Frost Dates for Boulder, CO 80305
Boulder has 6 ZIP codes. The Boulder, CO frost page lists all of them with their dates.
Last spring frost May 5 · First fall frost October 1 · 149 frost-free days · USDA Zone 6a
ZIP 80305 has about 16,300 people, one of 6 ZIP codes in Boulder.
Moderate season: 149 frost-free days. Spring frost risk spans April 24–May 18 (24 days). Fall frost risk spans September 18–October 14 (26 days).
Frost probability for 80305
| Frost | 10% | 50% | 90% |
|---|---|---|---|
| Spring (last frost) | May 18 | May 5 | April 24 |
| Fall (first frost) | September 18 | October 1 | October 14 |
Dates come from NOAA 1971–2000 freeze probabilities at LONGMONT 2 ESE (25.9 km away); the 50% date is the median year.
Which crops fit a 149-day season in Boulder
In Boulder (149 frost-free days, a moderate season), 34 of 56 planner crops fit outright, 21 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 | 107 | fits: 80 days to maturity, 107 available after May 19. | Jun 1 |
| Pepper | Mar 24 | May 19–May 26 | 107 | fits: 75 days to maturity, 107 available after May 19. | Jun 6 |
| Lettuce | Mar 10 | Apr 7–Apr 14 | 184 | two-plantings: Plant Apr 7 and again by Aug 10; both mature before Oct 8. | Aug 10 |
| Carrots | — | Apr 14–Apr 21 | 191 | fits: 70 days to maturity, 191 available after Apr 14. | Jul 30 |
| Cucumber | Apr 14 | May 19–May 26 | 121 | fits: 60 days to maturity, 121 available after May 19. | Jul 5 |
| Beans | — | May 12–May 26 | 135 | two-plantings: Plant May 12 and again by Jul 12; both mature before Sep 24. | Jul 12 |
| Peas | — | Mar 10–May 19 | 212 | fits: 65 days to maturity, 212 available after Mar 10. | Jul 21 |
| Zucchini | Apr 21 | May 19–May 26 | 121 | fits: 55 days to maturity, 121 available after May 19. | Jul 10 |
| Onion | Feb 10 | Apr 7–Apr 21 | 184 | fits: 100 days to maturity, 184 available after Apr 7. | — |
| Spinach | — | Mar 10–Apr 28 | 226 | two-plantings: Plant Mar 10 and again by Aug 29; both mature before Oct 22. | Aug 29 |
| Kale | Feb 24 | Apr 7–Apr 21 | 198 | two-plantings: Plant Apr 7 and again by Aug 14; both mature before Oct 22. | Aug 14 |
| Broccoli | Mar 10 | Apr 7–Apr 21 | 184 | fits: 80 days to maturity, 184 available after Apr 7. | Jul 6 |
| Cabbage | Mar 10 | Apr 7–Apr 21 | 198 | fits: 70 days to maturity, 198 available after Apr 7. | Jul 30 |
| Radish | — | Mar 10–Jun 2 | 212 | two-plantings: Plant Mar 10 and again by Aug 30; both mature before Oct 8. | Aug 30 |
| Beets | — | Mar 10–Jun 2 | 212 | two-plantings: Plant Mar 10 and again by Jul 31; both mature before Oct 8. | Jul 31 |
| Potatoes | — | Apr 21–Apr 28 | 170 | fits: 90 days to maturity, 170 available after Apr 21. | — |
| Garlic | — | Oct 1 | 254 | overwinter: Plant Oct 1 to Oct 22; harvest the following summer. | Oct 22 |
| Strawberry | — | Mar 31–May 26 | 205 | 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 | 121 | fits: 75 days to maturity, 121 available after May 12. | Jun 13 |
| Pumpkin | Apr 21 | May 19–May 19 | 114 | fits: 100 days to maturity, 114 available after May 19. | — |
| Cauliflower | Mar 24 | Apr 28–May 12 | 163 | fits: 75 days to maturity, 163 available after Apr 28. | Jul 11 |
