Hardy unusual roots such as salsify, scorzonera and Jerusalem artichoke go in as soon as the soil can be worked, usually March into April. Tender ones such as oca and yacon wait until after the last spring frost, which at the Chicago airport reference station falls between April 11 and April 22. Fall radishes like daikon go in during the second half of summer. Where you garden can shift every one of these dates by a week or two.
The short answer for Illinois
Start with what the frost dates actually say. The NOAA 1991-2020 Climate Normals put the last spring frost at the Chicago reference station between April 11 and April 22. That range is the 30 to 70 percent band of 32°F freeze probability, so inside it a freeze is still a live possibility in any given year. The first fall frost lands between October 24 and November 3, measured the same way. These are airport figures for one city. Suburban and rural gardens usually see the last spring frost later and the first fall frost earlier, often by one to two weeks, and the gap is larger at elevation.
That framing sorts the unusual roots into groups, and each group answers to a different date. The table gives the timing by type, and the longer guide to Growing Unusual Root Crops in Illinois covers each crop in more depth.
| Type | Examples | When to plant |
|---|---|---|
| Hardy seed-sown roots | Salsify, scorzonera, parsnip, Hamburg parsley | Sow in March and April as soon as the soil is workable |
| Perennial tubers and crowns | Jerusalem artichoke, Chinese artichoke, horseradish | Plant in early spring as the ground thaws, or in fall |
| Transplanted slow roots | Celeriac | Start seed indoors in late winter, set out around the last frost |
| Tender tropical roots | Oca, yacon, mashua | Plant out in May, well after the frost band has passed |
| Fall roots | Daikon, black radish | Sow from late July through August |
When do you sow hardy unusual roots in Illinois?
Early. Salsify, scorzonera and parsnip seed germinates in cool soil and the seedlings shrug off light frost, so there is no reason to wait for the April 11 to April 22 band to close. Sowing in March, when the soil is no longer frozen or sodden, gives these slow roots the longest possible run before summer heat arrives. Jerusalem artichoke tubers and horseradish crowns follow the same rule. Pushed into thawed ground in early spring, they will be up well before the last freeze, and a late cold snap only nips the first shoots.
When do you plant tender roots like oca and yacon?
After the frost band, with a margin. A single 32°F night kills the tops of oca, yacon and mashua. Most Illinois gardeners wait until the soil has warmed and the last frost has passed, and later in the northern part of the state or in a cold low spot. Start the tubers or cuttings in pots indoors in March or April so they have a head start when they go out. Because the first fall frost can arrive as early as the last week of October at the reference station, and earlier in a rural garden, every week of delay in May is a week taken from the end of the crop.
What will not work here
Planting tender roots in early April fails. The reference-station band runs through April 22, and a freeze after that date is still possible in a normal year. A planting of oca or yacon that looks lush on April 20 can lose its tops in a single night. Row cover helps against a light frost, but it will not save a bed through a hard freeze.
Yacon is the tightest fit for the frost calendar. It wants most of a full frost-free season to build large storage roots, and the gap between the last spring frost band and the first fall frost band is not generous, especially for a garden that sees frost one to two weeks outside the airport dates. Oca forms its tubers as days shorten in fall, so a cold snap early in the fall frost window cuts the harvest short. Gardeners in frost-prone spots should treat oca as an experiment, not a staple, and dig the plants right after the first hard frost blackens the tops.
Do slow roots fit a short spring in Illinois?
Some do, but only when they are sown early. Salsify and scorzonera are slow to size up, so give them a long season from sowing to a usable root. Sown in late May, they run into summer heat while still tiny. Parsnip germinates slowly and unevenly in warm soil, and the seed loses viability quickly, so use fresh seed every year. Celeriac sown directly outdoors in Illinois has little chance to size up. It does far better as an indoor-started transplant.
Can you water weekly and close-space in an Illinois summer?
Not reliably. Illinois summers run hot and humid, with dry spells in between. A weekly watering routine leaves seed beds of parsnip and salsify to crust over and dry out during the long germination period. Tight spacing is the other trap. In humid weather, crowded foliage stays wet, and fungal leaf spots and root rots follow. Give daikon and celeriac enough room for air to move, mulch to hold moisture steady, and water deeply when rain stalls rather than on a fixed day.
