Boulder, Colorado planting calendar and frost dates
Boulder is in USDA hardiness zone 6a. The last spring frost typically comes around May 6 and the first fall frost around October 5, giving about 151 frost-free days a year.
- Hardiness zone
- Zone 6a
- Typical last frost
- May 6
- Typical first frost
- October 5
- Frost-free season
- 151 days
Frost dates in Boulder
Last spring frost
- April 24Risky: frost later than this in 9 years out of 10
- May 6Typical: a later frost in half of years
- May 18Safe: a later frost only 1 year in 10
First fall frost
- September 21Risky: frost earlier than this 1 year in 10
- October 5Typical: an earlier frost in half of years
- October 18Late: frost before this in 9 years out of 10
The growing season between the typical last and first frosts is about 151 days. Frost here means a low of 32°F at the weather station. A hard freeze (28°F), which damages most hardy crops too, typically ends around April 27 and returns around October 15.
NOAA U.S. Climate Normals 1991-2020 for Boulder, 1.6 miles S of the city center and 165 feet higher. Low-lying and rural spots often frost later in spring and earlier in fall than airports and city centers.
What to plant each month in Boulder
January
Start indoors: onions
February
Start indoors: broccoli, cabbages, cauliflower, collard greens, kale, leeks, onions, oregano, parsley, petunias, ranunculus, rosemary, snapdragons
March
Start indoors: black-eyed susans, broccoli, cabbages, cauliflower, celery, chives, cilantro, collard greens, eggplants, ginger, kale, leeks, lettuce, marigolds, nasturtiums, oregano, parsley, peppers, petunias, ranunculus, rosemary, sage, snapdragons, spinach, swiss chard, tomatoes
Plant or sow outdoors: black-eyed susans, blueberries, garlic, grapes, grass seed, hollyhocks, onions, peas, poppies, raspberries, rhubarb, strawberries, turnips, wildflowers
April
Start indoors: basil, cantaloupes, cilantro, cosmos, cucumbers, eggplants, marigolds, nasturtiums, okra, peppers, pumpkins, ranunculus, spinach, swiss chard, tomatoes, watermelons, winter squash, zucchini
Plant or sow outdoors: asparagus, beets, black-eyed susans, blackberries, blueberries, broccoli, cabbages, carrots, cauliflower, chives, cilantro, collard greens, dahlias, dill, figs, garlic, gladioli, grapes, grass seed, green onions, hollyhocks, kale, leeks, lettuce, onions, parsley, peas, poppies, potatoes, radishes, ranunculus, raspberries, rhubarb, roses, snapdragons, spinach, strawberries, swiss chard, turnips, wildflowers
May
Start indoors: cantaloupes, cosmos, cucumbers, okra, pumpkins, watermelons, winter squash, zinnias, zucchini
Plant or sow outdoors: asparagus, basil, beets, cantaloupes, carrots, chives, cilantro, cucumbers, dahlias, dill, eggplants, figs, garden mums, ginger, gladioli, grapes, green beans, green onions, kentucky bluegrass, lavenders, leeks, lettuce, lilies, marigolds, mints, nasturtiums, oregano, parsley, peppers, perennial ryegrass, petunias, potatoes, pumpkins, ranunculus, rosemary, roses, sage, snapdragons, spinach, sunflowers, sweet corn, sweet peas, swiss chard, tall fescue, thyme, tomatoes, watermelons, winter squash, zucchini
June
Start indoors: zinnias
Plant or sow outdoors: asparagus, basil, bermudagrass, cantaloupes, celery, centipedegrass, cosmos, cucumbers, eggplants, garden mums, green beans, green onions, okra, peanuts, peppers, pumpkins, st. augustinegrass, sweet potatoes, tomatoes, watermelons, winter squash, zinnias, zoysiagrass, zucchini
Plant for fall: brussels sprouts
July
Plant or sow outdoors: bermudagrass, centipedegrass, green onions, st. augustinegrass, zinnias, zoysiagrass
Plant for fall: bearded irises, beets, broccoli, brussels sprouts, cabbages, carrots, cauliflower, collard greens, lettuce, peas, radishes, snapdragons, spinach, swiss chard, turnips
August
Plant for fall: azaleas, bearded irises, beets, broccoli, cabbages, carrots, cauliflower, clematis, coneflowers, crocuses, daffodils, daylilies, grass seed, hostas, hydrangeas, kale, lettuce, lilies, ornamental alliums, peonies, radishes, roses, saffron, snapdragons, spinach, swiss chard, tulips, turnips
September
Plant for fall: crocuses, daffodils, daylilies, grass seed, hostas, hyacinths, kale, lilies, ornamental alliums, peonies, radishes, saffron, tulips
October
Plant for fall: crocuses, daffodils, garlic, hyacinths, lilies, ornamental alliums, peonies, tulips
Frost dates and zones by ZIP code in Boulder
All 5 of Boulder's main ZIP codes take their frost dates from Boulder, so the dates on this page apply across the city. Every one of them is in zone 6a.