| Swiss Chard | Mar 10 | Apr 21–Apr 21 | 170 | two-plantings: Plant Apr 21 and again by Jul 31; both mature before Oct 8. | Jul 31 |
| Arugula | — | Mar 10–Jun 2 | 226 | two-plantings: Plant Mar 10 and again by Aug 29; both mature before Oct 22. | Aug 29 |
| Brussel Sprouts | Mar 10 | Apr 7–Apr 21 | 198 | fits: 100 days to maturity, 198 available after Apr 7. | Jun 30 |
| Butternut Squash | Apr 21 | May 19–May 26 | 114 | fits: 100 days to maturity, 114 available after May 19. | — |
| Eggplant | Mar 17 | May 19–May 26 | 107 | fits: 70 days to maturity, 107 available after May 19. | Jun 11 |
| Okra | Apr 7 | May 12–May 19 | 128 | two-plantings: Plant May 12 and again by Jul 10; both mature before Sep 17. | Jul 10 |
| Basil | Mar 31 | May 26–May 26 | 114 | fits: 60 days to maturity, 114 available after May 26. | Jul 5 |
| Cilantro | — | Apr 7–Apr 28 | 184 | two-plantings: Plant Apr 7 and again by Aug 5; both mature before Oct 8. | Aug 5 |
| Parsley | Feb 24 | May 12–May 19 | 149 | fits: 70 days to maturity, 149 available after May 12. | Jul 16 |
| Celery | Feb 24 | May 12–May 26 | 149 | fits: 120 days to maturity, 149 available after May 12. | May 27 |
| Scallions | Feb 24 | Apr 7–Apr 21 | 198 | two-plantings: Plant Apr 7 and again by Aug 9; both mature before Oct 22. | Aug 9 |
| Turnip | — | Apr 7–Apr 21 | 198 | two-plantings: Plant Apr 7 and again by Aug 19; both mature before Oct 22. | Aug 19 |
| Kohlrabi | — | Mar 24–Apr 7 | 212 | two-plantings: Plant Mar 24 and again by Aug 14; both mature before Oct 22. | Aug 14 |
| Bok Choy | Apr 7 | Apr 28–May 5 | 163 | two-plantings: Plant Apr 28 and again by Aug 10; both mature before Oct 8. | Aug 10 |
| Collards | Feb 24 | Apr 7–Apr 21 | 198 | two-plantings: Plant Apr 7 and again by Aug 9; both mature before Oct 22. | Aug 9 |
| Cantaloupe | Apr 21 | May 19–May 26 | 107 | fits: 85 days to maturity, 107 available after May 19. | May 27 |
| Chives | Feb 10 | May 12–May 19 | 163 | fits: 90 days to maturity, 163 available after May 12. | — |
| Dill | — | Mar 10–Jun 2 | 212 | two-plantings: Plant Mar 10 and again by Jul 31; both mature before Oct 8. | Jul 31 |
| Oregano | Feb 24 | May 12–May 19 | 163 | fits: 80 days to maturity, 163 available after May 12. | — |
| Rosemary | Feb 10 | May 12–May 19 | 149 | fits: 80 days to maturity, 149 available after May 12. | — |
| Sage | Mar 10 | May 12–May 19 | 163 | fits: 75 days to maturity, 163 available after May 12. | — |
| Thyme | Feb 24 | May 12–May 19 | 163 | fits: 90 days to maturity, 163 available after May 12. | — |
| Calendula | Mar 24 | Apr 21–May 5 | 170 | two-plantings: Plant Apr 21 and again by Aug 5; both mature before Oct 8. | Aug 5 |
| Cosmos | Apr 7 | May 5–May 19 | 135 | two-plantings: Plant May 5 and again by Jul 5; both mature before Sep 17. | Jul 5 |
| Lavender | Feb 10 | May 5–May 19 | 170 | fits: 90 days to maturity, 170 available after May 5. | — |
| Marigolds | Mar 24 | May 5–May 12 | 135 | two-plantings: Plant May 5 and again by Jul 15; both mature before Sep 17. | Jul 15 |
| Melons | Apr 21 | May 19–May 26 | 107 | fits: 85 days to maturity, 107 available after May 19. | May 27 |
