Delaware Planting Calendar: When to Sow and Plant, Frost Dates Included

Delaware’s frost calendar decides everything you plant this year. Last spring frost lands April 3 to 14, first fall frost October 26 to November 7, and that gap is your whole growing season. But those dates come from the Wilmington station, so if you’re gardening in Kent or Sussex, or your yard sits low, push both ends out by a week or two before trusting them. Check your own county’s pattern against that window before you put a single seed in the ground.

The frost window in Delaware

NOAA NCEI’s 1991-2020 climate normals, built from the 30 to 70 percent freeze-probability band for a 32°F freeze at the Wilmington-area station, put the last spring frost between April 3 and April 14 and the first fall frost between October 26 and November 7. These are 30-year averages, not a promise for any single year, but they’re the most reliable public estimate of when the freeze risk drops to a level most gardeners are willing to work with.

Those dates hold closest to the reference station itself. Gardens outside Wilmington, particularly the flatter, more open farmland of Kent and Sussex counties or any yard that sits in a low spot, tend to see the last spring frost arrive later and the first fall frost arrive earlier than the airport figures, often by one to two weeks. Higher ground in the northern Piedmont edge of New Castle County can behave differently again. Treat the NOAA window as a starting point for the state and confirm the local pattern before committing tender transplants.

Growing season length

Counting from the latest spring frost date to the earliest fall frost date gives the most conservative estimate of the season: April 14 to October 26, a little over six months. Counting from the earliest spring date to the latest fall date gives the longest possible run, April 3 to November 7, closer to seven months. Most years fall somewhere between those two, which is why every planting date in this calendar is given as a range rather than a single day.

That’s a modest season next to a state like the one covered in our Florida Planting Calendar: When to Sow and Plant, Frost Dates Included, where large stretches of the peninsula see little or no frost at all. It’s a comfortable window compared to colder inland states, provided the gardener moves promptly once the soil has actually warmed rather than waiting on the calendar date alone, and it’s worth planning ahead across the whole season, starting with What to Plant in Delaware in January, moving through early groundwork like , and closing out the year with What to Plant in Delaware in November>What to Plant in Delaware in February, and on into the key transplanting window covered in What to Plant in Delaware in April>February and into the heart of the season with What to Plant in Delaware in June>February, and later into the season with What to Plant in Delaware in September>February, and carrying through to the season’s close with What to Plant in Delaware in October>, and on into the busier stretch covered in What to Plant in Delaware in May>, and on into the height of summer with What to Plant in Delaware in July>, and closing the year with What to Plant in Delaware in December>What to Plant in Delaware in February to late-season plantings such as those covered in What to Plant in Delaware in August.

Cool-season crops

Peas, spinach, and loose-leaf lettuce go into the ground first because their seeds germinate in cold soil and their seedlings shrug off a light frost. Gardeners typically sow these direct four to six weeks ahead of the last frost, a window covered in detail in our guide to What to Plant in Delaware in March. Counted against NOAA’s Wilmington-area window of April 3 to April 14, that places the practical sowing period in late February to mid-March for gardens near the reference station, and later for anywhere colder.

Timing by crop

Brassica transplants, broccoli, cabbage, kale, and Brussels sprouts started indoors, go in a bit later than the earliest peas, usually three to four weeks before the last frost rather than four to six, since a young transplant has less cold buffer than a seed still sitting in the soil. Against the same April 3-14 window, that lands transplanting in early to late March. Root crops split the difference: carrots, beets, radishes, and turnips germinate slowly in cold ground, so most gardeners wait until two to four weeks before the last frost, putting direct sowing in mid-March to early April.

Cold tolerance is what makes all of this possible. Peas, spinach, and most brassicas survive a dip into the high 20s Fahrenheit without dying, though growth simply stalls until warmer weather returns. That tolerance is a property of the plant, not a guarantee, so a hard, prolonged freeze after transplanting can still set young brassicas back hard even if it doesn’t kill them outright.

What to watch before sowing

• Squeeze a handful of soil: if it forms a muddy ball instead of crumbling, wait before working it.
• Watch the forecast for a hard freeze after transplanting brassicas, not just the average date.
• Radishes and spinach can be resown every two weeks through the cool window for a steady harvest instead of one glut.

Warm-season crops

Tomatoes, peppers, squash, beans, and corn need more than frost-free air. Their roots stall or rot in soil that’s still cold even after the risk of freezing has passed, which is why soil temperature, not the calendar date, decides when these crops actually go in. Most Delaware gardeners wait one to two weeks past NOAA’s April 3-14 last-frost window before setting out tomato and pepper transplants, once nights are reliably milder, which lands the practical transplant window in mid to late April.

Matching tomato varieties to the frost window

Days to maturity on a seed packet are counted from the transplant date, not from seed. Crossing that catalogue figure against NOAA’s Wilmington-area frost dates gives the set-out window, expected first harvest, and how much margin remains before the earliest fall frost on October 26.

The margin figures assume the earliest fall frost date NOAA gives for Wilmington, October 26. Gardens with a later average first frost, or a warm microclimate near a building or paved area, will see even more cushion than the table shows. On the disease side, Sakura, Edox, and Juliet all carry high resistance to Fusarium wilt races 1 and 2, useful in beds that have grown tomatoes for several years running, while Amalfi adds intermediate resistance to both early and late blight, which matters in a state where summer humidity keeps foliar disease pressure high.

