Set sweet potato slips in Delaware from mid-May through mid-June, once the soil is warm and nights stay mild. Earlier than that risks a chill the plants never shake off. Much later cuts into the growing time before fall frost. Two frost windows and a crop that wants roughly four months explain the whole calendar.
The short answer for Delaware
The Wilmington airport station puts the last spring frost between April 3 and April 14, and the first fall frost between October 26 and November 7. Those dates come from NOAA’s 1991-2020 Climate Normals. Each one is a probability of a 32°F freeze: the band where the chance of another freeze (in spring) or a first freeze (in fall) sits between 30 and 70 percent.
| Milestone | Reference-station window | What it means for sweet potatoes |
|---|---|---|
| Last spring frost | April 3 to April 14 | Slips still stay indoors or in the nursery |
| Planting window | Mid-May to mid-June | Soil warm, nights mild |
| First fall frost | October 26 to November 7 | Vines blacken; roots should be dug by then |
These are airport figures. Suburban and rural gardens usually see the last spring frost later and the first fall frost earlier, often by one to two weeks. A garden outside the city may have a last frost in late April and a first frost in mid-October. County extension advice is the check on that (more on it below).
Why wait until after the last frost date?
Sweet potatoes are a tropical vine grown as an annual. Frost kills the foliage outright, but cold soil does damage too. A slip set into soil below about 65°F sits there and makes little root. The usual rule is to plant two to three weeks after the last frost, which lands in the first half of May in most Delaware gardens and later in cool spots.
How long do sweet potatoes need in the ground?
Most varieties take roughly 90 to 120 days counted from setting out the slips, not from the day you start them. A slip planted on June 1 has a few days to spare before the first fall frost even at 120 days. The vines keep bulking up roots as long as nights stay above freezing, so every week of delay in spring is a week taken off the roots’ final size. The full growing routine is in Growing Sweet Potatoes in Delaware.
What will not work here
An April planting fails. The airport’s last frost may have passed by April 14, but the soil is still cold and the nights can drop back. Slips set out then often stall, turn yellow and shed leaves. Some recover by June, but by then a May planting has passed them.
Late planting fails for a different reason. A slip set out in early July leaves little margin before the October 26 to November 7 frost band, and less in a rural garden that frosts a week or two sooner. Varieties that need the full 120 days, or longer, in a warm season will produce small roots in that window. The slowest types suit longer summers. Delaware growers do better with the faster varieties covered in The Best Sweet Potatoes to Grow in Delaware.
Does Delaware summer heat help or hurt?
The Wilmington station averages 17.6 days a year at 90°F or more. That is enough warmth to grow sweet potatoes well, but it is not the long furnace of the Deep South. The crop gets plenty of heat in July and August. What it lacks is a long tail of warm weather in October. Cool nights in late September slow root growth, which is why a late start costs more here than it would farther south.
What does Delaware’s rainfall do to spacing and watering?
The reference station records about 13.1 inches of rain across June, July and August, out of 45.3 inches for the year. June brings 4.67 inches, July 4.41 and August 3.98, so August is the driest month and still not dry. In a summer that humid, tight spacing backfires. Vines set 6 inches apart form a dense mat of wet leaves that stays damp after every shower, and that invites rot and fungal trouble. A spacing of about 12 inches in the row, with 3 to 4 feet between rows, leaves room for air.
Weekly watering on a set schedule also misses the mark. In a normal summer the rain covers most of the crop’s needs. On sandy soil, though, an inch of rain can drain through within the day, so a sandy bed may still want water between storms. Clay holds that same inch for a week. Soil texture, the 90°F days and a rain gauge on your own plot decide the schedule, not the calendar. Mounded rows or ridges keep roots from sitting in water on heavy ground. A heavy storm after a dry spell can also split the roots, so even moisture beats feast and famine.
