In Arizona, elevation matters more than any calendar date. In the Phoenix low desert, frost almost never sets the schedule. Summer heat does, so celeriac, kohlrabi, turnips and their cousins are grown through the mild months. In Flagstaff and the high country, hard freezes return and the usual frost-based rules apply. Settle which Arizona you garden in, then choose the crop.
When the soil is warm enough in Arizona
The reference station here is Phoenix (NOAA station USW00023183, 1991-2020 Climate Normals). It records no freeze in most years. A rare cold snap can bring 32F between late December and mid January, and Phoenix reached that mark in only 6 of 30 springs. NOAA lists the frost-free season at 365 days. A frost date is a probability, not a promise, and in the low desert that probability is small enough to drop out of the planning.
For gardeners elsewhere, frost has always been a stand-in. Root crops don’t care about the air on a given night. They care about the soil, which warms after the last frost risk ends, not along with it. A gardener in a cold climate watches frost because it is the easiest signal to read. In the Phoenix area that signal tells you nothing, because the soil problem is reversed. The ground is too hot for much of the year, not too cold. The practical question becomes when the soil cools enough after summer for seed to germinate and roots to bulk up in mild weather.
Why this page has no frost-based calendar
Writing one for the low desert would be false precision. The limiting factor there is summer heat, which stops fruit set for months and makes cool-season roots a fall-through-spring project. A soil thermometer and your own observation of when nights ease will serve you better than a date lifted from a northern chart.
Flagstaff and the high country are a different climate entirely, and they do freeze hard. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) spans zones 4b to 10b across the state. That is a range, not one number, so any date advice for your own yard has to come from your own county. The University of Arizona Cooperative Extension is the place to ask about the varieties trialled where you live. This page names none, because it cannot source any, and a made-up variety name is the error local gardeners spot first.
For readers comparing climates, Growing Unusual Root Crops in Georgia and Growing Unusual Root Crops in New Jersey show how the same group of crops behaves where frost, not heat, draws the line. The overview of Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes covers the crops themselves.
How long the season lasts here
Do the subtraction the usual way and it fails. The low-desert facts give no reliable fall freeze, and the spring side is a rare event, not a date. NOAA’s 365-day frost-free figure for Phoenix is therefore an upper bound, not a gardening window. No pair of frost dates exists to subtract, and estimating one would be inventing it.
What that long frost-free stretch allows is unusual. Cold never ends the season in the low desert, so heat and your patience set the limit. Sowings can be staggered across the mild months instead of squeezed into one spring burst. A second sowing of a quick root is realistic. Staggering also protects you: if one batch bolts or stalls in a warm spell, the next is still coming. Transplanting is a choice rather than a necessity. A seedling started in a cooler spot and set out when the soil eases can be worth the root check, as long as the roots are handled gently, since most of these crops dislike disturbance.
Celeriac is the long one. In the low desert it needs to be started so that most of its growth falls in the mild months, not pushed into rising heat. Whether it can finish depends on how long the mild stretch runs where you are, and that varies a lot between a valley floor and a higher town. This page has no verified frost dates for the high country, so it won’t give a season length there. Gardeners at elevation, where freezes do bring a real last and first date, should get those dates from the extension service and then do the subtraction themselves. A short window makes the first sowing the only one, and that makes transplanting celeriac a very different decision from the low desert’s.
The succession sowing guide on this site explains how to space repeat sowings, and it applies well to quick roots such as turnips and kohlrabi once you know your own cool stretch.
Heat and water in Arizona
Two things govern this group of crops here: how long the cool season runs, and what the winter low does. Heat and rain are the third and fourth factors. This page has no verified count of days above 90 F or figure for summer rainfall, so it won’t quote either. It also won’t pretend the desert is a mild place in July. The caution on the climate data is blunt: summer heat is the limiting factor in the low desert.
