USDA Hardiness Zone 6a
Frost Dates for 86001 — Flagstaff, Arizona
NOAA 1991–2020 normals from Fort Valley, AZ (7,347 ft), 1 miles southwest.
Updated
In Flagstaff, the last spring frost (32°F) is around June 11 in half of years and has passed by June 30 in nine years out of ten. The first fall frost comes around September 25, as early as September 8 one year in ten, leaving a typical freeze-free season of 99 days (NOAA, Fort Valley, AZ).
- Last spring frost
- June 11
- First fall frost
- September 25
- Growing season
- 99 days
- Hardiness zone
- 6a
50% probability
50% probability
freeze-free, 50%
-10 to -5°F (-23.3 to -20.6°C) coldest night
- First snow
- November 9
- Last snow
- April 29
- Winters with snow
- 100%
- Snow per year
- 72.6"
median, 1991–2020
median, 1991–2020
21 of 21 observed
NOAA normal
Frost risk by date in Flagstaff
Half of springs at Fort Valley, AZ see their last 32°F night by June 11; only one in ten is past it by May 26, and one in ten still has one after June 30. Planting tender crops after June 19 leaves a 30% risk of a frost on them, after June 24 20%, and after June 30 10%. The 19 days between the even-odds date and the one-in-ten date is narrower than at 61% of the 98 Arizona stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 25 in half of years, as early as September 8 one year in ten, and holds off past October 6 one year in ten. A hard freeze of 28°F follows 9 days later, around October 4; in spring the one-in-ten date for 28°F is June 14, 16 days before the 32°F one. The freeze-free season lasts 99 days in a typical year, shorter than at 99% of the 98 Arizona stations; it reaches 122 days one year in ten and falls short of 70 one year in ten. On average 244 nights a year drop to 32°F or below. Severe cold of 16°F or below comes every winter, usually between November 9 and March 30.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | May 26 | May 31 | Jun 5 | Jun 7 | Jun 11 | Jun 14 | Jun 19 | Jun 24 | Jun 30 |
| 28°F | May 6 | May 13 | May 18 | May 22 | May 27 | May 31 | Jun 3 | Jun 7 | Jun 14 |
| 24°F | Apr 19 | Apr 24 | Apr 29 | May 2 | May 5 | May 9 | May 13 | May 18 | May 27 |
| 20°F | Apr 1 | Apr 7 | Apr 11 | Apr 15 | Apr 19 | Apr 22 | Apr 26 | Apr 30 | May 6 |
| 16°F | Mar 6 | Mar 12 | Mar 17 | Mar 22 | Mar 30 | Apr 4 | Apr 7 | Apr 11 | Apr 18 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | Sep 8 | Sep 16 | Sep 19 | Sep 22 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 6 |
| 28°F | Sep 21 | Sep 26 | Sep 30 | Oct 2 | Oct 4 | Oct 6 | Oct 10 | Oct 13 | Oct 19 |
| 24°F | Sep 30 | Oct 4 | Oct 7 | Oct 11 | Oct 14 | Oct 18 | Oct 21 | Oct 26 | Nov 1 |
| 20°F | Oct 10 | Oct 16 | Oct 20 | Oct 24 | Oct 28 | Nov 1 | Nov 4 | Nov 9 | Nov 14 |
| 16°F | Oct 21 | Oct 28 | Nov 1 | Nov 5 | Nov 9 | Nov 13 | Nov 17 | Nov 20 | Nov 25 |
Freeze probabilities 1991–2020 at Fort Valley, AZ, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 86001 →Compare Flagstaff with another place →
Weather by month in Flagstaff
July is the warmest month at Fort Valley, AZ, averaging 84°F by day and 49°F at night; December is the coldest at 46°F and 16°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 77% of years.
Warmth for growing adds up to 1,730 growing degree days a year (base 50°F), the fewest of the 102 Arizona stations. 26% of them (445) have built up by the end of June, and July adds the most, 510. Precipitation totals 20.6 inches a year, most in August (3.4") and least in June (0.2"); May to September bring 44% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 46 | 17 | 2.1 | 100% | 30 | 0 |
| Feb | 48 | 20 | 2.5 | 100% | 28 | 0 |
| Mar | 54 | 24 | 1.8 | 100% | 31 | 1 |
| Apr | 62 | 27 | 0.9 | 100% | 28 | 16 |
| May | 71 | 33 | 0.9 | 100% | 23 | 106 |
| Jun | 82 | 40 | 0.2 | 77% | 10 | 322 |
| Jul | 84 | 49 | 2.8 | 10% | 1 | 510 |
| Aug | 81 | 49 | 3.4 | 5% | <1 | 473 |
| Sep | 76 | 41 | 1.8 | 70% | 8 | 257 |
| Oct | 66 | 30 | 1.3 | 100% | 26 | 44 |
| Nov | 55 | 23 | 1.3 | 100% | 29 | 1 |
| Dec | 46 | 16 | 1.6 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Fort Valley, AZ (7,347 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Walnut Canyon Nm, AZ, 14 miles southeast of Flagstaff and 652 feet lower, has a last frost 29 days earlier (May 13), a first frost 11 days later (October 6), a season 46 days longer.
When it snows in Flagstaff, Arizona
The first measurable snow (0.1 inch or more) in Flagstaff usually arrives around November 9, and the last of the season around April 29. Those are the middle dates across the winters of 1991–2020 at Bellemont WFO, AZ, 6.6 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 43 days after the last snow.
Planting calendar for 86001
| When | What to do |
|---|---|
| April 2 – April 16 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| April 16 – April 30 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| May 14 – May 28 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| June 11 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 3 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 11 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 25 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 86001 is 1 mile from Fort Valley, AZ, which stands at 7,347 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 86001 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.