Frost Dates for Longmont, CO 80503
Last spring frost May 5 · First fall frost September 28 · 146 frost-free days · USDA Zone 6a
ZIP 80503 has about 35,800 people, one of 3 ZIP codes in Longmont.
Moderate season: 146 frost-free days. Spring frost risk spans April 24–May 18 (24 days). Fall frost risk spans September 16–October 10 (24 days).
Frost probability for 80503
| Frost | 10% | 50% | 90% |
|---|---|---|---|
| Spring (last frost) | May 18 | May 5 | April 24 |
| Fall (first frost) | September 16 | September 28 | October 10 |
Dates come from NOAA 1971–2000 freeze probabilities at LONGMONT 2 ESE (11.0 km away); the 50% date is the median year.
Which crops fit a 146-day season in Longmont
In Longmont (146 frost-free days, a moderate season), 37 of 56 planner crops fit outright, 18 get two plantings, 1 crop is perennial.
| Crop | Start indoors | Plant out / sow | Days you have | Verdict | Last sow date |
|---|---|---|---|---|---|
| Tomato | Mar 31 | May 19–May 26 | 104 | fits: 80 days to maturity, 104 available after May 19. | May 29 |
| Pepper | Mar 24 | May 19–May 26 | 104 | fits: 75 days to maturity, 104 available after May 19. | Jun 3 |
| Lettuce | Mar 10 | Apr 7–Apr 14 | 181 | two-plantings: Plant Apr 7 and again by Aug 7; both mature before Oct 5. | Aug 7 |
| Carrots | — | Apr 14–Apr 21 | 188 | fits: 70 days to maturity, 188 available after Apr 14. | Jul 27 |
| Cucumber | Apr 14 | May 19–May 26 | 118 | fits: 60 days to maturity, 118 available after May 19. | Jul 2 |
| Beans | — | May 12–May 26 | 132 | fits: 60 days to maturity, 132 available after May 12. | Jul 9 |
| Peas | — | Mar 10–May 19 | 209 | fits: 65 days to maturity, 209 available after Mar 10. | Jul 18 |
| Zucchini | Apr 21 | May 19–May 26 | 118 | fits: 55 days to maturity, 118 available after May 19. | Jul 7 |
| Onion | Feb 10 | Apr 7–Apr 21 | 181 | fits: 100 days to maturity, 181 available after Apr 7. | — |
| Spinach | — | Mar 10–Apr 28 | 223 | two-plantings: Plant Mar 10 and again by Aug 26; both mature before Oct 19. | Aug 26 |
| Kale | Feb 24 | Apr 7–Apr 21 | 195 | two-plantings: Plant Apr 7 and again by Aug 11; both mature before Oct 19. | Aug 11 |
| Broccoli | Mar 10 | Apr 7–Apr 21 | 181 | fits: 80 days to maturity, 181 available after Apr 7. | Jul 3 |
| Cabbage | Mar 10 | Apr 7–Apr 21 | 195 | fits: 70 days to maturity, 195 available after Apr 7. | Jul 27 |
| Radish | — | Mar 10–Jun 2 | 209 | two-plantings: Plant Mar 10 and again by Aug 27; both mature before Oct 5. | Aug 27 |
| Beets | — | Mar 10–Jun 2 | 209 | two-plantings: Plant Mar 10 and again by Jul 28; both mature before Oct 5. | Jul 28 |
| Potatoes | — | Apr 21–Apr 28 | 167 | fits: 90 days to maturity, 167 available after Apr 21. | — |
| Garlic | — | Sep 28 | 251 | overwinter: Plant Sep 28 to Oct 19; harvest the following summer. | Oct 19 |
| Strawberry | — | Mar 31–May 26 | 202 | perennial: A perennial: plant Mar 31; it fruits from the following year and keeps producing for several years. | — |
| Corn | Apr 7 | May 12–May 19 | 118 | fits: 75 days to maturity, 118 available after May 12. | Jun 10 |
| Pumpkin | Apr 21 | May 19–May 19 | 111 | fits: 100 days to maturity, 111 available after May 19. | — |
| Cauliflower | Mar 24 | Apr 28–May 12 | 160 | fits: 75 days to maturity, 160 available after Apr 28. | Jul 8 |
| Swiss Chard | Mar 10 | Apr 21–Apr 21 | 167 | two-plantings: Plant Apr 21 and again by Jul 28; both mature before Oct 5. | Jul 28 |
| Arugula | — | Mar 10–Jun 2 | 223 | two-plantings: Plant Mar 10 and again by Aug 26; both mature before Oct 19. | Aug 26 |
| Brussel Sprouts | Mar 10 | Apr 7–Apr 21 | 195 | fits: 100 days to maturity, 195 available after Apr 7. | Jun 27 |
| Butternut Squash | Apr 21 | May 19–May 26 | 111 | fits: 100 days to maturity, 111 available after May 19. | — |
