Deconstruct SunBoulder, CO: Hardiness Zone 6a & Sunlight by Season
Boulder is in USDA hardiness zone 6a. Here’s what that means for winter, plus sunrise, sunset, daylight hours and the sun’s path through the year.
Zone 6a in Boulder
On the 2023 USDA Plant Hardiness Zone Map, Boulder falls in zone 6a. That means the coldest night of a typical winter drops to between −10 and −5 °F. Perennials, shrubs and trees labeled for zone 6a or colder (a lower number) should survive the winter here.
A zone only covers winter cold. For a plant to do well it also needs the right amount of sun, and that varies far more from one corner of a yard to the next than the zone does across town.
This is not the official USDA Plant Hardiness Zone Map. Zones are derived from the official 2023 dataset, resampled to about 2 km.
Nearby cities
- Westminster, CO Zone 6a
- Arvada, CO Zone 6a
- Thornton, CO Zone 6a
- Lakewood, CO Zone 6a
- Denver, CO Zone 6a
- Aurora, CO Zone 6a
- Greeley, CO Zone 5b
- Fort Collins, CO Zone 5b
The sun over Boulder
On the June solstice the sun rises in the northeast at 5:32 AM and climbs to 73° above the horizon at 1:02 PM. On the December solstice the noon sun reaches only 27°, so a 20-foot tree casts a noon shadow about 40 feet long. In June the same tree’s noon shadow is only 6 feet.
Sunrise, sunset & daylight by month
| Date | Sunrise | Sunset | Daylight | Noon sun |
|---|---|---|---|---|
| Jan 21 | 7:17 AM | 5:07 PM | 9h 49m | 30° |
| Feb 21 | 6:45 AM | 5:44 PM | 10h 58m | 40° |
| Mar 21 | 7:02 AM | 7:14 PM | 12h 12m | 50° |
| Apr 21 | 6:14 AM | 7:45 PM | 13h 32m | 62° |
| May 21 | 5:40 AM | 8:15 PM | 14h 35m | 70° |
| Jun 21 | 5:32 AM | 8:33 PM | 15h 01m | 73° |
| Jul 21 | 5:49 AM | 8:24 PM | 14h 35m | 70° |
| Aug 21 | 6:18 AM | 7:49 PM | 13h 31m | 62° |
| Sep 21 | 6:47 AM | 6:59 PM | 12h 12m | 50° |
| Oct 21 | 7:18 AM | 6:12 PM | 10h 54m | 39° |
| Nov 21 | 6:53 AM | 4:40 PM | 9h 47m | 30° |
| Dec 21 | 7:19 AM | 4:38 PM | 9h 20m | 27° |
Local time (Denver), including daylight saving time. “Noon sun” is the sun’s height above the horizon at solar noon.
Daylight through the year
What “full sun” means in Boulder
Full sun is 6+ hours of direct sun. In Boulder an open spot has 15.0 hours of daylight to work with in June and 9.3 in December, so a bed that clears 6 hours in summer can easily drop to part shade in winter once the low sun slips behind trees and houses.
Measure the sun in your Boulder yard
Daylight is the ceiling. Deconstruct Sun traces the trees and buildings around one exact spot and tells you its real direct-sun hours, today and at every solstice and equinox.
Questions
What hardiness zone is Boulder, CO in?
Boulder, Colorado is in USDA plant hardiness zone 6a on the 2023 map. The average coldest winter night is −10 to −5 °F (−23.3 to −20.6 °C).
How many hours of daylight does Boulder get?
About 15.0 hours on the June solstice (sunrise 5:32 AM, sunset 8:33 PM) and 9.3 hours on the December solstice (sunrise 7:19 AM, sunset 4:38 PM). Around the equinoxes it is close to 12 hours.
How many hours of sun does my Boulder yard get?
Up to the daylight hours above, minus whatever trees, buildings and fences block. In winter, when the sun stays low, that can be most of it. The Deconstruct Sun iPhone app measures the real direct-sun hours for one exact spot by having you trace what is around it.
When is the best time to plant in Boulder?
Zone 6a tells you which perennials survive winter, not planting dates. For timing, use your local average frost dates from your county extension office, and choose plants rated for zone 6a or colder.
Source: USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture, planthardiness.ars.usda.gov. Underlying data © PRISM Group, Oregon State University, prism.oregonstate.edu, accessed 2026-08-30. Sun times calculated for 40.024, -105.251.