Around Philadelphia, the only city where NOAA has published exact numbers for this state, the last spring frost typically falls between March 29 and April 6, and the first fall frost lands between November 5 and November 16. That single data point does not describe the whole state. The northern tier and the Allegheny highlands run two to four weeks colder in spring, which changes everything about when it’s safe to plant.
The frost dates for Pennsylvania
The figures above come from one weather station: Philadelphia International Airport, listed by NOAA as USW00013739. It’s the only site in this state with a full 32°F freeze-probability table published in the 1991-2020 U.S. Climate Normals, so it’s the reference point, not a stand-in for the whole commonwealth.
Here’s what the numbers actually mean. NOAA doesn’t publish a single “frost date.” It publishes a probability curve, and the range cited here (March 29 to April 6 in spring, November 5 to November 16 in fall) is the 30-to-70% band. On the early end of that spring window, there’s still roughly a 30% chance of a 32°F freeze. By the later date, the odds have only climbed back to about 70%, which means even after the window closes, frost risk hasn’t fully disappeared.
| Event | Date range (Philadelphia station) | What the band means |
|---|---|---|
| Last spring frost (32°F) | March 29 to April 6 | 30% to 70% chance of freeze on those dates |
| First fall frost (32°F) | November 5 to November 16 | 30% to 70% chance of freeze on those dates |
Now the caution that matters most: Philadelphia sits in the mildest corner of the state, tucked into the Delaware Valley with a long frost-free stretch relative to Everywhere else. The northern tier, the Poconos, and the Allegheny Plateau run two to four weeks later on both ends of that window. Nobody has published a matching table for those areas at the level of detail cited here, so treat the Philadelphia figures as a floor for how early spring can arrive statewide, not a ceiling.
Pennsylvania’s USDA Plant Hardiness Zone map (2023 edition, built on 1991-2020 averages) spans 5a to 7b across the state. That’s a wide range for one state, and it’s worth remembering this system measures something different from frost timing: it’s based purely on the average annual extreme minimum winter temperature, not on when frost starts or stops. A location can share a hardiness zone with Philadelphia and still see frost weeks earlier in fall, because the two measurements track different things.
How long the growing season actually is here
At the Philadelphia station, stacking the two frost windows together gives a frost-free season running somewhere between 213 and 232 days, depending on which end of each probability band you land on. Count from the latest spring frost date (April 6) to the earliest fall frost date (November 5) and you get 213 days. Count from the earliest spring date (March 29) to the latest fall date (November 16) and you get 232. That 19-day spread is the built-in uncertainty of working with probability bands instead of fixed calendar dates.
Why zone and season length aren’t the same thing
It’s tempting to look at a 5a-7b hardiness range and assume it maps neatly onto a shorter or longer growing season, but that’s not how the USDA system works. Zone 7b, the warmer end found near the southeastern corner, reflects milder winter lows, not necessarily a longer frost-free stretch. A spot could carry a relatively mild winter minimum and still get an early fall frost if it sits in a valley that traps cold air. The two measurements answer different questions: one is about how cold winter gets, the other is about when the growing window opens and closes.
What that means practically: a 213-to-232-day season is generous by northeastern U.S. standards, and it’s specific to the Philadelphia area. Regions in the northern tier or up in the Alleghenies, where spring frost lingers two to four weeks past the Philadelphia dates, are working with a season that’s shorter on both ends. No published figure here quantifies exactly how much shorter for those specific areas, which is exactly why leaning on a single reference station for statewide planning gets people into trouble. If you garden outside the southeastern corner, the honest move is to treat your own frost history, or your county extension office’s records, as the real authority rather than stretching the Philadelphia numbers to fit.
Turning these dates into a sowing date
Frost dates are only useful once you turn them into a planting calendar, and the process is the same no matter where in the state you garden. Seed packets and planting guides typically express timing as a number of weeks relative to your last spring frost date, for example, six to eight weeks before that date for starting tomatoes indoors, or two weeks before it for direct-sowing cold-tolerant greens. The trick is plugging in the right local date instead of the Philadelphia figure by default.
