Wilmington sits at 39.67 degrees north, and on the longest day of the year it gets 14 hours and 49 minutes of daylight. That single number decides which onions will actually form a bulb in a Delaware garden and which ones will just sit there making leaves. It puts the state squarely in long-day and intermediate-day territory, with 18 documented varieties whose published range covers this latitude and eight that flatly will not work here.
How long your longest day is
Everything about growing onions comes down to one mechanism: bulbing is triggered by day length, not by heat or soil temperature. An onion plant reads the number of daylight hours the way a thermostat reads temperature, and once it hits a variety-specific threshold, it switches from growing leaves to swelling a bulb. Short-day types flip that switch around 12 hours of daylight. Long-day types wait until they see 14 to 16 hours. Intermediate-day types sit in between, usually 13 to 14 hours.
At 14 hours 49 minutes on the summer solstice, Wilmington clears the short-day threshold by almost three hours and comfortably reaches into intermediate-day and long-day range. That rules out short-day onions entirely, they’d start bulbing while still seedling-sized, and it opens the door to almost every intermediate and long-day variety on the market. This is the whole story in one sentence: Delaware’s summer days are long enough for long-day onions, too short for short-day ones, and right in the sweet spot for intermediate types.
One caution worth repeating: 39.67 degrees is the latitude of the NOAA weather station near Wilmington, not of the entire state. Delaware is small, but it still stretches from the Pennsylvania line down past Fenwick Island, a few tenths of a degree in either direction. That’s not enough to flip a long-day onion into failure, but if you garden near the Maryland line in Sussex County, or up near the Delaware Water Gap, it’s worth checking your own coordinates against this figure before you order seed. A quick way to do it: search your town name plus “latitude,” and compare the result to the 39.67 degrees used here. Anyone within half a degree north or south is functionally in the same day-length group as Wilmington. Anyone further out should treat the numbers on this page as a starting point, not gospel.
Which varieties bulb in Delaware
Eighteen documented varieties have a published latitude range that covers 39.67 degrees north, which means their catalog data says they’re built to bulb at this day length. That’s a long list, and it spans both long-day and intermediate-day types, so a Delaware gardener genuinely has options for color, flavor, and storage life.
- Long-day: Ailsa Craig, Barolo, Blush, Bridger, New York Early, Patterson, Red Long of Florence, Rossa di Milano, Walla Walla Sweet
- Intermediate-day: Cabernet, Calibra, Candy, Elsye, Expression, Monastrell, Red Long of Tropea, Scout, Sierra Blanca
Two more, Frontier and Red Carpet, sit within a degree and a half of their published lower bound of 40 degrees north. They may bulb in Delaware, but a gardener planting either one is working at the edge of the documented range, not comfortably inside it. Treat these as a gamble worth trying in a small trial bed, not a variety to plant wall-to-wall.
The varieties that will not bulb here
This is the part that actually matters if you’ve ever bought a bag of onion sets at a garden center without checking the label closely. Eight varieties documented on this site have latitude ranges that don’t reach Wilmington’s, and planting any of them here wastes a season.
- Too far south (short-day types): Gabriella, Madalyn (both rated 20-35N), Red Rock (30-36N), White Castle (30-38N). Planted in Delaware, these begin bulbing while the plant is still small, because the day length trigger they’re waiting for arrives here too early in the season, before the top growth is big enough to feed a real bulb. The result is a fully formed but miniature onion.
- Too far north (long-day types): Powell (43-47N), Redwing (43-65N), Safrane (42-50N), Talon (42-50N). These need a summer day length that Delaware’s 14 hours 49 minutes never reaches. The plant doesn’t stall or shrink, it just keeps making leaves indefinitely, then rots in the ground with no bulb at all once the season turns.
If you’ve grown a supermarket onion set before and ended up with a handful of scallion-like stalks and rot at the base come fall, this is almost certainly why. It’s a variety mismatch, not something you did wrong in the garden.
When to plant onions in Delaware
Delaware’s planting calendar rides on the state’s mild-to-moderate winters, which is why growers here plant differently from someone in a hard-freeze northern state, and differently from a Deep South garden that plants onions in fall for a spring harvest. Wilmington’s own frost data pins the last spring freeze between April 3 and April 14, and the first fall freeze between October 26 and November 7.
Because that spring frost window arrives relatively early and the fall frost holds off into late October or even November, Delaware gardeners typically start onions indoors in late winter, around six to eight weeks before that early-April frost date, then transplant seedlings or sets outdoors once the ground can be worked and hard freezes are unlikely, usually late March into early April. That gives long-day and intermediate-day types the full stretch of lengthening spring days they need to bulk up leaf growth before the June solstice flips the bulbing switch.
Fall planting, common in true short-day country further south, isn’t the standard approach here. Delaware’s winters aren’t reliably mild enough for onion sets to overwinter as a rule, and since short-day varieties don’t work at this latitude anyway, there’s little reason to plant in October the way a Georgia or Florida gardener would.
These frost dates are averages pulled from thirty years of data at one airport weather station, not a promise for any specific yard. Suburban and rural properties around Wilmington, and especially those further from the coast or at slightly higher elevation, commonly see their last spring frost run one to two weeks later than the station average, and their first fall frost arrive one to two weeks earlier. That’s enough of a swing to matter for tender transplants set out too early. Check your own county extension office’s frost guidance before committing a full tray of seedlings to the ground, and treat the April 3 to April 14 window as the middle of the range, not the edge of it.
Sweet onions or onions that keep
The variety list above splits cleanly into two camps, and the split isn’t random, it’s chemistry. Pungency in an onion comes from volatile sulfur compounds, and those same compounds do double duty as a natural preservative. A bulb loaded with them bites hard on a cutting board and also survives months in a cool pantry. A bulb with fewer of them tastes mild and sweet raw, but breaks down fast once stored.
University of Missouri Extension states it plainly: as a rule, sweet onions do not store as well as the more pungent types. That’s not marketing language, it’s a documented trade-off, and it lines up with the day-length pattern running through this whole page. Short-day types, grown in the Deep South, tend to run sweet and short-keeping. Long-day types, grown further north, tend to run sharp and long-keeping. Delaware’s intermediate-day options land in between.
Practically, that means a Delaware garden growing something like Candy or Cabernet on the intermediate-day side will get onions with decent size and moderate storage life, good for using through fall and into early winter, but not the kind that lasts until spring planting rolls around again. Lean toward the long-day side of the list, Patterson, Ailsa Craig, or Bridger, and you trade a little sweetness for onions that hold in a cool, dark, dry spot well into winter, sometimes to March.
What that means for the pantry: a Delaware gardener who plants a mix, say Candy for fresh eating in late summer and Patterson or Ailsa Craig for the cellar, gets both. Plant only sweet types and expect to be buying onions again by January. Plant only the sharpest long-day keepers and you’ll have onions on hand well past the last frost, but nothing worth slicing raw into a salad in August.