The winter figure is verified. The mean daily minimum of the coldest month at the Phoenix reference station is 45.3F. That is a mean, not a record low, but it shows the ground in the low desert doesn’t freeze solid. The hardy roots and the perennials can usually stay in the ground through winter, and nobody needs to lift them before a freeze. Mulch there serves a different purpose. It keeps roots cooler and slows evaporation instead of insulating against frost.
Heat stress and watering
Roots that bulk up in heat tend to turn woody, pithy or bitter, and bolting is the common failure. Shade at the root zone and steady moisture help. Drip irrigation delivers water at the root and keeps the foliage drier, and a layer of mulch holds moisture and shades the soil. Erratic watering splits roots and stalls growth. Watering needs to be regular, not generous one week and absent the next.
Turnip and rutabaga are brassicas. They share their troubles with cabbage, kale and mustard, so rotating them into a cabbage bed rotates nothing. Pests and diseases that build up in one carry to the others. The rotation page on this site explains how to space them out.
In the high country, winter matters in the opposite direction. With the hardiness range reaching zone 4b, the question of leaving roots in the ground under mulch or lifting them before the soil freezes is real there. Local winter lows vary widely by elevation, so the extension service is the source for your own site. Wherever roots are lifted for storage, inspect every one. A soft patch, a weeping spot or a moldy crown ends the question for that root. Looking sound does not make a stored root safe, and no keeping time can be promised.
Where summers run cool and damp, the trouble is different: mildew, slow ripening and disease. Arizona’s low desert is not that place, but some high-country spots in a wet monsoon spell can lean that way. If your garden does, disease resistance moves higher on the list.
Which these root crops suit Arizona
Choose by type, and by the Arizona you actually have. Variety names are deliberately missing here. The right variety is the one trialled in your region, and the University of Arizona Cooperative Extension is where those trials are reported. A variety that thrives in a Flagstaff garden can fail in Yuma, so any name from a distant list would mislead.
| Type | Low desert (Phoenix area) | High country (Flagstaff area) |
|---|---|---|
| Turnip and rutabaga | Mild-season crop; heat is the risk | Frost-timed; hard freezes shape the window |
| Kohlrabi | Quick type suits the cool stretch | Earliest types suit a short season |
| Celeriac | Needs a long mild period to finish | Season length decides whether it can finish |
| Horseradish | Needs shade and steady water | Hardy; can stay in the ground under mulch |
| Jerusalem artichoke | Grows readily; siting is the issue | Hardy; siting is the issue |
A short season favors the earliest types, which matters in the high country. A hot one favors types that tolerate airflow and shade at the root, which describes the low desert. Humid spells make disease resistance the first criterion.
Site Jerusalem artichoke before anything else. Once planted it is effectively permanent, because every fragment of tuber left in the ground regrows. It is the single most regretted planting in a vegetable garden. Give it a bed of its own, away from beds you plan to dig and replant, or a contained spot you will never want for anything else. Only after that decision does the growing method matter. It also stores inulin instead of starch, and many people get digestive discomfort from it. A small first portion is the sensible way to find out. No claim is made here that cooking or frost removes the effect.
Horseradish needs care at the kitchen counter. Grating a fresh root releases compounds that burn the eyes and airways. Ventilate the room and keep your eyes well away from the bowl. This is a genuine irritant and no joke.
Common questions
Is there a last frost date for planting root crops in Arizona?
In the low desert, not in a useful sense. Phoenix (NOAA station USW00023183) has no freeze in most years, and reached 32F in only 6 of 30 springs. In the high country, freezes are routine and the extension service can give local dates.
Can I leave roots in the ground over winter?
In the low desert, usually yes. The mean daily minimum of the coldest month at the Phoenix reference station is 45.3F. In colder parts of the state, whether to mulch or lift depends on how hard your soil freezes.
Should I rotate turnips into a cabbage bed?
No. Turnip and rutabaga are brassicas and share pests and diseases with cabbage, kale and mustard. Use the rotation page on this site and give them a different plant family.
Where do I find varieties that work in my part of Arizona?
Ask the University of Arizona Cooperative Extension. Its local trials are the only recommendation that means anything for your elevation, and this page deliberately names none.