When to Plant Unusual Roots in Delaware

Most unusual roots in Delaware go in the ground in spring, a few weeks either side of the last frost. Tender types such as sweet potatoes wait until after it, and fast fall roots like daikon go in late summer. The exact dates shift with where your garden sits, sometimes by two weeks. Here is how the calendar works, where it breaks, and how to adjust it for your own yard.

The short answer for Delaware

Hardy unusual roots can be sown from early April, and tender ones should wait until the frost window has clearly passed. At the Wilmington reference station (NOAA NCEI 1991-2020 Climate Normals, station USW00013781), the last spring frost falls between April 3 and April 14. The first fall frost falls between October 26 and November 7.

Those dates are a probability, not a deadline. They mark the band where the chance of a 32°F freeze sits between 30% and 70%. On April 3 you still have roughly even odds of another freeze. By April 14 the odds have dropped, but they have not hit zero. The same logic runs in reverse in fall.

The other caution matters just as much. These numbers come from an airport weather station. Suburban and rural gardens typically see the last spring frost later and the first fall frost earlier, often by one to two weeks, and more at elevation. Treat the dates as the reference-city figure, not a state-wide guarantee.

Different root types hang their calendar on different ends of that window:

If you are still choosing what to grow, The Best Unusual Roots to Grow in Delaware covers the varieties. For the wider picture, start with Growing Unusual Root Crops in Delaware.

What will not work here

The frost dates leave roughly six to seven months between the last spring freeze and the first fall one at the reference station. That is a decent stretch, and it still defeats a few common plans. The biggest is planting a slow root late. Salsify or scorzonera sown in midsummer will not size up before the first fall frost band arrives, because these roots need months, not weeks. A garden that frosts one to two weeks earlier than the airport has even less room.

The second failure is treating April 3 as a green light for everything. At the early edge of the band, a 32°F freeze is still close to a coin flip at the reference station, and your garden may be later. Hardy roots shrug off a light freeze. Sweet potato slips, yacon and oca do not. A single night at 32°F can end a planting made too eagerly. The fix is patience: the gap between hardy and tender sowings is the whole point of the calendar.

Third, a winter figure can mislead in both directions. The mean daily minimum of the coldest month is 25.6°F at Wilmington. That is cold enough to freeze the top inch or two of unprotected soil on many nights, yet mild enough that hardy roots such as parsnips, salsify and burdock can often be left in the ground and dug as needed. Mulch helps them. Tender roots are another matter. Sweet potatoes and yacon will not overwinter in the ground, so a plan that counts on leaving them in will fail. Lift them before frost, and store them where they stay cool but safe from freezing.

Fourth, fall sowing has a hard edge. Fast roots like daikon need time to fill out, and days shorten quickly after summer ends. Sowing them in the final weeks before the first fall frost band leaves you with leaves and no root worth pulling. Count backward from October 26, then subtract the one to two weeks that suburban and rural sites often lose.

Finally, a note on what this page does not have. The facts behind it do not include Delaware summer humidity or rainfall figures, so it does not claim a spacing or watering rule from them. For those, rely on county guidance and what your own beds do in July.

What changes across Delaware

Delaware is a small state, but it is not one garden. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) puts the state across zones 7a to 8a. There is no single answer for hardiness here. The spread is real, and it affects which marginal roots will make it through a winter without protection.

The zone system rests on one variable only: the average annual extreme minimum temperature. It says nothing about summer heat, rainfall, soil type, or how late the last frost lingers. Two gardens in the same zone can still differ by a couple of weeks at the frost dates, which is why the calendar and the zone map answer different questions.

For the dated recommendation, this page uses the reference city, Wilmington, and not the whole range. A gardener in a cooler, more inland or higher spot should shift in two directions. Push spring sowings later, since the last frost typically arrives one to two weeks after the airport figure, and often more at elevation. Pull fall sowings earlier by a similar margin. A gardener in a milder spot can lean toward the early side of each window, but should not treat it as permission to skip frost protection on tender plants.

Which zone you are in is not something this page can tell you. Look up your location on the USDA Plant Hardiness Zone Map, which lets you search by ZIP code, and read the zone for your own address. Then compare it with the reference-city figures above to judge how far to shift. A zone 7a garden sits on the colder end of the state’s range, and an 8a garden on the milder end, so winter survival of borderline roots will differ between them.

Where to check this for your own county

Your state extension service is the next stop. For Delaware, that is University of Delaware Cooperative Extension, part of the land-grant system. Delaware has three counties (New Castle, Kent and Sussex), and the extension office serves each one with advice tuned to local conditions.

When you get there, look for two things. First, the local planting calendar for vegetables in your county, which tells you when soil is typically workable and when frost risk drops. Second, any varieties trialled in your county. A variety that performed well in a trial near you is better evidence than any list written for the whole state, because it has already faced your soil and your seasons.

A county recommendation beats a state one, plainly. This page is the state one. It gives a defensible starting point built on a single reference station, plus the logic for adjusting it. It cannot see your soil, your elevation, or the cold pocket at the bottom of your yard.

One honest limit on what is here: the facts behind this page do not include a Delaware extension calendar specific to unusual root crops. Extension calendars in general are usually written per crop or per species, not one per state, so look for the crop you actually grow instead of assuming a general vegetable chart covers it. If the office has nothing on salsify or scorzonera, ask what it recommends for parsnips or carrots, which share the same hardy-root timing, and treat that as a close stand-in.

Checking this against your own garden

Your own observations outrank every date above. Three are worth watching, and none needs equipment.

Soil warmth by feel. Push a hand a few inches into the bed in the morning and hold it there for a moment. If the soil feels cold and clammy, tender roots will sulk. If it feels neutral or mildly warm, you are close. A bed that warms sooner than the neighbor’s tells you about drainage and sun exposure. A bed that stays cold tells you the opposite.

First true leaves. Sow a short test row of something quick, such as a radish, a few weeks before your main sowing. Watch how fast the first true leaves, the ones after the seed leaves, appear. Slow, stunted growth says the soil or air is not ready. If the test row stalls, your slow roots would have too.

The first male flowers. If you grow squash, the first flush of male blooms tends to come as the weather warms up for good. A neighbor’s early flowers and your late ones point to a real difference between your microclimates. A yard that flowers later is likely to fall behind the airport dates by the full one to two weeks, or more.

Write down what you see each year. After two or three seasons, your own notes will give you a calendar tighter than any state page can.

Common questions

When is the earliest I can plant unusual roots in Delaware?

Hardy roots like salsify, scorzonera, parsnip and burdock can go in as soon as the soil is workable, around the early part of the April 3 to April 14 frost window at the reference station. Your garden may be later, so check the county.

Do the April 3 to April 14 dates mean frost is over by then?

No. They mark the band where the chance of a 32°F freeze is between 30% and 70% at the Wilmington reference station. Frost can still occur after that, especially in suburban and rural gardens.

Can I leave unusual roots in the ground over winter?

Hardy ones often can. The mean daily minimum of the coldest month is 25.6°F at Wilmington, which hardy roots like parsnips and burdock tolerate, ideally under mulch. Tender roots such as sweet potatoes and yacon must be lifted before frost.

Which hardiness zone is Delaware in?

The USDA Plant Hardiness Zone Map (2023) shows zones 7a to 8a across the state. There is no single zone, so check the map for your own location.