In Phoenix and the low desert, herbs go in the ground on a heat calendar, not a frost calendar: cool-season herbs from October through February, heat-tolerant perennials from February through April. Above 5,000 feet, in Flagstaff and the high country, the opposite is true, and a hard freeze still runs the show.
The short answer for Arizona
Start with what the frost data actually says, because it changes everything else. NOAA’s 1991-2020 climate normals for the Phoenix station list a 365-day frost-free season. Looking at the daily record behind that number, a freeze at 32°F showed up in only 6 of the last 30 springs, arriving anywhere from late December to mid-January when it did happen at all. That is not a last-frost date you can build a calendar around. It is a rare event, not a season boundary.
So the low-desert planting calendar for herbs runs on a different clock: the coldest month’s average nightly low sits at a mild 45.3°F, well above freezing, which means the limiting factor for most of the year isn’t cold at all. It’s the long stretch of extreme summer heat that stops germination, wilts young transplants, and shuts down flavor-oil production in tender leaf herbs.
That gives two practical windows. Cool-season herbs that hate heat, cilantro, dill, chervil, parsley, arugula-type leaves, get sown directly from October through February, while the 45.3°F winter minimum keeps them alive and growing slowly all winter. Heat-tolerant perennials, rosemary, thyme, oregano, sage, along with basil as an annual, go in as transplants from February through April, giving them roots established before the punishing part of the year arrives. Once summer hits, the job shifts from planting to simply keeping perennials alive until the fall reset.
None of this applies to a first-frost date either, for the same reason: fall freezes are just as unreliable as spring ones at this station. If you’re gardening in the low desert, the useful question isn’t “when’s my last frost” but “when does the heat let up.”
What will not work here
The single most common mistake is importing a generic, frost-based planting calendar into a place where frost barely exists. Most seed packets and gardening advice tell you to sow cool-season herbs “as soon as the soil can be worked in spring” and wait for warm-season herbs until “after the last frost.” Follow that logic in Phoenix and you’ll plant cilantro or dill in March, right as the calendar above says they should already be finishing up. They’ll germinate fine and bolt within weeks once the heat arrives, because a spring-latitude rule was applied to a place where spring is already too late for those herbs.
Watering habits imported from wetter, cooler regions fail here too, but in the opposite direction from what most gardeners expect. Mediterranean perennials like rosemary, thyme, and sage are genuinely built for this climate. They don’t fail from underwatering; they fail from a lawn-style weekly watering schedule that keeps their roots wet in fast-draining desert soil, which rots them out. These herbs want deep, infrequent watering with real drying time in between, not a fixed weekly ritual.
Basil sits in an odd middle ground. It’s a warm-season herb, so it belongs in the February-through-April planting window, but it still isn’t built for the full intensity of a low-desert summer. Without afternoon shade during the hottest stretch, established basil plants stop producing new leaves and go leggy and bitter, even though the plant technically survives.
And the standard advice about wide spacing to fight humidity and fungal disease, common guidance for gardeners in the Southeast or Midwest, isn’t the risk here at all. Arizona’s dry air means fungal pressure is low; the real spacing concern is exposed soil losing moisture fast in direct sun, which argues for tighter plantings and mulch rather than the wide gaps a humid-climate guide would recommend.
What changes across Arizona
Arizona’s USDA hardiness zone range runs from 4b to 10b, a spread of roughly six full zones inside one state’s borders. That’s wider than the zone range of many entire multi-state regions, and it means the heat-driven calendar described above is really a Phoenix-and-low-desert calendar, not an Arizona calendar. The USDA system is built on one number, the average annual extreme minimum temperature, so a 4b-to-10b range tells you the coldest nights a garden is likely to see, not its summer heat, but it’s still the clearest signal of how differently the seasons behave from one part of the state to another.
At elevation, Flagstaff and the high country sit at the cold end of that range, and they operate on a completely conventional frost-based calendar: real, hard freezes every winter, a compressed growing season, and cool-season herbs sown in spring after the actual last frost rather than in fall. A gardener there planting by the October-through-February window described earlier would lose most of a season’s herbs to freeze, because that window was built on Phoenix’s 45.3°F winter minimum, a number that simply doesn’t apply once you’re a few thousand feet higher.
Because this is a state-wide range and not one zone, the practical move isn’t to guess which end applies to a given garden. It’s to check the USDA Plant Hardiness Zone Map directly for the specific location, since a garden twenty miles apart in elevation can sit two or three zones apart on that map, and the whole shape of the planting calendar, fall-and-winter versus spring-and-summer, flips depending on where that reading lands.
Where to check this for your own county
State-level patterns come from one reference station’s climate data. A county-level recommendation from a land-grant university extension office will always beat that, because it’s built on trial plots and local weather records specific to that county rather than a single station standing in for a state that spans six hardiness zones.
The University of Arizona Cooperative Extension is the official source for that local detail: extension.arizona.edu. Once there, look specifically for two things. First, a county-level planting calendar, since the office publishes guidance broken out by region precisely because a Yuma garden, a Phoenix garden, and a Flagstaff garden don’t share a season. Second, look for variety trial results or recommended cultivar lists for herbs, since extension offices test which varieties of basil, cilantro, or rosemary actually hold up to local heat, soil, and water conditions, information a general state-wide page like this one simply cannot provide.
If the county office you find publishes a specific sowing date range for your herb, or names varieties that performed well in local trials, treat that as the better answer. State-level data is useful for understanding the pattern, why the calendar looks the way it does, but county extension guidance is built for the actual dirt a reader is planting into.
Checking this against your own garden
Every date describes an average built from decades of station data. A reader’s own garden runs on its own microclimate, shaped by nearby pavement, wall reflection, shade patterns, and soil type, and that microclimate outranks any calendar. Three simple checks tell you more than a date ever will.
Feel the soil a couple of inches down before sowing anything. If it’s still cool and heavy to the touch in what should be a warm planting window, the local soil is lagging behind the regional average, likely from shade or recent rain, and waiting a week or two costs nothing but saves a poor germination rate.
Watch for the first true leaves on anything already sown. Seedlings that push past their initial round leaves quickly, within the timeframe expected for that herb, confirm the soil and air temperature are actually matched to what that plant needs right now, regardless of what the calendar says the date should be.
And watch how fast heat-sensitive herbs like cilantro try to bolt, sending up a flower stalk early. Early bolting is the plant reporting real heat stress in that specific spot, often weeks before the seasonal average would predict it, and it’s the clearest sign that a particular garden runs hotter than the reference station.
Some gardeners also time this by the Moon: when to plant herbs by the moon, as almanac tradition rather than fact.