Pennsylvania sits at 39.87°N latitude (measured at Harrisburg, the reference point for this page), where the longest day of the year delivers 14 hours and 50 minutes of daylight. That number puts the state squarely in long-day and intermediate-day onion territory. Short-day varieties, the type that dominates southern seed racks, will not bulb properly here. Here’s what actually works, and what to skip.
The short answer for Pennsylvania
An onion doesn’t care how warm your soil is in May. What flips the switch from leaf-growth to bulb-growth is day length, and at 14 hours 50 minutes on the solstice, Pennsylvania clears both the roughly 12-hour threshold that triggers short-day types and the higher 14-to-16-hour window that long-day varieties are built for. That’s why this state can grow both intermediate-day and long-day onions successfully, a wider window than states farther south get.
Eighteen documented varieties have published latitude ranges that include 39.87°N, which means their catalogue data covers this exact spot on the map. Among the long-day group: Ailsa Craig, Barolo, Blush, Bridger, New York Early, Patterson, Rossa di Milano, and Walla Walla Sweet all list ranges that stretch down to include central Pennsylvania. On the intermediate-day side, Cabernet, Calibra, Candy, Elsye, Expression, Monastrell, Red Long of Florence, Red Long of Tropea, Scout, and Sierra Blanca all cover this latitude as well.
Two more varieties, Frontier and Red Carpet, sit in what’s really a blurred zone rather than a clean yes. Both are long-day types with published ranges starting at 40°N, and Harrisburg’s 39.87°N falls just under that line. They may bulb here, but a gardener planting either one is working at the edge of the documented range, not in the middle of it.
The practical upshot: a Pennsylvania gardener isn’t choosing between “works” and “doesn’t work” the way someone in Georgia or Minnesota might be. There’s a genuine menu here, split between sweeter intermediate-day types and sharper, longer-keeping long-day types. That choice is worth its own section below, because it isn’t just about taste.
What will not work here
Two groups of onions are excluded from a Pennsylvania garden, and they fail for opposite reasons. The first group is short-day varieties bred for latitudes well south of here. Their published ranges max out between 30°N and 38°N, nowhere near Harrisburg’s 39.87°N. Planted in Pennsylvania, a short-day onion starts bulbing the moment day length crosses roughly 12 hours, which happens in late spring here, long before the plant has grown enough leaf mass to support a decent bulb. The result isn’t a small onion from a feeding mistake. It’s a small onion because the variety was told to stop growing leaves too soon.
- Gabriella (short-day, 20-35°N): bulbs far too early for a Pennsylvania season, giving an undersized onion.
- Madalyn (short-day, 20-35°N): same mismatch, bulbing triggered well before the plant has bulked up.
- Red Rock (short-day, 30-36°N): the trigger point comes weeks too soon at this latitude.
- Red Sensation (short-day, 25-35°N): built for a much shorter summer day than Pennsylvania provides.
- Texas Grano 1015Y (short-day, 25-35°N): a Gulf Coast onion in a mid-Atlantic day-length pattern.
- Vidora (short-day, 25-35°N): bulbs prematurely for the same reason as the others in this group.
- White Castle (short-day, 30-38°N): even at its upper range, it falls short of Harrisburg’s latitude.
The second excluded group fails for the opposite reason: these are long-day onions bred for latitudes north of Pennsylvania, where summer days stretch even longer than 14 hours 50 minutes. Planted here, the day never gets long enough to trip their bulbing trigger. The plant doesn’t struggle along and produce a small bulb. It keeps making leaves all season, then rots in the ground with nothing underneath.
- Powell (long-day, 43-47°N): needs a day length Pennsylvania’s summer never reaches.
- Redwing (long-day, 43-65°N): built for far-northern summers, well beyond this state’s latitude.
- Safrane (long-day, 42-50°N): requires more daylight than even Pennsylvania’s northern edge provides.
- Talon (long-day, 42-50°N): same story, the trigger point sits above the whole state.
