Delaware’s growing season brings two separate risks to pepper fruit set: heat that shuts pollen down and cool nights that do the same from the other direction. Wilmington logs 17.6 days a year at or above 90°F, enough heat load to stall a pepper crop mid-season if timing is off. When to Start Pepper Seed in Delaware matters here: start seed indoors 8 to 10 weeks before transplanting after the last frost (April 3 to April 14) and once soil hits 60°F, and watch your own nighttime lows closely.
The short answer
The heat problem
Wilmington averages 17.6 days a year at or above 90°F, based on NOAA NCEI Climate Normals from 1991 to 2020. That’s not a heat wave here and there. It’s more than two weeks of the growing season where daytime temperatures cross the threshold that Utah State University Extension identifies as one edge of pepper fruit-set failure. Above 90°F, pollen viability drops and blossoms can abort even on an otherwise healthy plant. The flowers open, look fine for a day, then drop without setting fruit. Gardeners in New Castle County, where Wilmington sits, should expect this to happen in stretches during July and August, not as a single freak afternoon.
The cold problem
The other edge of the same failure comes from nights below 55°F. Peppers are a warm-season crop through and through, and a cool snap in late spring or an early cold night in September can knock blossoms off just as effectively as a heat wave does. This site tracks daytime heat data for Delaware, specifically the count of days at 90°F or above, but it does not carry summer night minimum temperatures. That’s a real gap, and it matters here because coastal Delaware, inland Kent County, and the northern piedmont near Wilmington do not all cool down the same way after sunset. A gardener near the bay may see warmer overnight lows than one further inland, even at similar latitude. Don’t guess based on where you sit on a map. For a fuller breakdown of both thresholds, see Why Peppers Fail to Set Fruit in Delaware. Track your own nighttime temperatures with a basic min-max thermometer during the weeks your plants are flowering, and note any night that dips under 55°F. That’s the number that tells you whether a stretch of blossom drop is a cold-night problem or something else.
Put together, these two thresholds define a fairly narrow band. Delaware’s peppers need enough sustained warmth to flower and set fruit, but not so much heat that afternoons routinely clear 90°F during the flowering window. Timing your transplant and choosing when blossoms open becomes the real lever gardeners have, since nobody can turn down a heat wave once it arrives. Choosing from Pepper Varieties That Ripen in a Delaware Season can also help you work within that narrow window instead of against it.
When to start seed here
The arithmetic here is straightforward once you have the two anchor dates: the last spring frost and the point at which soil reaches 60°F. Utah State University Extension recommends transplanting only after frost has passed and soil temperature has reached that 60°F mark, and starting seed 8 to 10 weeks before that transplant date. Here’s how that plays out using Delaware’s own numbers.
- Find your local last-frost window. At the Wilmington reference station, NOAA NCEI’s 1991-2020 Climate Normals place the last spring frost (32°F) between April 3 and April 14, based on the 30-70% freeze probability band. That’s the airport-station figure, not a guarantee for every yard in the state.
- Adjust for where you actually garden. Suburban and rural properties away from the Wilmington station typically see their last frost land later, often by one to two weeks, and more if you’re at higher elevation or in a low-lying frost pocket. If you garden in southern Kent County or Sussex County, or anywhere away from the immediate coast, plan on the later end of that window or beyond.
- Confirm soil temperature separately from air temperature. Frost passing doesn’t mean the soil is warm enough. Soil at 60°F usually trails the last frost date by one to three weeks in this region, since soil warms more slowly than air. A simple soil thermometer pushed 2 to 4 inches down, checked in the morning over a few consecutive days, is the only reliable way to know.
- Count back 8 to 10 weeks from your expected transplant date. If your soil is likely to hit 60°F by late April to early May, based on the adjustments above, that puts your indoor seed-starting date somewhere in late February to early March.
