Sow hardy unusual roots such as parsnips, salsify and scorzonera as soon as the soil can be worked in spring. Set tender ones like sweet potato slips out only after the last freeze has passed. At the Des Moines reference station, the last spring freeze falls between April 12 and April 24, and the first fall freeze between October 14 and October 25. Which crop goes in when depends on its type, and The Best Unusual Roots to Grow in Iowa covers the choices.
The short answer for Iowa
Hardy roots go in once a handful of soil crumbles instead of smearing. Tender roots wait until after the last freeze and until the soil has warmed, which usually means May. Fall decides the rest: every crop needs to finish, or at least be dug, ahead of the first hard freeze.
The frost dates come from the NOAA NCEI 1991-2020 Climate Normals for the Des Moines airport station. The last spring frost runs April 12 to April 24, and the first fall frost runs October 14 to October 25. Each date is a probability of a 32°F freeze, taken from the 30% to 70% band. In spring it is the chance that a freeze still comes on or after that date. In fall it is the chance that one has already arrived by then. A freeze after April 24 still happens in some years.
The airport is the optimistic end of the range. Iowa State Extension county figures for 1950 to 2021 run about four days later in spring and ten days earlier in fall. Add four days to the spring dates and take ten off the fall ones if your garden sits away from a city’s warmth.
| Type | Examples | Goes in | Comes out |
|---|---|---|---|
| Hardy, direct-sown | Parsnip, salsify, scorzonera, Hamburg parsley | Soil workable, around the April 12 to April 24 band | After fall frost; before the ground freezes hard |
| Hardy perennial | Jerusalem artichoke tubers, horseradish crowns | Early spring, soil workable | After the tops die back |
| Transplanted | Celeriac | Around the last freeze, hardened off first | Before a hard freeze |
| Tender, warm-season | Sweet potato slips | After the last freeze, warm soil | Before the October 14 to October 25 band |
| Late-summer sown | Daikon, black radish | Counted back from the first fall freeze | Before the ground freezes |
When do you sow parsnips and salsify?
Early. Their seed is slow and sits in cool soil for weeks, and the seedlings shrug off light frost once they are up. Waiting for May shortens a season these crops already use fully. Packet days to maturity count from sowing for direct-sown roots.
When do sweet potatoes and other tender roots go in?
Slips go out once the last freeze has passed and the soil feels warm to a bare hand. A cold May wastes them faster than a late planting does. Late planting costs a little yield, while chilled slips can stall for weeks.
What will not work here
The slowest roots run out of season. The outdoor window between the last spring freeze and the first fall freeze is the whole stretch these crops get, and the county figures cut it shorter at both ends. Ginger and turmeric need a long, hot season, so outdoors they stay small unless started indoors early and grown in pots. Yacon wants the same long runway. Oca forms its tubers as the days shorten in autumn, so a freeze around October 14 to October 25 can kill the tops just as the crop begins to size up.
Do roots left in the ground survive an Iowa winter?
The hardy ones do. The mean daily minimum of the coldest month at Des Moines is 13.8°F, and parsnips, salsify, horseradish and Jerusalem artichoke sit through that without trouble, especially under a layer of mulch. That figure is an average, so single nights go lower. The tender roots, meaning sweet potatoes, yacon and oca, cannot overwinter outdoors at all. Dig them before the first hard freeze and store them indoors.
What goes wrong in a humid summer?
Close spacing. Celeriac and Hamburg parsley crowded together hold moisture in their leaves through long humid spells, and leaf spots spread from plant to plant. Give each root the spacing on its packet, and err wider. Irrigating in a shallow soak also fails. Dry spells in July and August stress shallow-watered roots, and a sudden soaking after drought splits and forks them. A deep, even supply works better than a weekly splash.
Heavy, wet spring soil is the last common failure. Parsnip and scorzonera seed rots in cold, waterlogged ground, and a crust on the surface blocks the seedlings that survive. Wait a few days after a hard rain, loosen the bed deeply, and cover the row with a thin layer of fine compost instead of the native clay.
