16
Peppers	warm	Start seeds indoors: March 14 · Transplant out: May 23
Cucumbers	warm	Direct sow: May 16
Zucchini / squash	warm	Direct sow: May 16
Bush beans	warm	Direct sow: May 16
Sweet corn	warm	Direct sow: May 16
Basil	warm	Transplant out: May 16
Peas	cool	Sow (spring): April 4 · Sow (fall): August 7
Spinach	cool	Sow (spring): March 28 · Sow (fall): August 7
Lettuce	cool	Sow (spring): April 11 · Sow (fall): August 7
Kale	cool	Transplant (spring): April 18 · Sow (fall): July 24
Broccoli	cool	Transplant (spring): April 25
Carrots	cool	Sow (spring): April 18 · Sow (fall): July 24
Onions	cool	Transplant (spring): April 11
Potatoes	cool	Plant seed potatoes: April 25
Radishes	cool	Sow (spring): April 11 · Sow (fall): August 21

Dates are computed from Castle Rock's average frost dates using standard cool-/warm-season timing. They're a starting point — a cold spring, a warm microclimate, or a fast-maturing variety can shift them by a week or two.

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Gear that helps you plant on time

Some links below are Amazon affiliate links (tag theboringthin-20). If you buy through them we may earn a small commission at no extra cost to you — it keeps this almanac free. We only list generic, genuinely-useful categories.

Soil thermometer →Know when the ground is warm enough to sow — the honest signal, not the calendar.Seed-starting trays + dome →Start tomatoes & peppers indoors on the schedule below.Seedling heat mat →Warms the soil for faster, more even germination indoors.Floating row cover / frost cloth →Buys you 4–8°F to beat an early or late frost.Raised garden bed kit →Warms and drains faster in spring than in-ground soil.Garden gloves →Basic kit for transplant day.

Nearby cities in Colorado

Parker, CO Littleton, CO Aurora, CO Colorado Springs, CO Denver, CO Arvada, CO Westminster, CO Commerce City, CO

Sources & method. Hardiness zone: 2023 USDA Plant Hardiness Zone Map (PRISM Climate Group, Oregon State University). Frost dates & growing season: NOAA/NCEI 1991–2020 U.S. Climate Normals, nearest station with published freeze normals — Castle Rock, CO, 8.8 mi (confidence: high). Monthly weather normals: Castle Rock, CO, 8.8 mi (confidence: high). ZIP centroid from U.S. Census 2023 ZCTA data. Frost dates are the 50%-probability date — half of years frost occurs outside the stated date. Nothing here is invented; where the nearest station is far, we say so. Last updated 2026-07-22.