What changes across Pennsylvania
Harrisburg’s 39.87°N is a useful reference point, but Pennsylvania is not one dot on a map. The state runs from roughly 39.7°N along its southern border with Maryland and Delaware up to about 42.3°N at the New York line, a span of more than two and a half degrees. That’s enough to shift which onions sit comfortably in range and which are riding the edge.
| Location | Approx. latitude | What it changes |
|---|---|---|
| Southern PA (Maryland/Delaware border) | ~39.7°N | Nearly identical to the reference figure; intermediate-day types like Candy and Elsye sit close to their own upper latitude limit here |
| Harrisburg (reference) | 39.87°N | Baseline for every figure on this page |
| Northern PA (Erie / New York border) | ~42.3°N | Slightly longer summer days than the reference; blurred-zone varieties like Frontier and Red Carpet (40-50°N) are a more comfortable fit here than farther south |
A gardener in Erie or the northern tier isn’t gardening under exactly the same daylight pattern as one in Chambersburg or Lancaster, even though both are technically “Pennsylvania.” The difference is small compared to, say, the gap between Florida and Minnesota, but it’s enough to matter for a variety sitting right at a published boundary. If you’re within thirty or forty miles of either state line, it’s worth checking your own coordinates rather than assuming Harrisburg’s numbers apply exactly to your backyard.
None of this changes the exclusions from the section above. Short-day onions fail across the entire state, from the southern border to the New York line, because even the southern edge of Pennsylvania sits well north of where those varieties are built to grow. The far-northern long-day exclusions (Powell, Redwing, Safrane, Talon) also hold statewide, since even Erie’s slightly longer days don’t reach the 43°N-plus range those varieties need.
Sweet now, or onions that last
Because Pennsylvania sits in the overlap zone between intermediate-day and long-day varieties, the choice a gardener faces isn’t about whether an onion will grow. It’s about what happens after harvest. According to University of Missouri Extension, sweet onion varieties do not store as well as the more pungent types, and that’s not a marketing quirk. The compounds responsible for an onion’s sharp bite are volatile sulfur compounds, and those same compounds act as a natural preservative in storage. Fewer of them means a milder onion on the cutting board and a shorter shelf life in the pantry.
Intermediate-day varieties on the Pennsylvania-suitable list, things like Candy, Cabernet, and Calibra, tend to run sweeter and lighter on those sulfur compounds. They’re excellent eaten fresh through summer and into early fall, but they’re not the onion you want to be relying on in January. Long-day varieties like Patterson, Bridger, and New York Early carry more of that sulfur punch, which means more bite raw but also a bulb that holds up in a cool, dry basement or garage for months.
Picture two pantries in March. One belongs to a gardener who planted mostly Candy and Elsye for sweet summer eating, those onions are long gone by now, used up by the holidays. The other belongs to someone who planted Patterson or Bridger alongside their sweeter picks, and that grower still has firm, storable onions on the shelf. Pennsylvania’s day-length window makes both approaches possible in the same garden, which is more flexibility than a strictly short-day or strictly long-day climate allows. The trade-off doesn’t disappear, it just becomes a decision instead of a limitation.
Checking this against your own garden
Every figure on this page traces back to a single station near Harrisburg. That’s a solid baseline for most of Pennsylvania, but a state-wide number is never a promise for every individual yard. Before ordering onion sets or transplants, run through a short checklist specific to your own location.
- Look up your exact latitude if you garden near the New York border, the Maryland/Delaware line, or the Ohio state line, all of which sit farther from Harrisburg’s 39.87°N than the center of the state does.
- Confirm your local last-frost and first-frost dates through your county extension office rather than relying on a statewide average.
- If you’re within a degree or two of a variety’s published boundary, whether that’s Frontier and Red Carpet’s blurred zone or an intermediate-day type’s upper limit, treat the planting as a trial rather than a guarantee.
- Ask your county extension office which onion varieties local growers have had consistent results with, since they track regional performance beyond published catalogue ranges.
- Note your microclimate: a south-facing slope, an urban heat pocket, or a low frost-prone hollow can shift your effective growing conditions even within the same county.