Iowa can grow the whole group: celeriac, kohlrabi, horseradish and Jerusalem artichokes. The calendar is the easy part. At the NOAA airport station, the last spring freeze falls between April 12 and April 24, and the first fall freeze between October 14 and October 25. The harder part is that soil temperature, not the date, sets the real start. Celeriac needs nearly the whole window, and the two perennials need a plan for winter. The full crop-by-crop picture is in Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes.
When the soil is warm enough in Iowa
Start with the number most gardeners ask for. In the NOAA NCEI 1991-2020 Climate Normals, the 30 to 70 percent probability band for the last 32°F freeze runs from April 12 to April 24 (station data here). That is a probability, not a promise, and an Iowa April has broken plenty of promises.
One caution matters if you garden outside Des Moines. Iowa State Extension county figures for 1950-2021 run about 4 days later in spring and about 10 days earlier in fall than the airport station. The airport numbers are the optimistic end of the range, so a garden in a cold pocket or on the northern side of the state should lean toward the later dates.
Frost is also only a proxy. These root crops wait for warm soil, and soil warms after the frost risk ends, not with it. A bed can be frost-free on April 24 and still sit cold and soggy, which stalls seed and sulks transplants. A soil thermometer pushed to planting depth in the morning tells you more than the calendar does. Cool-season types such as kohlrabi tolerate cooler ground than celeriac transplants, which want settled, warm conditions. The sowing temperatures for each crop are best taken from your extension office rather than from a general page like this one.
Site the permanent ones first
Before planting anything else, decide where Jerusalem artichokes go, because they are effectively permanent once planted. Every fragment of tuber left in the ground regrows, and digging them out never gets every piece. This is the single most regretted planting in a vegetable garden. Give them a bed with edges you can live with: a corner by a fence, a strip against a driveway, or a spot you will not want for anything else. Never put them in the middle of a bed meant for rotation.
Horseradish behaves in a similar way. Root pieces left behind will resprout, so it also deserves a defined spot. Both go in as soon as the ground is workable, which is earlier than celeriac transplants go out.
This page names no varieties, on purpose. The reasoning is at the end, in the section on choosing types.
How long the season lasts here
Subtract the dates and the window appears. Pairing the earliest spring date with the earliest fall date (April 12 to October 14) gives about 185 days. Pairing the latest with the latest (April 24 to October 25) gives about 184. The extremes are April 24 to October 14, roughly 173 days, and April 12 to October 25, roughly 196 days.
Apply the extension offsets, 4 days later in spring and 10 days earlier in fall, and about 14 days come off. The window then runs from roughly 159 to 182 days. For planning, the lower half of that range is the safer one to trust.
What does that length allow? First, a second sowing. Quick types such as kohlrabi can go in twice, once in spring and again in midsummer for a fall harvest. Count backward from the first fall freeze using the days to maturity on the seed packet, and add a margin, because growth slows as autumn light shortens. Cool-season roots also finish well past the first light frost, so the frost-to-frost window understates their real season.
Second, the window decides whether transplanting is worth the root disturbance. Most taprooted crops resent being moved and are better sown where they will grow. Celeriac is the exception. It is the long one, and it only fits Iowa’s window when started indoors and set out as a transplant. Kohlrabi also moves well from cell trays. For celeriac, take the days to maturity on the packet and subtract them from the days between transplant date and first fall freeze. If the result is negative, the plant will not finish.
The succession sowing guide on this site lays out how to stagger the sowings so a bed is never empty and never glutted.
Heat and water in Iowa
The two numbers that usually anchor this section are the count of days above 90°F and the summer rainfall total. This page does not carry either one, and it will not guess. Your state extension office and the NOAA station data both publish them, and they are worth a quick look for your county before you plan watering.
