Philadelphia sits at 39.87 degrees north and gets 14 hours and 50 minutes of daylight on the longest day of the year. That single number rules out a whole category of onion before you’ve even picked up a trowel. It also explains why so many gardeners who grow from grocery-store sets end up with green tops and no bulb at all. Eighteen documented varieties have latitude ranges that cover this territory. Eight do not, and planting them anyway is the single most common mistake made across Pennsylvania gardens.
How long your longest day is
Two numbers matter more than soil, more than fertilizer, more than the calendar: 39.87 degrees north, the latitude of Philadelphia, and 14 hours 50 minutes, the amount of daylight the region gets on the summer solstice. Onions don’t bulb because the weather warms up. They bulb because the length of the day crosses a threshold that tells the plant to stop building leaves and start building a bulb. Clemson Cooperative Extension puts that threshold at roughly 12 hours for short-day types; long-day types wait for 14 to 16 hours before they’ll switch over at all.
At 14 hours 50 minutes, Pennsylvania clears the short-day threshold by a wide margin and lands squarely in intermediate-day and long-day territory. That’s the whole story in one sentence: this state can grow intermediate-day and long-day onions, and it cannot grow short-day onions. For the specific dates that follow from this, see When to Plant Onions in Pennsylvania. Everything below this point in the article is detail on top of that fact.
Why day length beats warmth
A short-day onion planted here will start bulbing while the plant is still a few inches tall, because the day already exceeds its 12-hour trigger by late spring. The result is a small, unimpressive bulb, not a failure of care. A long-day onion planted somewhere the summer day never reaches 14 hours faces the opposite problem: it never gets the signal to switch modes, so it keeps making leaves until it eventually rots in the ground. Neither outcome has anything to do with fertilizer, watering, or soil pH.
Pennsylvania is a long state, north to south and across its western Alleghenies, and 39.87 degrees describes Philadelphia specifically, not the whole commonwealth. A gardener near the New York border sits at a noticeably higher latitude and gets a longer summer day than someone in the southeastern corner. If you’re near the state’s northern edge, it’s worth checking your own latitude before assuming the Philadelphia figure applies to your yard.
Which varieties bulb in Pennsylvania
Eighteen of the twenty-eight onion varieties documented on this site have published latitude ranges that include 39.87 degrees north. That means the day length here falls comfortably inside their bulbing window, whether they’re classed as intermediate-day or long-day types. For the full breakdown, see The Best Onion Varieties for Pennsylvania.
| Variety | Day-length type | Latitude range |
|---|---|---|
| Ailsa Craig | Long-day | 38-60N |
| Barolo | Long-day | 38-55N |
| Blush | Long-day | 38-55N |
| Bridger | Long-day | 35-50N |
| Cabernet | Intermediate-day | 35-45N |
| Calibra | Intermediate-day | 35-45N |
| Candy | Intermediate-day | 33-40N |
| Elsye | Intermediate-day | 32-45N |
| Expression | Intermediate-day | 32-45N |
| Monastrell | Intermediate-day | 35-46N |
| New York Early | Long-day | 38-50N |
| Patterson | Long-day | 38-55N |
| Red Long of Florence | Intermediate-day | 38-50N |
| Red Long of Tropea | Intermediate-day | 35-48N |
| Rossa di Milano | Long-day | 38-50N |
| Scout | Intermediate-day | 32-46N |
| Sierra Blanca | Intermediate-day | 32-45N |
| Walla Walla Sweet | Long-day | 35-55N |
Two more sit close enough to a published bound that they’re worth mentioning as edge cases rather than a flat yes or no: Frontier (long-day, 40-50N) and Red Carpet (long-day, 40-45N). Both fall within a degree and a half of their lower limit at Philadelphia’s latitude, so they may bulb here, particularly in the state’s northern tier, but a gardener trying either one is working at the margin, not in the middle of the range.
The varieties that won’t survive here
This is the part that explains a lot of past disappointment. A gardener who’s already planted a supermarket set and watched it bolt into leaves with no bulb underneath will recognize this list.
- Gabriella (short-day, 20-35N), bulbs too early, stays tiny
- Madalyn (short-day, 20-35N), bulbs too early, stays tiny
- Red Rock (short-day, 30-36N), bulbs too early, stays tiny
- White Castle (short-day, 30-38N), bulbs too early, stays tiny
- Powell (long-day, 43-47N), day never gets long enough, plant rots
- Redwing (long-day, 43-65N), day never gets long enough, plant rots
- Safrane (long-day, 42-50N), day never gets long enough, plant rots
- Talon (long-day, 42-50N), day never gets long enough, plant rots
The four short-day varieties fail because Philadelphia’s summer day crosses their 12-hour trigger while the plants are still small, so they bulb early and stay small. The four long-day varieties on the northern end fail for the opposite reason: the day here never reaches the 14-plus hours they’re waiting for, so the plant just keeps producing leaves until it eventually rots in the ground. That’s not a small onion, it’s no onion at all, and no amount of feeding or watering changes it.
When to plant onions in Pennsylvania
Planting season for onions tracks the day-length group a region falls into, which is why the calendar looks completely different in Georgia than it does here. Mild-winter areas growing short-day onions plant in autumn and let the bulbs mature through a warm winter. Pennsylvania doesn’t have that option, and doesn’t need it: intermediate-day and long-day onions here go in from late winter into early spring, as sets or transplants, timed around the last spring frost rather than around autumn.
NOAA’s 1991-2020 climate normals for the Philadelphia station put the last spring frost (32F) between March 29 and April 6, and the first fall frost between November 5 and November 16. Onion sets and young transplants tolerate a light frost, so gardeners in the mildest part of the state can go in a few weeks ahead of that spring window rather than waiting for it to close.
Only ranges, never safe dates
Those frost dates describe Philadelphia, and the caution attached to them matters: this is the mildest corner of Pennsylvania. The northern tier and the Alleghenies run two to four weeks later in spring, which pushes planting further into April for those gardens. Treat both dates as a 30-to-70 percent probability band, not a guarantee, and check a local frost record before committing seedlings to open ground, especially if you’re gardening at elevation or well north of the Philadelphia line.
Sweet onions or onions that keep
Every onion variety trades sweetness for storage life, and that trade-off is chemistry, not marketing. University of Missouri Extension notes that sweet onions do not store as well as pungent types, because the same volatile sulfur compounds that give an onion its bite are what protect the bulb once it’s out of the ground. Fewer sulfur compounds means a milder onion for raw eating and a shorter shelf life. More sulfur compounds means a sharper onion in the kitchen and a bulb that holds up in storage for months.
This lines up with the day-length groups covered above. Short-day types generally run sweeter and shorter-keeping, long-day types generally run sharper and longer-keeping, and intermediate-day types sit somewhere between the two. Since Pennsylvania’s day length puts it in intermediate and long-day territory rather than short-day, the state’s gardens lean toward the keeping end of that spectrum by default, without ruling out sweeter intermediate varieties for gardeners who’d rather have onions to eat now than onions to store.
What that means for your pantry
A Pennsylvania garden planted mostly with long-day varieties from the list above should produce onions with enough pungency to hold through winter storage into March, provided they’re cured and kept properly. Lean instead toward the intermediate-day varieties on that same list, and the harvest skews milder and won’t last nearly as long in a basement or pantry. Neither choice is wrong. It’s a question of whether you’d rather have a sweeter onion to use up by early winter or a sharper one still good on the shelf when the next season’s planting window opens back up.