- Build in a buffer, not a shortcut. Because Delaware’s frost window itself spans nearly two weeks at the reference station alone, and local conditions push that further, err toward starting seed on the earlier side of your calculated window rather than the later side. A pepper transplant that sits ready a week early is far less costly than one that goes into cold soil too soon.
None of this arithmetic is unique to Delaware. It’s the same formula Utah State University Extension applies everywhere peppers are grown. What changes state to state, and county to county within Delaware, are the two dates you plug in: your last frost and your soil-warming timeline. Redo the math with your own county’s numbers rather than borrowing Wilmington’s outright.
What goes wrong here
Delaware’s climate produces a specific, repeatable set of pepper failures. Each one has a distinct cause and a distinct fix, and mixing them up wastes a growing season.
- Blossom drop from a hot spell. When daytime temperatures push past 90°F, which happens on average 17.6 days a year at Wilmington, pepper flowers can open and drop within a day or two without setting fruit. The remedy isn’t to fight the heat directly. Time your transplant so that peak flowering, which typically follows several weeks after transplant, falls before the hottest stretch of mid-summer where possible, and keep plants consistently watered during hot spells since drought stress compounds heat stress on flowering.
- Blossom drop from a cool night. The same symptom, dropped flowers with no fruit, can come from the opposite direction: a night below 55°F, particularly in late spring or early fall. Since this site doesn’t track nighttime minimums for Delaware, the fix starts with your own thermometer. If you see a cold night in the forecast during flowering, consider row cover overnight to hold in ground heat, and avoid transplanting so early that your plants are flowering while nights are still routinely dipping into the 50s.
- Sunscald on exposed fruit. Strong sun combined with Delaware’s summer heat can scald pepper fruit that’s left uncovered by foliage, showing up as a pale, papery patch on the sun-facing side. This usually follows pruning that removes too much leaf cover, or plants that are naturally sparse. Leave foliage in place rather than pruning it back hard, and avoid harvesting so aggressively that fruit further down the plant loses its leaf shade.
- Blossom-end rot on developing fruit. This shows up as a dark, sunken spot on the blossom end of the fruit and is driven by inconsistent watering, which interferes with calcium uptake even when soil calcium levels are adequate. It gets worse during Delaware’s hot, dry stretches when soil moisture swings between soaked and parched. Even, regular watering, along with mulch to hold soil moisture steady, addresses most cases.
Checking this against your own county
Everything above is built on data from a single reference station and a single set of statewide averages. Your yard is not the Wilmington airport, and the difference matters more for peppers than for a lot of other crops, given how narrow the temperature band for fruit set really is. Soil type, drainage, tree cover, and even which side of a hill you’re on can shift your effective frost dates and your nighttime lows by more than a week in either direction.
The most reliable way to check your own numbers against what’s published here is through the University of Delaware Cooperative Extension. Their office can speak to conditions specific to your county, including planting windows and locally observed pest and disease pressure that a statewide climate normal simply can’t capture.
Common questions
What temperature stops peppers from setting fruit?
Pepper fruit set fails at both ends of the temperature range: nights below 55°F and days above 90°F, according to Utah State University Extension. Either extreme can cause flowers to drop without producing fruit, even on a plant that otherwise looks healthy.
How many days a year does Delaware see 90°F or hotter?
At the Wilmington reference station, the 1991-2020 NOAA NCEI Climate Normals put the average at 17.6 days a year at or above 90°F. That’s the figure to weigh against your flowering window when planning transplant timing.
When should I transplant peppers outdoors in Delaware?
Transplant after your last frost and once soil has reached 60°F. At the Wilmington station, the last spring frost typically falls between April 3 and April 14, though suburban and rural gardens often see frost end one to two weeks later than that airport figure.
Does this site track nighttime low temperatures for Delaware?
No. This site carries daytime heat data, specifically days at or above 90°F, but not summer night minimums. Since cool nights below 55°F can stall pepper fruit set just as heat does, track your own overnight lows during flowering rather than estimating them from latitude or from winter temperature data.