Michigan suits most unusual root crops, with two conditions: wait for warm soil instead of the calendar, and give celeriac every day of the season. Kohlrabi, horseradish and Jerusalem artichokes handle the climate well. The state runs from USDA zone 4a to 6b, though, so a Detroit-area bed and a northern one are different gardens. The dates, the winter low and the one planting gardeners regret come first.
When the soil is warm enough in Michigan
The last spring frost falls around April 30 in the Detroit area and in early to mid May across most of the Lower Peninsula, according to MSU Extension’s frost-free dates. That is a probability, not a promise: some years a frost lands after the date, and the northern part of the state sits at the colder end of the 4a to 6b range and runs later still.
Frost is only a proxy for what these crops want. They wait for warm soil, and soil lags the air. It warms after the frost risk ends, not with it, and a cold, wet bed in the first week of May can hold seed in place for weeks or rot it outright. A cheap soil thermometer pushed in a few inches tells you more than any date. Compare its reading with the minimum germination temperature on the seed packet, and plant when the two agree.
The crops respond differently. Kohlrabi, turnips and rutabagas are cool-season brassicas and tolerate cooler ground than most vegetables, so they can go in earlier. Celeriac is started indoors and transplanted, and it should wait until frost has passed and the bed has warmed, because a cold snap can stall young plants. Horseradish crowns and Jerusalem artichoke tubers go in as soon as the soil can be worked. Both are dormant pieces of root, and they start growing on their own schedule.
Site the Jerusalem artichoke before you plant it
Jerusalem artichoke is effectively permanent once planted. Every fragment of tuber left in the ground regrows, so digging the patch never really finishes it. This is the planting gardeners most often regret. Choose a corner you will never want back, at the edge of the property or in a bed with a hard barrier such as a mowed strip, pavement or a deep edging, and keep it away from vegetable rotations. Only then think about planting.
Some of the vegetable’s reputation comes from what happens at the table. It stores inulin rather than starch, and many people get digestive discomfort from it. Start with a small first portion to find out how you respond.
For varieties, ask your state extension service rather than a catalog. Cultivars trialled in Michigan soils are the ones worth buying, and this page names none for that reason. More on that in the last section. For the wider picture of how these crops behave, see Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes.
How long the season lasts here
MSU Extension’s Michigan Garden Calendar puts the first fall frost between October 6 and October 21 for zone 6a. Count from the Detroit-area last frost of April 30 and the window runs 159 to 174 days. The table shows the arithmetic.
| Last spring frost (Detroit area) | First fall frost (zone 6a) | Frost-free window |
|---|---|---|
| April 30 | October 6 | 159 days |
| April 30 | October 21 | 174 days |
Two cautions. The fall dates belong to zone 6a, while the spring date belongs to the Detroit area. A garden with a last frost in mid May loses those days straight off the front of the window. And this page has no fall frost dates for the colder zones, so it estimates nothing for them. Look up your own county’s dates before you plan.
What does a window this long allow? A second sowing, for one thing. Quick roots such as kohlrabi and turnips can go in again in summer for a fall harvest, and the cooling nights of September often suit them better than July’s heat. The succession sowing guide on this site lays out how to space those rounds, and it matters more here than in a short-season state.
In the north, where the window is shorter, the first sowing is the only one. Everything is planted once and harvested once, so choose earlier types and don’t spend part of the window waiting on a second round. The same shortness changes the transplanting question. Celeriac is the only one of these crops that really needs transplanting, and the head start is the point. Kohlrabi can be transplanted too, but the other root crops, especially turnips and rutabagas, resent root disturbance, and it rarely pays to risk it. Where the window is tight, the transplant is worth the trouble only for celeriac, and only if the roots are handled gently.
Celeriac is the long one, and it is the crop that tests the window. Read the days to maturity on the packet of whatever you buy, count back from the first fall frost, and remember that the count starts at transplanting, not at sowing indoors. If the sum doesn’t fit, the answer is a different crop, not hope.
Heat and water in Michigan
The figures this page was meant to use for heat and rain, days above 90 F and summer rainfall, are not among the verified facts for Michigan, so none is quoted here. A hot-climate paragraph about scorched roots and irrigation schedules would be guesswork, and Michigan is not a desert garden anyway. The more useful frame is the Great Lakes summer: humid, often damp, with warm days that give way to cool nights.
