In Arizona’s low desert, sow turnips in the cool season, from fall into winter, and not in spring. Phoenix reached 32°F in only 6 of 30 springs, so there is no frost date to count from. In the high country, where hard freezes are normal, the logic flips. This page gives the Phoenix-based window, what fails, and where to confirm it.
The short answer for Arizona
For the low desert, sow turnips when the summer heat breaks and keep sowing through the mild winter. Skip spring and summer. The sourced numbers on this page come from Phoenix, which is the reference station for the state’s low desert. They describe that climate, not every county.
What the frost numbers are a probability of
Most planting calendars start from two dates: the last spring frost and the first fall frost. Those dates are the odds of reaching 32°F on or after a given day, measured over 30 years of records. For Phoenix (NOAA station USW00023183, 1991-2020 Climate Normals), the last spring frost reads “no freeze in most years.” In the few years that do freeze, a cold snap can bring 32°F between late December and mid January. The station reached that mark in only 6 of 30 springs. The first fall frost is listed as “no reliable fall freeze.” NOAA lists the frost-free season at 365 days. You can check the raw data through NOAA NCEI’s normals service.
A date that does not exist cannot anchor a calendar. So this page does not offer a “sow 6 weeks before last frost” rule for the low desert. It would be a number invented to fill a gap.
What sets the window instead
Heat does. Turnips are a cool-season root crop, and in the low desert the cool season is the winter. The coldest month there averages a daily minimum of 45.3°F. That is cool enough for turnips to grow and mild enough that a bed of seedlings is rarely at risk. So the window opens when summer’s heat finally eases, runs through the winter, and closes as warmth returns in spring.
This page does not have a sourced month-by-month set of turnip sowing dates, and I won’t guess at them. What it does have is the structure. Sow after the heat breaks, give the crop its whole cool stretch, and stop when the weather turns warm again. The county calendar section below tells you where to get exact dates.
What will not work here
The most common failure is a borrowed calendar. Gardening books written for the Midwest or Northeast tell you to sow turnips in early spring, as soon as the soil can be worked. In Phoenix, early spring is when the cool season is already ending. A turnip sown then gets a few pleasant weeks before the heat arrives. The plants stall, run to seed, or produce roots that turn woody and sharp-tasting. The crop looks fine at the thinning stage and disappoints at harvest.
Rutabaga has it worse. It needs a longer stretch of cool weather than a turnip to size up, which means a late sowing runs straight into rising heat. A fast turnip can sneak in under the wire. A rutabaga that goes in too late cannot. If you want to try one in the low desert, sow it at the very start of the window, not the middle. Our guide to The Best Turnips and Rutabaga for Arizona covers which types mature fast enough to be worth the risk.
Weekly watering fails too. Turnip seed is small and is sown shallow, so the seedbed lives in the top inch or so of soil. In a hot, dry climate that layer can dry out in a day or two, well before the next weekly soak. Seed that sprouts and then dries out dies, and you get a gappy row. Fall sowings are the ones most exposed to this, since fall days can still be hot while the soil needs to stay damp for germination. Light, more frequent watering until the seedlings are up works better than a deep weekly one, then you can back off as roots reach down.
A subtler failure is waiting for frost. Northern gardeners often leave turnips in the ground for a hard frost to sweeten them. With 365 frost-free days and a freeze in only 6 of 30 springs, the low desert will not hand you that. Cool nights do most of the work here, and harvesting on time matters more than waiting.
Finally, the frost-based calendar fails in reverse in the high country. There, hard freezes are real, and a calendar that ignores them will lose plants. Flagstaff and the high country are a different climate entirely, and the Phoenix numbers above should not be carried there.
What changes across Arizona
Arizona is not one garden. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places the state in zones 4b to 10b. That is six full zones, and each full zone is a 10°F step in the average annual extreme minimum temperature. So the spread is large. The zone system uses a single variable, the coldest night of a typical year. It says nothing about summer heat, which is the thing that limits turnips in the low desert.
