Most unusual roots go into Ohio ground in two waves: hardy ones in early spring, tender ones after the last frost. The Ohio State University Extension Ohio Gardening Calendar puts zone 6a’s last spring frost at May 5 to May 15 and its first fall frost at October 5 to October 15. Nearly every sowing date for these crops hangs on those two windows.
The short answer for Ohio
Sow hardy roots such as parsnip, salsify and scorzonera as soon as the soil can be worked in spring. Hold tender ones, sweet potato slips above all, until the May 5 to May 15 window has closed. Roots meant for fall harvest go in during late summer, timed backward from the October 5 to October 15 window.
Those dates are a probability, not a deadline. They describe when the last spring frost typically arrives, based on past records for zone 6a. In any given year frost can slip in before or after the window. That is why May 15 is the safer bet for anything a cold night would kill, and why May 5 is a gamble you can sometimes win.
Counting between the two windows gives a frost-free season of about 143 days (May 15 to October 5) at the short end and about 163 days (May 5 to October 15) at the long end. Every tender root has to fit inside that span. For the wider picture of these crops, start with Growing Unusual Root Crops in Ohio.
Sorting roots by frost tolerance
Frost tolerance, more than the calendar date itself, decides when each type goes in. This table lays out the split.
| Type | Examples | When it goes in |
|---|---|---|
| Hardy, slow-germinating | Parsnip, salsify, scorzonera, burdock | Early spring, weeks ahead of the May 5 to May 15 window, once soil is workable |
| Tender, warm-season | Sweet potato | After the window closes; May 15 is the cautious date |
| Fall quick roots | Daikon, black radish, rutabaga | Late summer, counted back from the October 5 to October 15 window |
| Hardy perennials | Jerusalem artichoke, horseradish | Early spring or fall; frost is not a threat to them |
Counting back from fall
Read the days to maturity on the seed packet, then subtract that number from October 5, the early end of the fall window. A root listed at 60 days, for example, should be sown by about August 6 to finish before the earliest typical frost. Many fall roots shrug off a light frost, so the cautious count leaves you some slack. Tender crops get no such slack.
What will not work here
The slowest tropical roots lose the arithmetic. Yacon, oca, ginger and turmeric all want a long, warm run, and 143 to 163 frost-free days is short for them. Oca tends to bulk up as days shorten, which is just when the October 5 to October 15 window arrives. Expect small harvests from direct planting, and treat ginger and turmeric as container plants started indoors, not as field crops.
Jumping the gun fails too. A warm stretch in April feels like permission, but the last spring frost can still land as late as May 15. Sweet potato slips set out at the early edge of the window into cold, wet soil tend to stall rather than grow. They do not die politely; they sit there for weeks.
Winter is a separate test. The NOAA 1991-2020 normals for the Columbus reference station show a mean daily minimum of 22.0°F in the coldest month. That is an average night low, so individual nights run colder. It is mild enough for hardy perennials like horseradish and Jerusalem artichoke to stay in the ground, and it is far too cold for tender roots left in the soil. Don’t bet on a sweet potato overwintering outdoors.
Spacing and watering are where this page runs out of numbers. It has no rainfall or humidity figures for Ohio, so the advice here is general, not measured. Still, Ohio summers are often muggy, and tightly packed foliage stays wet longer and invites rot. Give long roots room and keep air moving between rows.
Soil is the other trap. Many Ohio yards have heavy clay, which forks and stunts long taproots such as parsnip and salsify. Loosen the bed deeply before sowing, or choose shorter, blunter types. The Best Unusual Roots to Grow in Ohio sorts the options by shape and season length.
What changes across Ohio
Ohio is not one garden. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places the state in zones 5b through 6b, three half-zone steps. There is no single answer for the whole state, and any single date you read, including the ones above, belongs to a particular spot inside that spread.
The frost dates quoted here are for zone 6a. A garden in a colder pocket should lean toward the later end of the spring window and the earlier end of the fall one, which shortens the budget for slow crops. A garden in a warmer pocket can lean the other way. The 22.0°F figure comes from one reference station at Columbus, so it describes that place, not the state.
Hardiness zones and frost dates also measure different things. The zone is built on one variable, the average annual extreme minimum temperature, and it tells you whether a perennial root survives winter. It says nothing direct about how many frost-free days you get. Two gardens in the same zone can differ by weeks in when spring frost ends.
If you garden in the north, near the lake, or on higher ground and in cold hollows, shift your sowing later in spring and your fall deadlines earlier. Favor the faster roots, and start tender ones indoors so they enter the soil as established transplants. To find your own zone, look up your location on the USDA Plant Hardiness Zone Map and read the result for yourself. This page can’t tell you which zone you are in.
Where to check this for your own county
Your state extension service is the next stop. Ohio State University Extension runs county offices across the state, and their staff and Master Gardener volunteers know what actually happens in local soil and weather.
Look for two things there. The first is a local planting calendar, ideally with frost dates for the weather station nearest you instead of a state-wide average. The second is any variety trial results from your county or a neighboring one, which show what grew well in comparable conditions and not just what a seed catalog promises.
If your county office has no trial data on a particular unusual root, ask. They may know of results from a nearby county or a grower who has tried it. A negative answer is useful too, since it tells you the crop is an experiment on your plot.
A county recommendation beats a state one, and this page is the state one. It has no county-level dates, and it won’t pretend to. Use it to frame the season, then let local numbers set the final date.
Checking this against your own garden
Your own observations outrank every date on this page, because your yard has its own microclimate. Three signals are worth learning.
The first is soil warmth by feel. Push your bare hand a few inches into the bed in the morning. Soil that feels cold and clammy is not ready for sweet potato slips, however the calendar reads. A bed that feels pleasantly cool, not chilly, is closer. A south-facing spot or a raised bed will usually warm days before a shaded low corner, and that gap is your microclimate showing.
The second is the first flush of male flowers on squash or zucchini. Males open before females, and their arrival tells you real summer warmth has settled in. If yours appear well ahead of a neighbor’s, your garden runs warm. If they lag, plan on a tighter season for slow roots.
The third is the first true leaves on seedlings. The seed leaves come first; the true leaves that follow mark a plant actually growing. Sow a few radishes or beans as a test and watch how quickly those leaves show up in different beds. Fast beds suit demanding crops, and slow ones suit the hardy, patient roots.
Common questions
Can I plant unusual roots in Ohio in the fall?
Yes, within limits. Fast roots like daikon, black radish and rutabaga are sown in late summer, counted back from the October 5 to October 15 first-frost window. Perennial roots such as horseradish and Jerusalem artichoke can also go in during fall.
Is May 15 a safe date for sweet potatoes?
It is the late end of the last-frost window for zone 6a, so it is the safer choice, though not a guarantee. Watch the forecast and keep a cover handy for slips already planted.
Do I need to dig everything before the first frost?
Not the hardy ones. Parsnip, salsify and horseradish can stay in the ground past the October 5 to October 15 window. Tender roots like sweet potato should come out before frost blackens the vines.
Do these dates apply in northern Ohio?
Not exactly. The dates are for zone 6a, and the state runs from zone 5b to 6b. A colder garden should lean later in spring and earlier in fall, and the county office can give you the real dates.