| Mint | — | May 5–May 19 | 170 | fits: 90 days to maturity, 170 available after May 5. | — |
| Nasturtiums | Apr 7 | May 5–May 12 | 135 | two-plantings: Plant May 5 and again by Jul 10; both mature before Sep 17. | Jul 10 |
| Pole Beans | — | May 12–May 26 | 128 | fits: 65 days to maturity, 128 available after May 12. | Jun 30 |
| Spaghetti Squash | Apr 21 | May 19–May 26 | 114 | fits: 90 days to maturity, 114 available after May 19. | May 29 |
| Sunflowers | — | May 5–May 19 | 135 | fits: 70 days to maturity, 135 available after May 5. | Jun 25 |
| Tomatillos | Mar 10 | May 12–May 19 | 114 | fits: 75 days to maturity, 114 available after May 12. | Jun 6 |
| Watermelon | Apr 21 | May 12–May 19 | 114 | fits: 80 days to maturity, 114 available after May 12. | Jun 1 |
| Yellow Squash | Apr 21 | May 19–May 26 | 121 | fits: 55 days to maturity, 121 available after May 19. | Jul 10 |
| Zinnias | Apr 7 | May 5–May 19 | 135 | two-plantings: Plant May 5 and again by Jul 5; both mature before Sep 17. | Jul 5 |
Month by month in Boulder
| 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 27), Celery (May 27), Melons (May 27), Spaghetti Squash (May 29). |
| June | Last sowing dates: Tomato (Jun 1), Watermelon (Jun 1), Pepper (Jun 6), Tomatillos (Jun 6), Eggplant (Jun 11), Corn (Jun 13), +3 more. |
| July | Last sowing dates: Basil (Jul 5), Cosmos (Jul 5), Cucumber (Jul 5), Zinnias (Jul 5), Broccoli (Jul 6), Nasturtiums (Jul 10), +13 more. |
| August | Last sowing dates: Calendula (Aug 5), Cilantro (Aug 5), Collards (Aug 9), Scallions (Aug 9), Bok Choy (Aug 10), Lettuce (Aug 10), +6 more. |
| September | First fall frost approaching (around October 1). Transplant fall brassicas. Sow spinach, arugula, and radishes. |
| October | Direct sow from: Garlic (Oct 1). First frost around October 1. Last sowing dates: Garlic (Oct 22). |
| November | Harvest remaining cold-hardy crops: kale, carrots, and beets. Plant garlic bulbs for next year. |
| 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 Boulder, CO (ZIP 80305)?
The median last spring frost date for Boulder, CO (ZIP code 80305) 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 Boulder, CO?
The median first fall frost date for Boulder, CO is around October 1. 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 18 in the earliest one-in-ten years, so cover tomatoes and peppers from then.
How long is the growing season in Boulder, CO?
The frost-free growing season in Boulder is about 149 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 Boulder (149 frost-free days, a moderate season), 34 of 56 planner crops fit outright, 21 get two plantings, 1 crop is perennial.
What USDA hardiness zone is Boulder, CO?
Boulder, 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 Boulder, CO?
Start tomatoes indoors Mar 31 and plant out May 19 in Boulder. The planner uses the local median last frost and this crop's indoor-start and transplant offsets.
How many plantings of lettuce fit in Boulder?
Two: plant Apr 7 and again by Aug 10; both mature before Oct 8.
When do I plant garlic in Boulder?
Oct 1 to Oct 22; it overwinters and is ready the following summer.
Nearby and related
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