When is the last frost in Longmont, CO?

The average last spring frost (50% probability of 32°F) in Longmont is around May 7, based on NOAA/NCEI 1991–2020 normals at the Longmont 2 Ese, CO station (1.3 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 Longmont?

The average first fall frost in Longmont, CO is around October 5, giving roughly a 151-day frost-free growing season.

What planting zone is Longmont, CO?

Longmont 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 Longmont?

In Longmont, CO the warmest month is July (avg high 90.8°F) and the coldest is January (avg low 14.4°F). It averages about 15.2″ of precipitation a year.

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Longmont, CO Planting Calendar & Frost Dates

USDA hardiness zone 5b · ZIP 80501 · elevation 1509′ · NOAA & USDA data

Longmont is in USDA hardiness zone 5b (winters can dip to -15 to -10°F), with a growing season of about 151 days between the average last spring frost (May 7) and first fall frost (October 5). The warmest month is July (avg high 90.8°F); the coldest is January (avg low 14.4°F). It averages 15.2″ of precipitation a year. 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 7

avg (50% prob, 32°F)

First fall frost

October 5

avg (50% prob, 32°F)

Growing season

151 days

frost-free window

Warmest month

July

avg high 90.8°F

Month-by-month typical weather in Longmont

Average conditions from the NOAA/NCEI 1991–2020 U.S. Climate Normals at Longmont 2 Ese, CO (1.3 mi away). Temperatures °F, precipitation & snow in inches.

Month	Avg high	Avg low	Mean	Precip (in)	Snow (in)
January	44.5	14.4	29.4	0.6	–
February	46.8	17.1	32	0.6	–
March	56.8	26.1	41.5	1.2	–
April	63.2	32.3	47.8	2	–
May	72.7	41.4	57.1	2	–
June	83.9	50.5	67.2	1.8	–
July	90.8	55.4	73.1	1.7	–
August	87.8	53.7	70.8	2	–
September	80.3	45.8	63	1.3	–
October	66.2	32.9	49.5	1	–
November	53.5	23.5	38.5	0.8	–
December	44.8	14.5	29.6	0.5	–

What to plant when in Longmont

Crop	Season	What to do & when
Tomatoes	warm	Start seeds indoors: March 26 · Transplant out: May 14
Peppers	warm	Start seeds indoors: March 12 · Transplant out: May 21
Cucumbers	warm	Direct sow: May 14
Zucchini / squash	warm	Direct sow: May 14
Bush beans	warm	Direct sow: May 14
Sweet corn	warm	Direct sow: May 14
Basil	warm	Transplant out: May 14
Peas	cool	Sow (spring): April 2 · Sow (fall): August 10
Spinach	cool	Sow (spring): March 26 · Sow (fall): August 10
Lettuce	cool	Sow (spring): April 9 · Sow (fall): August 10
Kale	cool	Transplant (spring): April 16 · Sow (fall): July 27
Broccoli	cool	Transplant (spring): April 23
Carrots	cool	Sow (spring): April 16 · Sow (fall): July 27
Onions	cool	Transplant (spring): April 9
Potatoes	cool	Plant seed potatoes: April 23
Radishes	cool	Sow (spring): April 9 · Sow (fall): August 24

Dates are computed from Longmont'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

Boulder, CO Loveland, CO Broomfield, CO Thornton, CO Westminster, CO Arvada, CO Commerce City, CO Denver, 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 — Longmont 2 Ese, CO, 1.3 mi (confidence: high). Monthly weather normals: Longmont 2 Ese, CO, 1.3 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.