Longmont, CO planting zone & frost dates
Longmont is in USDA hardiness zone 5b. The last spring frost usually comes around May 7 and the first fall frost around October 5, for a growing season of about 151 days.
Planting calendar for Longmont
Dates are counted from the average last spring frost (May 7) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Mar 12 – Mar 26 Transplant outdoors: May 14 – May 21 |
| Peppers | Start seeds indoors: Feb 26 – Mar 12 Transplant outdoors: May 21 – May 28 |
| Eggplant | Start seeds indoors: Feb 26 – Mar 12 Transplant outdoors: May 21 – May 28 |
| Broccoli | Start seeds indoors: Feb 26 – Mar 12 Transplant outdoors: Apr 9 – Apr 23 |
| Cabbage | Start seeds indoors: Feb 26 – Mar 12 Transplant outdoors: Apr 9 – Apr 23 |
| Onions (sets) | Plant outdoors: Apr 9 – Apr 23 |
| Peas | Sow outdoors: Mar 26 – Apr 9 |
| Spinach | Sow outdoors: Mar 26 – Apr 9 |
| Lettuce | Sow outdoors: Apr 9 – Apr 23 |
| Carrots | Sow outdoors: Apr 9 – Apr 23 |
| Potatoes | Plant outdoors: Apr 9 – Apr 23 |
| Beans | Sow outdoors: May 14 – May 21 |
| Sweet corn | Sow outdoors: May 14 – May 21 |
| Cucumbers | Sow outdoors: May 14 – May 21 |
| Squash & zucchini | Sow outdoors: May 14 – May 21 |
| Basil | Start seeds indoors: Mar 26 – Apr 9 Transplant outdoors: May 14 – May 21 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 10 (8 weeks before the average first fall frost on Oct 5).
Useful for zone 5b gardens
- Seed starting kit with dome — to start seeds indoors before the last frost
- LED grow light for seedlings — stops leggy seedlings
- Frost cloth / plant covers — for late spring and early fall frosts
- Soil thermometer — warm-season crops need soil above 60°F
- Cold frame / mini greenhouse — adds weeks to a short season
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Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Apr 24 | May 7 | May 22 |
| First 32°F frost in fall is before | Oct 16 | Oct 5 | Sep 23 |
How to read it: in spring there is a 10% chance of a frost after May 22, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 23. A hard freeze (28°F) usually ends around April 28.
Hardiness zone by ZIP code in Longmont
- 80501 zone 5b
- 80502 zone 5b
- 80503 zone 6a
- 80504 zone 5b
Nearby cities
Questions
What growing zone is Longmont, CO?
Longmont is in USDA Plant Hardiness Zone 5b (average extreme winter low -15°F to -10°F), according to the USDA 2023 map. Some ZIP codes of the city fall in zone 6a.
When is the last frost in Longmont?
The average last spring frost (32°F) in Longmont is May 7. There is still a 10% chance of frost until May 22.
When can I plant tomatoes in Longmont?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 14 – May 21. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Longmont 2 Ese, CO (USC00055116), 2.1 km (1.3 mi) from the city center, elevation 1509 m, based on 17 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.