What a cool, damp summer does to these roots is fairly predictable. Mildew shows up on foliage, ripening slows, and fungal and bacterial problems spread in crowded beds. Celeriac and horseradish crowns sit in wet soil and can rot. The sensible response is airflow: spacing wider than the packet’s minimum, weeding so leaves dry quickly, mulching to keep soil from splashing onto foliage, and watering at the base in the morning. Avoid overhead sprinkling in the evening. If you want to compare a humid garden with a dry one, Growing Unusual Root Crops in Massachusetts and Growing Unusual Root Crops in Utah show how much the same crops change with climate.
Turnip and rutabaga are brassicas, and so is kohlrabi. They share their troubles with cabbage, kale and mustard, so a gardener who rotates them into a cabbage bed has rotated nothing. Follow the rotation page on this site and give them a bed that has been free of the whole family.
The winter low decides what stays in the ground
The coldest-month figure is more useful here. NOAA’s 1991–2020 climate normals give a mean daily minimum of 19.2 F for the coldest month at Detroit. That is the average of the coldest nights in the coldest month, not the record low, so a cold snap will go below it. The USDA Plant Hardiness Zone Map (2023) is built on a different variable, the average annual extreme minimum, and it spans 4a to 6b across the state. Use Detroit as the reference and expect colder gardens in the north.
Horseradish and Jerusalem artichokes are perennial and hardy, so they can overwinter in the ground under mulch. Celeriac is the opposite: lift it before the soil freezes and store it. When you pull stored roots out, inspect each one. A soft patch, a weeping spot or a moldy crown ends the question. A root that looks sound on the outside has not been proven safe, so don’t trust appearance alone.
If you grate horseradish, do it with ventilation, and keep your eyes away from the bowl. Fresh-grated root releases compounds that burn the eyes and airways, and this is not a joke. An open window and a fan help, and so does standing back.
Which these root crops suit Michigan
This page names no cultivars. The varieties that matter are the ones trialled in Michigan conditions, and the verified facts here don’t include any trial results. An invented name is the mistake a gardener spots at once, and a recommendation made from memory of a catalog isn’t evidence. MSU Extension is the place to ask, because its county offices know what has done well locally and what has not.
What this page can do is reason from the territory’s facts and choose by type.
| Michigan fact | What it favors |
|---|---|
| A frost-free window of 159 to 174 days in the Detroit area, shorter in the north | Earlier-maturing types for kohlrabi, turnips and rutabagas; celeriac only if the days to maturity fit your own window |
| Humid summers with disease pressure | Disease resistance as the first criterion, plus wide spacing and mulch |
| A coldest-month mean minimum of 19.2 F at Detroit | Horseradish and Jerusalem artichokes left in place under mulch; celeriac lifted and stored |
The first row says that a short season favors the earliest types, so in the north read the maturity figure on every packet. The second says that a humid climate makes disease resistance the first thing to look for in a variety description, ahead of flavor or size. Ask the extension office which types have held up against local mildew and rot. The third matters less for selection and more for planning, since it tells you which crops can stay and which must be dug.
For a first Michigan planting, kohlrabi is the forgiving one: quick, cool-season, and fine in a modest window. Horseradish is easy to grow and demanding to site, a patch you want to keep because it spreads from root pieces. Celeriac is the stretch crop and the one to attempt only after checking the days to maturity against your own frost dates. Jerusalem artichokes grow readily, and that is the risk.
Common questions
Is April 30 the date to plant in Michigan?
No. April 30 is the approximate last frost in the Detroit area, and most of the Lower Peninsula runs into early to mid May. These roots want warm soil, which arrives after the frost risk ends. Use a soil thermometer, not the date.
Can I leave these roots in the ground over winter?
Horseradish and Jerusalem artichokes can stay in under mulch. Celeriac should be lifted before the soil freezes and stored. Detroit’s mean daily minimum in the coldest month is 19.2 F (NOAA 1991–2020), but northern gardens run colder.
Can I put rutabagas where cabbage grew?
Not if you want a real rotation. Rutabagas, turnips and kohlrabi are brassicas, and they share pests and diseases with cabbage, kale and mustard. Give them a bed free of the whole family.
How do I check a stored root?
Look and feel each one. A soft patch, a weeping spot or a moldy crown ends the question, and a root that looks fine is not thereby safe. No page can promise a keeping time, since it depends on the root and on how it was stored.