| ZIP code | Zone | Weather station | Typical last frost | Typical first frost |
|---|---|---|---|---|
| 80301 | 6a | Boulder, 1.6 mi S | May 6 | Oct 5 |
| 80302 | 6a | Boulder, 1.6 mi S | May 6 | Oct 5 |
| 80303 | 6a | Boulder, 1.6 mi S | May 6 | Oct 5 |
| 80304 | 6a | Boulder, 1.6 mi S | May 6 | Oct 5 |
| 80305 | 6a | Boulder, 1.6 mi S | May 6 | Oct 5 |
Boulder also supplies the frost dates for Longmont, so their planting calendars match Boulder's.
Planting dates for popular crops in Boulder
| Plant | Start indoors | Plant or sow outdoors | Fall planting |
|---|---|---|---|
| Tomatoes | Mar 30 to Apr 13 | May 25 to Jun 8 | — |
| Strawberries | — | Mar 25 to Apr 8 | — |
| Lettuce | Mar 11 to 25 | Apr 29 to May 13 | Jul 6 to Aug 10 |
| Cilantro | Mar 25 to Apr 8 | May 6 to 13 | — |
| Basil | Apr 6 to 20 | May 25 to Jun 1 | — |
| Potatoes | — | Apr 24 to May 1 | — |
| Garlic | — | Mar 25 to Apr 8 | Oct 5 to 19 |
| Onions | Jan 14 to Feb 25 | Apr 8 to 22 | — |
| Cucumbers | Apr 27 to May 11 | May 25 to Jun 8 | — |
| Pumpkins | Apr 27 to May 11 | May 25 to Jun 8 | — |
| Peppers | Mar 9 to Apr 6 | May 25 to Jun 8 | — |
| Carrots | — | Apr 15 to May 6 | Jul 13 to Aug 3 |
| Sweet Potatoes | — | Jun 9 to 16 | — |
| Watermelons | Apr 20 to May 4 | May 25 to Jun 8 | — |
| Sweet Corn | — | May 6 to 20 | — |
| Green Beans | — | May 25 to Jun 8 | — |
| Zucchini | Apr 27 to May 11 | May 25 to Jun 8 | — |
| Broccoli | Feb 18 to Mar 11 | Apr 8 to 22 | Jul 20 to Aug 17 |
| Peas | — | Mar 25 to Apr 8 | Jul 13 to 27 |
| Spinach | Mar 25 to Apr 1 | Apr 29 to May 13 | Jul 27 to Aug 31 |
| Sunflowers | — | May 6 to 27 | — |
| Zinnias | May 31 to Jun 7 | Jun 28 to Jul 12 | — |
| Tulips | — | — | Aug 24 to Oct 19 |
| Daffodils | — | — | Aug 24 to Oct 5 |
| Grass Seed | — | Mar 25 to Apr 8 | Aug 24 to Sep 7 |
Nearby cities
- Broomfield, CO zone 6a, last frost May 2
- Longmont, CO zone 5b, last frost May 6
- Westminster, CO zone 6a, last frost May 2
- Arvada, CO zone 6a, last frost May 5
- Thornton, CO zone 6a, last frost May 2
- Lakewood, CO zone 6a, last frost May 5
Planting in ColoradoAbout zone 6aPrintable calendar for Boulder
Questions
When is the last frost in Boulder?
Around May 6 in a typical year, based on NOAA's 1991-2020 records for Boulder. There is still a 1 in 10 chance of frost after May 18.
What planting zone is Boulder in?
USDA hardiness zone 6a (2023 map). The zone tells you which perennials survive the winter; planting dates depend on frost dates instead.
When should I plant tomatoes in Boulder?
Start seeds indoors March 30 to April 13 and set plants out May 25 to June 8.
When is the first frost in Boulder?
Around October 5 in a typical year, and before September 21 about 1 year in 10.