| Eggplant | Mar 17 | May 19–May 26 | 104 | fits: 70 days to maturity, 104 available after May 19. | Jun 8 |
| Okra | Apr 7 | May 12–May 19 | 125 | two-plantings: Plant May 12 and again by Jul 7; both mature before Sep 14. | Jul 7 |
| Basil | Mar 31 | May 26–May 26 | 111 | fits: 60 days to maturity, 111 available after May 26. | Jul 2 |
| Cilantro | — | Apr 7–Apr 28 | 181 | two-plantings: Plant Apr 7 and again by Aug 2; both mature before Oct 5. | Aug 2 |
| Parsley | Feb 24 | May 12–May 19 | 146 | fits: 70 days to maturity, 146 available after May 12. | Jul 13 |
| Celery | Feb 24 | May 12–May 26 | 146 | fits: 120 days to maturity, 146 available after May 12. | May 24 |
| Scallions | Feb 24 | Apr 7–Apr 21 | 195 | two-plantings: Plant Apr 7 and again by Aug 6; both mature before Oct 19. | Aug 6 |
| Turnip | — | Apr 7–Apr 21 | 195 | two-plantings: Plant Apr 7 and again by Aug 16; both mature before Oct 19. | Aug 16 |
| Kohlrabi | — | Mar 24–Apr 7 | 209 | two-plantings: Plant Mar 24 and again by Aug 11; both mature before Oct 19. | Aug 11 |
| Bok Choy | Apr 7 | Apr 28–May 5 | 160 | two-plantings: Plant Apr 28 and again by Aug 7; both mature before Oct 5. | Aug 7 |
| Collards | Feb 24 | Apr 7–Apr 21 | 195 | two-plantings: Plant Apr 7 and again by Aug 6; both mature before Oct 19. | Aug 6 |
| Cantaloupe | Apr 21 | May 19–May 26 | 104 | fits: 85 days to maturity, 104 available after May 19. | May 24 |
| Chives | Feb 10 | May 12–May 19 | 160 | fits: 90 days to maturity, 160 available after May 12. | — |
| Dill | — | Mar 10–Jun 2 | 209 | two-plantings: Plant Mar 10 and again by Jul 28; both mature before Oct 5. | Jul 28 |
| Oregano | Feb 24 | May 12–May 19 | 160 | fits: 80 days to maturity, 160 available after May 12. | — |
| Rosemary | Feb 10 | May 12–May 19 | 146 | fits: 80 days to maturity, 146 available after May 12. | — |
| Sage | Mar 10 | May 12–May 19 | 160 | fits: 75 days to maturity, 160 available after May 12. | — |
| Thyme | Feb 24 | May 12–May 19 | 160 | fits: 90 days to maturity, 160 available after May 12. | — |
| Calendula | Mar 24 | Apr 21–May 5 | 167 | two-plantings: Plant Apr 21 and again by Aug 2; both mature before Oct 5. | Aug 2 |
| Cosmos | Apr 7 | May 5–May 19 | 132 | fits: 60 days to maturity, 132 available after May 5. | Jul 2 |
| Lavender | Feb 10 | May 5–May 19 | 167 | fits: 90 days to maturity, 167 available after May 5. | — |
| Marigolds | Mar 24 | May 5–May 12 | 132 | two-plantings: Plant May 5 and again by Jul 12; both mature before Sep 14. | Jul 12 |
| Melons | Apr 21 | May 19–May 26 | 104 | fits: 85 days to maturity, 104 available after May 19. | May 24 |
| Mint | — | May 5–May 19 | 167 | fits: 90 days to maturity, 167 available after May 5. | — |
| Nasturtiums | Apr 7 | May 5–May 12 | 132 | two-plantings: Plant May 5 and again by Jul 7; both mature before Sep 14. | Jul 7 |
| Pole Beans | — | May 12–May 26 | 125 | fits: 65 days to maturity, 125 available after May 12. | Jun 27 |
| Spaghetti Squash | Apr 21 | May 19–May 26 | 111 | fits: 90 days to maturity, 111 available after May 19. | May 26 |
| Sunflowers | — | May 5–May 19 | 132 | fits: 70 days to maturity, 132 available after May 5. | Jun 22 |
| Tomatillos | Mar 10 | May 12–May 19 | 111 | fits: 75 days to maturity, 111 available after May 12. | Jun 3 |
| Watermelon | Apr 21 | May 12–May 19 | 111 | fits: 80 days to maturity, 111 available after May 12. | May 29 |
| Yellow Squash | Apr 21 | May 19–May 26 | 118 | fits: 55 days to maturity, 118 available after May 19. | Jul 7 |
| Zinnias | Apr 7 | May 5–May 19 | 132 | fits: 60 days to maturity, 132 available after May 5. | Jul 2 |
Month by month in Longmont
| Month | What to do |
|---|---|
| January | Plan your garden layout and order seeds. Research varieties suited to your frost dates. |