Beans, corn, and squash skip the indoor step entirely. Direct-sow them once the last frost date has passed and the soil has warmed, typically at or shortly after NOAA’s April 3-14 window, and continue with succession sowings every two to three weeks into early summer for a longer harvest. Tomato and pepper seeds started indoors are usually begun six to eight weeks before the transplant date. Working back from a mid-to-late April set-out, that puts indoor seed-starting in late February to mid-March for most Delaware gardeners near the reference station.

The fall planting window

Fall planting dates are worked out backward from the first fall frost rather than forward from summer. NOAA NCEI’s 1991-2020 normals put that date at October 26 to November 7 for the Wilmington-area station. Take a crop’s days to maturity from the seed packet, add 10 to 14 days to cover the slower growth that comes with shortening daylight, then subtract the total from the earliest fall frost date for a safety margin.

Counting back from the fall frost

• Find the crop’s days to maturity on the seed packet.
• Add 10 to 14 days for slower autumn growth as daylight shortens.
• Subtract that total from NOAA’s earliest fall frost date for Wilmington, October 26, to find the latest safe sowing date.

Run that math on a fast crop and a slower one and the difference shows up clearly. A 40-day fall spinach, plus a 14-day buffer, needs to go in the ground roughly 54 days ahead of the frost window, landing sowing somewhere in early to mid-September. A 60-day kale, with the same buffer, needs about 74 days, pushing sowing back into mid to late August. Both examples use the earliest NOAA frost date for the safety margin and the latest for the outer edge of the range.

Some crops don’t just tolerate a light frost, they improve after one. Kale, collards, Brussels sprouts, carrots, and parsnips all convert stored starches to sugars when temperatures dip, which is why gardeners often leave them in the ground through the first couple of frosts rather than rushing the harvest. Spinach behaves the same way, holding flavor and texture better after a light freeze than it does in the last warm days of September.

Reading a planting calendar properly

A state-wide planting calendar is a starting point, not an instruction. NOAA’s frost dates come from one station near Wilmington, and Delaware’s geography changes enough over a short distance to matter. The Piedmont edge in the north sits slightly higher than the flat coastal plain that covers most of Kent and Sussex counties, the urban heat held by Wilmington and Newark keeps nearby yards a bit warmer than open farmland further out, and low spots anywhere in the state can collect cold air on a still night regardless of which county they’re in. The 30 to 70 percent probability band behind NOAA’s dates also means a freeze after the average date is a normal outcome, not a rare exception.

What changes it in your own yard

The fix isn’t a better calendar, it’s local observation. Keep a cheap soil thermometer in the bed you plan to plant and track it over a few mornings rather than trusting a single reading. Note which corners of the yard hold frost longest, since a frost pocket near a fence line or at the bottom of a slope can run consistently colder than the rest of the property. Keep a simple frost log from year to year. After two or three seasons it tells you more about your own garden than any published date ever will. The same principle shows up in states with far more dramatic swings, like the one covered in our Missouri Planting Calendar: When to Sow and Plant, Frost Dates Included, where northern and southern counties can differ by several weeks.

Local help

The University of Delaware Cooperative Extension keeps county-level planting guidance and runs soil testing that a state-wide calendar can’t replace, since freeze data from one airport station will never capture every backyard’s frost pocket. Contact the county office nearest you before finalizing a transplant date, especially if your garden sits outside New Castle County or in a low spot. For more state-specific growing advice, browse our other Delaware guides.

Common questions

Can I plant tomatoes before the last frost date if I use row covers?

Row covers raise the local temperature a few degrees but don’t guarantee protection against a hard freeze, so setting tomatoes out ahead of NOAA’s April 3-14 last-frost window is still a gamble. If you use covers to get an earlier start, keep extra layers ready for any night the forecast calls for a hard freeze rather than relying on the seasonal average.

What if my county isn’t Wilmington, does the frost date still apply?

Not exactly. NOAA’s April 3-14 and October 26-November 7 windows come from the airport station near Wilmington, and gardens farther from it, particularly in Kent and Sussex counties or in low-lying rural spots, tend to see later spring frosts and earlier fall frosts. Check with your local extension office for a closer estimate before you plan around the airport figures alone.

Is it too late to plant a fall garden if I missed the summer window?

Not necessarily. Work backward from the first fall frost window using the crop’s days to maturity plus a buffer for slower autumn growth. Fast crops like radishes and loose-leaf lettuce still fit even when sown in early September for gardens near Wilmington, and they improve in flavor after a light frost rather than being ruined by one.

Do I need to worry about frost after I’ve already hardened off my seedlings?

Yes. Hardened-off seedlings resist wind and sun better, but a hard freeze can still kill them, since NOAA’s frost window is a 30 to 70 percent probability band and not a hard cutoff. Keep frost cloth or buckets on hand through the back half of April just in case.

Why do seed packets and this calendar give different days-to-maturity numbers?

Seed packet days to maturity are usually counted from the transplant date for tomatoes and peppers, not from when the seed goes into starting mix, and the figure can shift by a week or two depending on how much heat and light the season delivers. Use the packet number as a guide crossed against your own last frost date, not as a fixed guarantee.

Related guides

• Maryland Planting Calendar: When to Sow and Plant, Frost Dates Included
• Pennsylvania Planting Calendar: When to Sow and Plant, Frost Dates Included
• Florida Planting Calendar: When to Sow and Plant, Frost Dates Included