USDA Hardiness Zone 6b

Frost Dates for 86004 — Flagstaff, Arizona

NOAA 1991–2020 normals from Walnut Canyon Nm, AZ (6,695 ft), 7 miles southwest.

Updated October 28, 2026

In Flagstaff, the last spring frost (32°F) is around May 13 in half of years and has passed by May 30 in nine years out of ten. The first fall frost comes around October 6, as early as September 26 one year in ten, leaving a typical freeze-free season of 145 days (NOAA, Walnut Canyon Nm, AZ).

Last spring frost: May 13
First fall frost: October 6
Growing season: 145 days
Hardiness zone: 6b

First snow: November 22
Last snow: April 10
Winters with snow: 100%
Snow per year: 52.4"

ShareX Facebook WhatsApp Email

Frost risk by date in Flagstaff

Half of springs at Walnut Canyon Nm, AZ see their last 32°F night by May 13; only one in ten is past it by April 27, and one in ten still has one after May 30. Planting tender crops after May 20 leaves a 30% risk of a frost on them, after May 24 20%, and after May 30 10%. The 17 days between the even-odds date and the one-in-ten date is narrower than at 80% of the 98 Arizona stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 6 in half of years, as early as September 26 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 12 days later, around October 18; in spring the one-in-ten date for 28°F is May 17, 13 days before the 32°F one. The freeze-free season lasts 145 days in a typical year, shorter than at 92% of the 98 Arizona stations; it reaches 166 days one year in ten and falls short of 126 one year in ten. On average 161 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 14 days longer, to about May 27, and return around September 29. Severe cold of 16°F or below comes every winter, usually between November 26 and February 28.

Spring: the chance of another night this cold after the date
Low	90%	80%	70%	60%	50%	40%	30%	20%	10%
36°F	May 11	May 16	May 20	May 24	May 27	May 30	Jun 1	Jun 4	Jun 10
32°F	Apr 27	May 2	May 6	May 10	May 13	May 16	May 20	May 24	May 30
28°F	Apr 8	Apr 14	Apr 19	Apr 23	Apr 28	May 2	May 6	May 11	May 17
24°F	Mar 18	Mar 25	Mar 31	Apr 3	Apr 7	Apr 10	Apr 15	Apr 21	May 1
20°F	Feb 27	Mar 5	Mar 8	Mar 11	Mar 15	Mar 19	Mar 24	Mar 30	Apr 6
16°F	Feb 5	Feb 13	Feb 20	Feb 25	Feb 28	Mar 4	Mar 8	Mar 12	Mar 20

Fall: the chance the first night this cold has come by the date
Low	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	Sep 17	Sep 21	Sep 24	Sep 27	Sep 29	Oct 1	Oct 3	Oct 5	Oct 9
32°F	Sep 26	Sep 30	Oct 2	Oct 4	Oct 6	Oct 9	Oct 12	Oct 16	Oct 21
28°F	Oct 4	Oct 7	Oct 11	Oct 15	Oct 18	Oct 21	Oct 25	Oct 29	Nov 4
24°F	Oct 14	Oct 19	Oct 24	Oct 28	Oct 31	Nov 4	Nov 7	Nov 11	Nov 17
20°F	Oct 25	Nov 1	Nov 6	Nov 10	Nov 15	Nov 18	Nov 21	Nov 25	Dec 1
16°F	Nov 7	Nov 16	Nov 20	Nov 23	Nov 26	Nov 29	Dec 3	Dec 7	Dec 15

Freeze probabilities 1991–2020 at Walnut Canyon Nm, 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 86004 →Compare Flagstaff with another place →

Frost risk through the year in Flagstaff

Frost curve for Flagstaff, Arizona (Walnut Canyon Nm, AZ, NOAA 1991–2020): the chance of another 32°F night falls from 90% on April 27 to 50% on May 13 and 10% on May 30; the chance that the first fall one has come rises from 10% on September 26 to 50% on October 6 and 90% on October 21. — In Flagstaff, the spring curve drops under 10% on May 30 and the fall curve passes 10% on September 26, 119 days later; that gap is shorter than at 85% of the 71 Arizona stations with frost dates. The spring risk falls from 90% to 10% over 33 days (April 27 to May 30) and the fall risk climbs from 10% to 90% over 25 days (September 26 to October 21): the last frost moves more from year to year than the first. At Fort Valley, AZ, 17 miles west, the same gap runs from June 30 to September 8, 49 days shorter.

Chance of a frost after each planting date in Flagstaff
Planted on	Another 32°F night	28°F night
Apr 29	86%	48%
May 6	70%	30%
May 1350% date	50%	17%
May 20	30%	under 10%
May 27	15%	under 10%
Jun 3	under 10%	under 10%

Weather by month in Flagstaff

July is the warmest month at Walnut Canyon Nm, AZ, averaging 86°F by day and 57°F at night; January is the coldest at 45°F and 20°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 83% of years.

Warmth for growing adds up to 2,486 growing degree days a year (base 50°F), fewer than at 93% of the 102 Arizona stations. 31% of them (769) have built up by the end of June, and July adds the most, 665. Precipitation totals 17.7 inches a year, most in August (3.4") and least in June (0.3"); May to September bring 48% of it.

Monthly normals 1991–2020 at Walnut Canyon Nm, AZ
Month	High °F	Low °F	Precip. in	32°F night	Nights ≤ 32°F	GDD 50°F
Jan	45	20	1.3	100%	30	0
Feb	49	23	1.7	100%	25	1
Mar	56	28	1.5	100%	23	12
Apr	62	34	0.9	100%	15	51
May	71	40	0.6	83%	5	211
Jun	82	51	0.3	7%	<1	494
Jul	86	57	2.6	0%	0	665
Aug	82	55	3.4	0%	0	580
Sep	78	46	1.6	22%	1	363
Oct	67	35	1.2	99%	11	99
Nov	55	28	1.4	100%	23	11
Dec	45	21	1.2	100%	29	0

Monthly normals 1991–2020 at Walnut Canyon Nm, AZ (6,695 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, Fort Valley, AZ, 17 miles west of Flagstaff and 652 feet higher, has a last frost 29 days later (June 11), a first frost 11 days earlier (September 25), a season 46 days shorter.

Frost dates for 86001 (Flagstaff) →

When it snows in Flagstaff, Arizona

The first measurable snow (0.1 inch or more) in Flagstaff usually arrives around November 22, and the last of the season around April 10. Those are the middle dates across the winters of 1991–2020 at Walnut Canyon Nm, 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 33 days after the last snow.

Planting calendar for 86004

What to do and when, from the 50% frost dates for Flagstaff, Arizona
When	What to do
March 4 – March 18	Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out.
March 18 – April 1	Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early.
April 15 – April 29	Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil.
May 13 onward	Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast.
by July 14	Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops.
by September 22	Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation.
October 6 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 86004 is 7 miles from Walnut Canyon Nm, AZ, which stands at 6,695 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 86004 in zone 6b, the band where the coldest night of the year averaged -5 to 0°F (-20.6 to -17.8°C) over 1991–2020; a zone describes winter lows, not frost dates.

Frost dates across Arizona →

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.