That means there is no single answer for the whole state. A gardener at the warm end of the range and one at the cold end are working with opposite problems. The low desert has too much heat and almost no freeze. The cold end has hard freezes, a short growing season, and a calendar that looks like the one in a northern state.
If you garden in the north or at altitude, shift your thinking in three ways:
- Frost dates matter again, so a calendar counted from the last spring frost and the first fall frost becomes usable.
- The season for turnips is the warm part of the year, not the winter, because the ground is frozen or too cold for much of the cool months.
- Rutabaga becomes harder, since a short season may not give it the time it needs.
This page does not carry sourced frost dates for Flagstaff or any other high-country town, so it will not invent them. Between the two extremes sit many mid-elevation towns that are neither desert nor mountain, and they can combine pieces of both patterns.
To see where you fall, look up your location on the USDA Plant Hardiness Zone Map rather than assuming a zone. Then read the zone as one clue about winter cold, not as a sowing date.
Where to check this for your own county
The state office that answers this question is University of Arizona Cooperative Extension. It is the land-grant extension service for the state, and it runs offices that serve individual counties. That local layer is where you should land.
When you get there, look for two things. The first is the local planting calendar for vegetables. Many Arizona extension materials are written by region or elevation, so find the one that matches your area, and look for turnips or root crops listed in it. The second is the list of varieties trialled in your county. A variety that did well in a test garden near you has already faced your heat, your soil and your water. That counts for more than a seed packet’s general claim.
If you cannot find turnips listed, ask the county office directly. Master Gardener programs attached to extension offices often field exactly this kind of question, and their volunteers know which sowing dates worked in your town last season.
A county recommendation beats a state one. This page is the state one. It gives the structure of the Arizona growing year and flags what breaks, but it averages across a territory that spans six hardiness zones. Treat anything here as a starting point and replace it with the county’s numbers once you have them. For the wider picture of the crop, see Growing Turnips and Rutabaga in Arizona.
Checking this against your own garden
Your own observations outrank every date on this page. Your yard has its own microclimate, shaped by walls, pavement, shade and slope. Three signals are free and need no equipment.
Soil warmth by feel. Push your bare hand into the bed in the morning and again in the afternoon. If the soil feels warm like bathwater even at dawn, it is too hot for turnip seed to germinate well. Wait until it feels cool or only mildly warm. A bed against a south-facing wall will stay warm longer than one in open shade, so you may sow later there than a neighbor does.
The first true leaves. Turnip seedlings put out a pair of seed leaves, then the first true leaves. If true leaves come quickly and the plants look sturdy, the timing suits your spot. If seedlings stall, wilt by afternoon or never develop true leaves, the bed is too hot or too dry, and a later sowing will do better.
The first male flowers on squash. If you grow squash or cucumbers, the first flush of male flowers means warm weather has taken hold in your yard. That is your cue to stop sowing turnips and to expect roots that are already in the ground to speed toward bolting. Make a note of the date each year. After two or three seasons, your own log will be a better calendar than any state page.
Some gardeners also time this by the Moon: when to plant turnips by the moon, as almanac tradition rather than fact.
Common questions
Can I sow turnips in the Phoenix area in summer?
Not with good results. Summer heat is the limiting factor in the low desert, and turnips are a cool-season crop. Wait for the heat to break in fall.
Does Phoenix have a last frost date?
Not a dependable one. NOAA’s 1991-2020 normals show no freeze in most years, and 32°F showed up in only 6 of 30 springs. The frost-free season is listed as 365 days. In years with a cold snap, it falls between late December and mid January.
Do turnips need frost to taste good?
They do not need it to grow. Some gardeners like a light chill for flavor, but the low desert rarely supplies a hard frost. The coldest month averages a daily minimum of 45.3°F, so cool nights are the realistic source of any sweetening. Harvest on time and keep the roots watered evenly.
Can I use this calendar in Flagstaff?
No. Flagstaff and the high country freeze hard and follow a different pattern, closer to a northern state’s. This page does not have sourced frost dates for those areas, so check with University of Arizona Cooperative Extension for your county.