Delaware gardeners can sow the hardiest unusual roots in early spring, but the frost date is only a starting line. At the Wilmington reference station the last spring freeze falls between April 3 and April 14, and the soil warms after that. The season that follows is long enough to give celeriac a fair try. Winters (USDA zone 7a-8a) let the tough roots stay put under mulch.
When the soil is warm enough in Delaware
The date first. According to the NOAA NCEI 1991-2020 Climate Normals, the last spring freeze at 32 F falls between April 3 and April 14 at station USW00013781, the Wilmington airport reference. That range is the 30-70% probability band, so it describes odds, not a promise. Some years bring a freeze after it.
Your garden is not the airport. Suburban and rural plots usually see the last spring frost later than the reference station, often by one to two weeks, so check your own county before you pencil in a date.
Frost is a proxy, soil is the test
Here is the catch. These root crops are waiting for warm soil, and soil warms after the frost risk ends, not along with it. A bed can be frost-free in mid-April and still be cold and wet at planting depth. Seed sown into that stalls, or rots. A cheap soil thermometer pushed in to planting depth in the morning tells you more than the calendar does. Your state extension service publishes the soil thresholds for each crop.
What goes in first, and what waits
The cool-tolerant group goes in earliest. Turnip, rutabaga and kohlrabi are all brassicas, and they cope with cooler soil better than celeriac does. Horseradish is set out as root cuttings in early spring. Celeriac is the opposite case. It is started indoors and moved out only after frost risk has passed and the soil has warmed.
Jerusalem artichoke needs a warning before any planting advice. Once planted, it is effectively permanent. Every fragment of tuber left in the ground regrows, and digging the bed over only scatters the pieces. It is the most regretted planting in many vegetable gardens. Choose a spot you could live with for good: a bordered corner, or a strip along a mown edge where stray shoots get cut. Keep it out of the main vegetable beds. Only then put tubers in, in early spring once the ground is workable.
How long the season lasts here
The same NOAA station gives the first fall freeze as October 26 to November 7. Count from April 3 to October 26 and you get 206 days. April 14 to November 7 gives 207. Pair the earliest spring date with the latest fall date and the window stretches to 218 days. Pair the latest spring date with the earliest fall date and it shrinks to 195. That is roughly 28 to 31 weeks between freezes at the airport station.
Take the caution seriously, though. A garden away from the airport can lose a week or two at each end, which trims the window by a couple of weeks or more. Count your own, using your county’s dates.
What a window this long allows
A short season makes the first sowing the only sowing. Delaware’s is not short, and that opens two doors.
The first is a second sowing. Quick brassica roots such as turnip and kohlrabi can go in again in late summer for a fall harvest, timed so they mature before the fall freeze. The succession sowing guide on this site explains how to space those sowings. The second door is transplanting. With this much room, starting celeriac indoors and setting out transplants beats gambling on direct seed. It is worth the minute it takes to check the root ball before planting, since a seedling that went in damaged gives back the weeks you were trying to save.
Neighboring climates tell a different story. Growing Unusual Root Crops in Indiana and Growing Unusual Root Crops in Nova Scotia work from different frost dates, and the comparison shows how much a few weeks change the plan.
Heat and water in Delaware
This page does not carry a verified count of days above 90 F or a summer rainfall total for Delaware, so it will not guess one. Those figures are published for the Wilmington reference station and by the state’s extension service, and they are worth looking up before you commit a bed to celeriac. Delaware sits in the humid mid-Atlantic, so expect warm, damp summers where leaf disease and uneven soil moisture are the likely troubles. Treat that as a general expectation, not a measured number.
Season length decides celeriac
Celeriac is the long one in this group, and the question is whether it can finish at all. A window of 195 to 218 days at the reference station gives it room, provided it starts indoors and the garden is not much shorter than the airport. Heat and water then decide how well it finishes. A thick mulch keeps the root zone cooler, and steady watering helps more than occasional soaking. Spacing for airflow matters in a humid summer.
The winter low decides what stays in the ground
The mean daily minimum of the coldest month at Wilmington is 25.6 F. That is an average, not a record low, but it tells you nights below freezing are the norm in midwinter. The USDA Plant Hardiness Zone Map (2023) puts Delaware at 7a-8a. That system rests on one variable, the average annual extreme minimum, and it covers a range, so other parts of the state differ from Wilmington.
Horseradish and Jerusalem artichoke are hardy perennials, and neither needs lifting in this climate. Hardy roots such as rutabaga can be left under a deep mulch and pulled as needed. If a hard freeze locks the soil, that plan ends. Celeriac is the one to lift before the soil freezes.
Anything you lift and store needs a hard look first. Inspect each root. A soft patch, a weeping spot or a moldy crown ends the question, and a root that merely looks sound is not thereby safe to keep. Discard it, because storage hides trouble rather than curing it.
Which these root crops suit Delaware
The choice comes down to type, and Delaware’s facts point three ways. The long window favors starting celeriac early. Warm summers favor types given airflow and shade at the root. Humidity makes disease resistance the first thing to ask about.
| Type | When it goes in | What matters most here |
|---|---|---|
| Turnip and rutabaga (brassicas) | Spring, and late summer for fall | Disease pressure in humidity; rotation |
| Kohlrabi (brassica) | Spring, and late summer for fall | Timing the harvest ahead of heat and the fall freeze |
| Celeriac | Started indoors, set out after the soil warms | Season length, steady moisture, mulch |
| Horseradish | Early spring, as root cuttings | Siting; hardy through winter |
| Jerusalem artichoke | Early spring, once sited | Permanence; siting before planting |
Turnip, rutabaga and kohlrabi are brassicas. They share their pests and diseases with cabbage, kale and mustard, so rotating them into a cabbage bed rotates nothing. Follow them with a different plant family, and use the rotation page on this site to plan the sequence.
Why no variety names appear here
Nobody has handed this page a list of varieties trialled in Delaware, so it names none. A recommendation is only worth something if someone grew the variety in your conditions, with Delaware’s humidity and your soil. The University of Delaware Cooperative Extension is where those trial results live. Ask there for disease-resistant types suited to your county. For the crops themselves, Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes covers each one in turn.
Two kitchen notes. Grating fresh horseradish releases compounds that burn the eyes and airways, so ventilate the room and keep your face away from the bowl. Jerusalem artichokes store inulin instead of starch, and many people get digestive discomfort from it. A small first portion is the sensible way to find out how you react.
Common questions
When can I plant unusual root crops in Delaware?
The Wilmington reference station’s last spring freeze falls between April 3 and April 14, but soil warmth is the real signal, and it arrives later. Rural and suburban gardens often see the last frost one to two weeks after the airport. Check soil at planting depth and confirm timing with your county’s extension office.
Can celeriac finish in Delaware?
The reference-station window of 195 to 218 days between freezes gives it room if you start indoors and transplant. A garden with a shorter window, or one that gets uneven water in a humid summer, makes it a tougher proposition.
Do I lift horseradish and Jerusalem artichokes for winter?
No. Both are hardy perennials and stay in the ground. Celeriac is the root to lift before the soil freezes. Anything stored needs checking, and a soft patch, a weeping spot or a moldy crown means it goes in the trash.
Can turnips follow cabbage in the bed?
Not usefully. Turnips and rutabagas are brassicas, like cabbage, kale and mustard, and they share the same pests and diseases. If a bed held any of them, plant something from another family next. Because Jerusalem artichoke cannot be rotated out at all, keep it in a bed of its own.