Here’s the practical sequence:
- Identify the closest matching frost record for your county rather than assuming the Philadelphia dates apply.
- Use the late end of the spring probability band (closer to April 6 for the Philadelphia reference) if you want a safety margin, since frost risk hasn’t fully dropped off even by the early end of the range.
- Count backward from that date for indoor seed-starting, following the weeks-before-frost guidance on the seed packet.
- For fall crops, count forward from your local last-frost estimate toward the first-frost window and work backward from there to figure out your last safe sowing date.
For anything planted directly in the ground, tender warm-season crops like tomatoes, peppers, and squash need the soil itself to have warmed up, not just the air. That’s a separate consideration from the frost date itself, and it’s not something the frost probability tables capture. Waiting until you’re past the higher end of the spring frost window (again, closer to April 6 for the Philadelphia reference point) gives both air and soil more time to catch up.
Because the published dates here only cover one station, anyone gardening outside the immediate Philadelphia area should treat Penn State Extension as the next stop for county-level planting calendars. Extension offices maintain records tailored to local conditions, which matters enormously in a state where spring can arrive three weeks apart between the Delaware Valley and the Allegheny highlands.
Why your own garden differs from this figure
A backyard rarely matches a weather station’s numbers exactly, and the reasons are mostly about geography, not luck. Cold air is heavy. It slides downhill and pools in low spots, which is why a garden at the bottom of a slope can frost while a bed thirty feet upslope stays clear. Pennsylvania’s terrain, with its ridges, river valleys, and plateau country, makes this effect more pronounced than in flatter states.
Elevation plays a similar role. Every added few hundred feet tends to push frost dates earlier in fall and later in spring, which is part of why the Allegheny highlands and the northern tier lag weeks behind the Philadelphia figures cited here. Urban and suburban settings work the opposite direction: pavement, buildings, and concentrated heat sources create a mild bubble that can delay the first fall frost and push the last spring frost earlier than what a rural station nearby would record.
Smaller features matter too. A south-facing wall that absorbs and releases heat overnight can shield nearby plants by several degrees. Dense tree canopy traps a bit of ground warmth on clear nights, while an open lawn radiates heat away faster and frosts sooner. Low-lying areas near creeks or ponds sometimes benefit from the water’s moderating effect, while other water-adjacent spots see more fog and dampness that encourages frost formation on calm nights.
None of this means the published frost windows are wrong. It means they describe conditions at one specific station, and your own yard is its own microclimate layered on top of that regional pattern. Tracking your own frost history over a few seasons, alongside your county extension’s data, gives a far more accurate planting signal than any single statewide figure ever could.
Common questions
What’s the earliest safe date to plant tomatoes outside in Pennsylvania?
There’s no single statewide date published here. For the Philadelphia area, waiting until past the later end of the spring frost window (around April 6) gives a margin of safety, since frost risk is still present even at the earlier end of that range. Gardeners elsewhere in the state should check with Penn State Extension for a local estimate, since spring arrives weeks later outside the southeastern corner.
Do frost dates differ between eastern and western Pennsylvania?
The only precise figures published here are for the Philadelphia station in the southeast. What’s documented is that the northern tier and the Allegheny highlands, which cover much of the state’s western and north-central sections, run two to four weeks later in spring than Philadelphia. Exact matching dates for those regions aren’t part of the dataset cited here.
What USDA hardiness zone is Pennsylvania in?
The state spans USDA Plant Hardiness Zones 5a through 7b, based on the 2023 map built from 1991-2020 temperature averages. That system measures the average annual extreme minimum winter temperature, not frost timing, so it shouldn’t be used as a substitute for a frost date.
Where can I find county-specific frost dates for Pennsylvania?
Penn State Extension maintains local planting resources tailored to specific counties, which matters given how much frost timing shifts between the Delaware Valley and the higher elevations to the north and west.