| February | Start indoors: Chives (Feb 10), Lavender (Feb 10), Onion (Feb 10), Rosemary (Feb 10), Celery (Feb 24), Collards (Feb 24), +5 more. |
| March | Start indoors: Broccoli (Mar 10), Brussel Sprouts (Mar 10), Cabbage (Mar 10), Lettuce (Mar 10), Sage (Mar 10), Swiss Chard (Mar 10), +8 more. Direct sow from: Arugula (Mar 10), Beets (Mar 10), Dill (Mar 10), Peas (Mar 10), Radish (Mar 10), Spinach (Mar 10), +1 more. Plant: Strawberry (Mar 31). |
| April | Start indoors: Bok Choy (Apr 7), Corn (Apr 7), Cosmos (Apr 7), Nasturtiums (Apr 7), Okra (Apr 7), Zinnias (Apr 7), +9 more. Direct sow from: Cilantro (Apr 7), Turnip (Apr 7), Carrots (Apr 14), Potatoes (Apr 21). Transplant from: Broccoli (Apr 7), Brussel Sprouts (Apr 7), Cabbage (Apr 7), Collards (Apr 7), Kale (Apr 7), Lettuce (Apr 7), +6 more. |
| May | Direct sow from: Sunflowers (May 5), Beans (May 12), Pole Beans (May 12). Transplant from: Cosmos (May 5), Lavender (May 5), Marigolds (May 5), Mint (May 5), Nasturtiums (May 5), Zinnias (May 5), +23 more. Last frost around May 5. Last sowing dates: Cantaloupe (May 24), Celery (May 24), Melons (May 24), Spaghetti Squash (May 26), Tomato (May 29), Watermelon (May 29). |
| June | Last sowing dates: Pepper (Jun 3), Tomatillos (Jun 3), Eggplant (Jun 8), Corn (Jun 10), Sunflowers (Jun 22), Brussel Sprouts (Jun 27), +1 more. |
| July | Last sowing dates: Basil (Jul 2), Cosmos (Jul 2), Cucumber (Jul 2), Zinnias (Jul 2), Broccoli (Jul 3), Nasturtiums (Jul 7), +13 more. |
| August | Last sowing dates: Calendula (Aug 2), Cilantro (Aug 2), Collards (Aug 6), Scallions (Aug 6), Bok Choy (Aug 7), Lettuce (Aug 7), +6 more. |
| September | Direct sow from: Garlic (Sep 28). First frost around September 28. |
| October | Last sowing dates: Garlic (Oct 19). |
| November | Clean up spent plants. Add compost to beds. Rest, review, and plan next year's garden. |
| December | Clean up spent plants. Add compost to beds. Rest, review, and plan next year's garden. |
Frequently Asked Questions
What is the last frost date for Longmont, CO (ZIP 80503)?
The median last spring frost date for Longmont, CO (ZIP code 80503) is around May 5. 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. One year in ten frost ends before April 24; one in ten it runs past May 18.
What is the first fall frost date in Longmont, CO?
The median first fall frost date for Longmont, CO is around September 28. Plan to harvest or protect frost-sensitive crops like tomatoes and peppers at least a week before this date to avoid losses. The first frost arrives around September 16 in the earliest one-in-ten years, so cover tomatoes and peppers from then.
How long is the growing season in Longmont, CO?
The frost-free growing season in Longmont is about 146 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. In Longmont (146 frost-free days, a moderate season), 37 of 56 planner crops fit outright, 18 get two plantings, 1 crop is perennial.
What USDA hardiness zone is Longmont, CO?
Longmont, CO is in USDA hardiness zone 6a (2023 USDA Plant Hardiness Zone Map). Zones come from the average coldest winter temperature, so they tell you which perennials survive the winter; the frost dates on this page tell you when to plant.
When should I start tomatoes indoors in Longmont, CO?
Start tomatoes indoors Mar 31 and plant out May 19 in Longmont. The planner uses the local median last frost and this crop's indoor-start and transplant offsets.
How many plantings of lettuce fit in Longmont?
Two: plant Apr 7 and again by Aug 7; both mature before Oct 5.
When do I plant garlic in Longmont?
Sep 28 to Oct 19; it overwinters and is ready the following summer.
Nearby and related
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