Cold wet soil in March has its own cost, since seed sown into saturated ground rots before it sprouts. Wait until a handful of soil crumbles instead of smearing. Jerusalem artichoke deserves one more warning: left alone, it comes back from every tuber fragment and spreads, so give it a bed with an edge.
What changes across Illinois
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) runs from zone 5a to zone 7b across Illinois, so there is no single answer for the state. Zone is built on one variable, the average annual extreme minimum temperature, so it tells you about winter survival and says nothing about when to plant. At the Chicago reference station, the mean daily minimum of the coldest month is 18.8°F, which matters for perennial roots left in the ground over winter.
For hardy roots, that cold is manageable. Horseradish, Jerusalem artichoke and parsnip winter in the ground in the northern part of the state and can be dug through the cold months whenever the soil is not frozen solid. A thick mulch of straw or leaves makes digging easier. Oca, yacon and mashua do not survive freezing soil anywhere in Illinois. They must be dug before the ground freezes and stored indoors in a cool, frost-free spot.
A gardener in the north or in a cold pocket should shift the whole calendar later in spring and finish sooner in fall, pushing tender roots deeper into May and favoring early-maturing types. Further south, spring arrives sooner, the fall frost comes later, and hardy roots can go in earlier in March. Summers are hotter there too, which favors fall sowings of daikon and black radish over spring ones that bolt in heat. Look up your ZIP code on the USDA map, then treat the dates above as a starting point. The Best Unusual Roots to Grow in Illinois goes into which types suit which parts of the state.
Where to check this for your own county
The dates above describe one airport in the northeast corner of the state. A county recommendation beats a state one. Your county office knows the local frost pockets, the soil type, and which varieties have actually been grown in your area.
University of Illinois Extension runs offices across the state and is the place to start. Look for a local planting calendar for vegetables, which gives sowing windows for your county rather than for Chicago, and a list of varieties trialled in your county. Master Gardeners attached to the county office can often tell you whether anyone nearby has grown salsify or oca successfully.
Ask about the typical last and first frost dates for your own town, not the nearest airport. Ask too about soil testing, since heavy clay and compacted ground cause forked, stunted roots in parsnip and salsify regardless of timing. Extension calendars are often written for common vegetables, so use the closest match, for example treating salsify like parsnip and daikon like a fall radish.
Checking this against your own garden
Your own observations outrank any date printed here. Two are worth watching, and neither needs an instrument.
- Soil warmth by feel. Push your hand two or three inches into the bed in the morning. If the soil still feels distinctly cold after a minute, it is too early for tender roots. A bed that stays cold well into May sits low or shaded.
- Where frost lingers. On the first frosty mornings of fall, note which parts of the yard stay white longest. Frost settles in low spots, so those beds run a week or two behind and end the season sooner. Give your tender roots the warmest, highest ground.
Keep a note of what you see each year. After two or three seasons, those notes become a calendar built for your own yard.
Common questions
Can I plant unusual roots in Illinois in fall?
Yes, for several. Daikon and black radish do best sown in late July through August so they mature in cool weather. Jerusalem artichoke tubers and horseradish crowns can go in during fall as well. Fall planting does not work for oca, yacon or mashua, which cannot take freezing soil.
What is the safest planting date for tender roots?
Later than the end of the frost band. At the Chicago reference station that band closes April 22, and suburban or rural gardens often see the last frost one to two weeks later. Watch the forecast and be ready to cover young plants if a cold night shows up.
Do Illinois winters kill roots left in the ground?
Hardy ones, no. Horseradish, Jerusalem artichoke, parsnip and salsify can stay in the ground under mulch. Oca, yacon and mashua are killed once the soil freezes, so dig them in fall.
Is the same planting calendar right for northern and southern Illinois?
No. Hardiness zones in the state run from 5a to 7b, so the north and south differ in spring warm-up, fall frost and winter cold. Shift the dates later in the north, and check your county extension office for the local frost dates. Because zone is built only on winter minimum temperature, two gardens in the same zone can still have very different growing seasons.