When to plant in Longmont, CO
USDA Zone 5bEverything below — frost dates, hardiness zone, and what to plant when in Longmont, Colorado — is derived from the closest NOAA station with complete climate normals.
Zone 5b means Longmont sees real winter cold, but a good range of perennials survive; the frost dates above govern when annual vegetables go out.
The average first fall frost in Longmont is now 8 days later than in the 1981–2010 normals. See how frost dates are shifting nationwide →
Frost probability
LONGMONT 2 ESE · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Longmont’s own odds, recorded at LONGMONT 2 ESE.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Jun 2 | May 18 | May 5 | Sep 14 | Sep 28 | Oct 8 |
| 32°F | May 22 | May 7 | Apr 24 | Sep 23 | Oct 5 | Oct 16 |
| 28°F | May 12 | Apr 28 | Apr 11 | Sep 30 | Oct 12 | Oct 25 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Full-year planting calendar
Each bar is the exact window to take a planting action in Longmont, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 30 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Longmont planting FAQ
When is the last spring frost in Longmont, CO?
Plan for the last spring frost in Longmont around May 7 (the date it has a 50% chance of a 32°F freeze). Anything frost-sensitive should go out after it.
When is the first fall frost in Longmont, CO?
Expect Longmont's first fall frost near October 5 — a 50% chance of 32°F by that date. Bring in or cover tender crops ahead of it.
What hardiness zone is Longmont in?
Longmont is in USDA hardiness zone 5b. In zone 5b, winters are cold — pick cold-hardy varieties and protect tender crops.
How long is the growing season in Longmont?
There are roughly 151 frost-free days in Longmont (a moderate growing season), running from the average last frost around May 7 to the first fall frost near October 5.
When should I plant tomatoes in Longmont?
In Longmont, start tomato seeds indoors around March 12–March 26, then transplant seedlings outdoors around May 14 once the danger of frost has passed.
Never miss a window in Longmont
An email when it’s time to start seeds, transplant, and sow — timed to Longmont’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reachFrost dates recorded at LONGMONT 2 ESE, 2 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Longmont, CO — Frost Dates & Zone 5b." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00055116. Retrieved from https://blissgarden.com/colorado/longmont.