What changes across Delaware
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Delaware in zones 7a to 8a. That is a spread, so no single answer fits the whole state. The zone rests on one variable, the average annual extreme minimum temperature. It says how cold the coldest winter night usually gets, and sweet potatoes never face winter in the ground. They face the last spring frost and the first fall frost, and those dates shift with the same geography that sorts the zones.
For a reader in the northern part of the state, or in a low hollow where cold air settles at night, the safe move is to push the planting date later. Wait toward the end of May rather than the middle, and expect fall frost to arrive early in the October 26 to November 7 band or ahead of it. Cold pockets can frost a week or two ahead of the airport, sometimes more. Gardens near open water or on warmer, sandier ground in the southern part of the state often run milder, so the early end of the window can work there. Delaware has little elevation, so the swings come from low spots, water and soil more than from hills.
For a zone reading, use the USDA map and look up your own ZIP code. A neighbor’s garden, a different county or a different side of town may sit in a different zone than the state’s overall range suggests, so check your own location rather than assume.
Where to check this for your own county
The University of Delaware Cooperative Extension is the state’s land-grant extension service, and it is the right next stop. Its staff work with growers in each county and know how the local soil and weather behave.
When you get there, look for three things:
- A local vegetable planting calendar that lists warm-season crops by county or region
- Sweet potato varieties that have been trialled in your county, with notes on yield and days to harvest
- Soil testing instructions, since pH and fertility shape root size as much as the planting date does
A county recommendation beats a state one. The windows in this article come from one reference station, and they describe the state as a whole. A county office can tell you how much later your area frosts in spring and how much earlier in fall, and which varieties have actually finished in a short local season. If the county advice and the calendar here disagree, follow the county.
Extension offices also tend to know where to find slips locally and when local suppliers ship them, which matters because slips shipped too early arrive before the soil is ready. Ask when slips are expected and plan your bed around that date.
Checking this against your own garden
Your own observations outrank every date above. Three of them are worth watching.
Soil warmth by feel. Push your hand 3 to 4 inches into the bed in the morning. If it feels cool and slightly raw, the soil is not ready. If it feels like comfortable bathwater, it is close. A bed under black plastic warms days sooner than bare ground, and one in shade warms later. That difference is your microclimate talking.
The slips’ first ten days. Healthy slips wilt a little on day one, then perk up and push out new leaves within a week or so. Slips that stay flat, yellow or stuck for ten days are telling you the soil or the nights are still too cold. A bed where slips take off fastest is your warmest spot, and next year it can go in first.
Frost on the lawn at dawn. Note the last morning in spring when the grass whitens, and where it whitens first. Low corners of the yard frost longest. In fall, the first tender plant to blacken, a basil plant or a squash vine, marks when to dig the roots. If your frosts come well before or after the airport’s, you now have your own dates.
The Moon calendar is a separate, older custom: see when to plant sweet potatoes by the moon, written as tradition rather than fact.
Common questions
Can I plant sweet potato slips in April in Delaware?
It is a gamble that rarely pays. The airport’s last spring frost band ends April 14, but a garden outside town usually frosts later, and April soil is too cold for slips to root well. Wait until the soil feels warm and nights stay mild, typically mid-May.
Is it too late to plant sweet potatoes in late June?
Late June is cutting it close. Most varieties want 90 to 120 days after setting out, and a late-June planting leaves little buffer before frost in the October 26 to November 7 band. It is closer still in a garden that frosts earlier than the airport. Choose a faster variety and plant as soon as slips are in hand.
When do I dig sweet potatoes in Delaware?
Dig before the first hard freeze, and check your own county for the usual date. If frost blackens the vines first, cut them back and dig soon after, since roots left in cold, wet soil can spoil. Handle them gently, because the skin bruises easily.
Do I need to water sweet potatoes in a Delaware summer?
Rain usually covers most of it, with about 13.1 inches falling across June, July and August. New slips need regular water for the first couple of weeks. After that, water during dry spells, especially on sandy soil that drains fast, and let a rain gauge in your own bed guide you.