What changes across Iowa
There is no single Iowa answer. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) puts the state across zones 4b to 6a. That map uses one variable only, the average annual extreme minimum temperature, so it tells you how cold the worst night gets. It does not tell you how long the frost-free season runs. Two gardens in the same zone can still have frost dates that differ.
The Des Moines dates and the 13.8°F winter minimum describe central Iowa. A gardener in the colder northern tier, near the 4b end, should lean later in spring and earlier in fall. Start by adding the county gap of about four days in spring and ten days in fall. Then favor the hardy roots, because they use the cool shoulders of the season. Parsnips, salsify, scorzonera, Hamburg parsley and horseradish all fit. Sweet potatoes and celeriac carry more risk there. Choose the earliest-maturing varieties and use black plastic or a raised bed to warm the soil sooner.
Readers toward the southern edge, nearer 6a, gain a little time at both ends. Tender roots like sweet potatoes get a more comfortable run, and fall-sown daikon and black radish have extra days before the first freeze. Winter cold alone does not make a southern garden safe for tropical roots like yacon, though, because the freeze that ends the growing season still comes in October.
Your own zone is not something to guess at. Enter your ZIP code on the USDA Plant Hardiness Zone Map and read the zone for your exact location. Low ground, river valleys, windbreaks and city lots all shift the result. Frost dates differ too, so check them for your county as well.
Where to check this for your own county
Iowa State University Extension publishes the local guidance that statewide dates can only approximate. Start with its home gardening and horticulture pages and look for three things. The first is a vegetable planting calendar broken out by region or county, with spring and fall dates for each crop. The second is a table of average frost dates for the nearest town, which will show how far your county sits from the Des Moines numbers. The third is any record of varieties tried in the county, which tells you what has done well in your soil and season instead of what a seed catalog promises.
A county recommendation beats a state one. Statewide dates average over ground that differs across Iowa, from the northern counties to the Missouri line. If the county calendar says a crop goes in a week later than the dates above, follow the county. Treat the state numbers as a starting frame for planning, with the local office setting the actual sowing date. The broader guide to Growing Unusual Root Crops in Iowa sets out what each crop needs once it is in the ground.
Checking this against your own garden
Dates on a calendar are averages, and your bed has its own record. Three observations outrank any printed date.
- Soil warmth by feel. Push a bare hand into the bed to the knuckles. If the soil aches cold, tender roots wait. A raised bed or a south-facing slope will always run ahead.
- First true leaves. Seedlings put out seed leaves first, then the true leaves that look like the mature plant. If an early parsnip or radish sowing took weeks to reach that stage, the soil stayed cool longer than the calendar suggested.
- Where frost lingers. Note which corner of the yard stays white longest on a cold morning. Low spots collect cold air and frost earlier in fall and later in spring. When basil or other tender annuals blacken, that marks the end of the season for tender roots in that spot.
Common questions
Can I sow parsnips before the last frost?
Yes. Sow once the soil is workable and no longer waterlogged. The seed is slow to sprout in cold ground, and seedlings tolerate light frost. A freeze can still burn tender new leaves, but the plants usually recover.
Do sweet potatoes have enough time?
Generally yes, when you plant slips instead of seed. Set the slips out after the last freeze once the soil has warmed, and dig them before the first fall freeze. The county figures put that freeze about ten days earlier than the airport’s October 14 to October 25 band, so choose an early variety and avoid a late start.
When do I plant roots for fall harvest?
Late summer, counting back from the first fall freeze. Subtract the packet days to maturity from your fall freeze date, then add a margin so the roots finish while the weather is cooling.
Do I need to dig roots before winter?
Parsnips, salsify, scorzonera and horseradish can stay in the ground under mulch, since the mean daily minimum of the coldest month at Des Moines is 13.8°F. Tender roots must come out before the first hard freeze. Dig the hardy ones in early spring before new growth starts.