What this page does have is the winter low and the season length, and between them they settle most of the questions for this group. The season length tells you whether celeriac can finish at all, and in Iowa it can, but only with an early indoor start. The winter low decides what stays in the ground. The mean daily minimum of the coldest month at Des Moines is 13.8°F. That is not the record low, and the extremes run well below it. The USDA Plant Hardiness Zone Map (2023) puts the state between zones 4b and 6a. That is a wide range, so the northern and southern ends of Iowa will not behave alike.
Left in the ground or lifted
Horseradish and Jerusalem artichokes are perennials, and under a layer of mulch both are generally treated as hardy across the zones Iowa spans. They can stay put. Celeriac and kohlrabi are not winter-hardy in the same way, and should come out before the soil freezes.
Lifted roots need honest inspection before they go into storage. Look for firm flesh with no soft patch, no weeping spot and no moldy crown. A root that is soft, wet or moldy has ended the question, whatever the rest of it looks like. A sound appearance alone does not make a root safe to keep. Check the stored roots again from time to time, and no storage duration is worth promising.
Without the heat and rainfall figures, the practical response to humid midwestern summers is the same for every crop. Mulch to hold even moisture, since celeriac especially punishes uneven watering. Space plants for airflow. Water the soil rather than the leaves. Leaf diseases move faster through crowded, damp foliage than through open rows.
Which these root crops suit Iowa
Choose by type, and let the territory’s facts do the narrowing. Iowa’s window is mid-length, so only the longest crop is pressed by it. A hot, humid summer favors types given airflow and shade at the root zone, and makes disease resistance the first criterion on a seed packet or plant tag.
| Type | What it asks of the season | Winter |
|---|---|---|
| Celeriac | Longest crop; indoor start, transplant after frost risk | Lift and store before the soil freezes |
| Kohlrabi | Quick; suits spring and midsummer sowings | Harvest before a hard freeze |
| Horseradish | Planted as root pieces in spring | Stays in the ground under mulch |
| Jerusalem artichoke | Planted as tubers in spring | Stays in the ground; regrows from any fragment |
Kohlrabi, turnips and rutabagas are brassicas. They share their pests and diseases with cabbage, kale and mustard, so a gardener who rotates them into a cabbage bed has rotated nothing. The rotation page covers how to space these families apart.
Two kitchen facts belong here as well. Grating fresh horseradish releases compounds that burn the eyes and airways. Ventilate the room, and keep your face well away from the bowl. And Jerusalem artichokes store inulin rather than starch, which causes digestive discomfort in many people. Start with a small first portion and see how you do.
No variety is named on this page, and that is the right answer rather than a gap. The only recommendation that counts is one from varieties trialled where you garden, and this page has no trial data for Iowa. Iowa State University Extension is where to find it. For contrast, Growing Unusual Root Crops in Prince Edward Island works the same crops through a cooler maritime season, and Growing Unusual Root Crops in Rhode Island covers a coastal setting.
Common questions
What is the planting date for celeriac in Iowa?
No single date works. The NOAA band for the last 32°F freeze is April 12 to April 24, but transplants wait for warm soil, which arrives later. Start seedlings indoors, and set them out once the frost risk has passed and a thermometer confirms the bed has warmed.
Can I leave roots in the ground over winter?
Horseradish and Jerusalem artichokes generally can, under mulch. Celeriac and kohlrabi should be lifted before the soil freezes. The coldest-month mean minimum at Des Moines is 13.8°F, and the state spans USDA zones 4b to 6a, so the north will be harder on anything left out.
Can I rotate kohlrabi into a bed that grew cabbage?
That rotates nothing. Kohlrabi, turnips and rutabagas are brassicas, and they share pests and diseases with cabbage, kale and mustard. Put them in a bed that has been free of that family, and check the rotation page for the spacing.
How do I keep Jerusalem artichokes from spreading?
Choose the site before planting, because any tuber fragment left in the soil regrows. Pick an edge or corner you will not want for other crops, and dig carefully at harvest. A smaller planting is easier to manage than a large one.