Last night’s frost warning for parts of the Harrisburg region remind that the 2025 growing season is winding down as we approach the point where an overnight freeze turns the tomato plants, annual flowers, and other tender plants to mush.
That drop-dead date varies from year to year, not to mention from area to area.
Higher elevations to the north of Peters Mountain and the region’s outlying rural areas can experience a killing frost weeks earlier, for example, than the heat-trapping areas in and around Harrisburg proper.
In general, our first fall frost dates have been happening later in the past two decades than during the late 1900s.
Last year, the first sub-32-degree night wasn’t recorded at Harrisburg International Airport (the source of our “official” National Weather Service readings) until Nov. 13.
That was almost two weeks later than the average date of Nov. 2, the norm since 2000.
The year before (2023), the first 32-degree night happened right on that average, while in 2022, the airport didn’t have a killing frost until Nov. 14. In 2020, frost didn’t wipe out the tender stuff until the week before Thanksgiving (Nov. 18).
Harrisburg-area gardeners haven’t seen 32 degrees this year since April 9, when the temperature dipped to 27 degrees. It’s been warmer than 32 degrees ever since.
April 11 is now Harrisburg’s 21st-century, last-spring-frost average, while May 10 is the area’s latest “official” spring frost since 2000.
The combination of earlier last spring frost dates and later first fall frost dates has been giving gardeners ever-longer growing seasons.
Last year’s growing season, for example, ran 232 days between the last spring frost of March 26 and the first fall frost of Nov. 13. If you’re math-minded, that means gardeners enjoyed nearly 64 percent of 2024 with temperatures above the freezing point.
Read more on our recent key weather stats for gardeners
Exactly when the growing fun ends each year is an educated, forecast-related guess, so it’s a good idea to be ready now if you’re a gardener who tries to milk every last day out of a season.
Some vegetable gardeners use cold frames and plastic-covered tunnels, for example, to nurse plants through mild freezes, while some flower-growers cover their petunias, begonias, and such with floating row covers and sheets to get the plants through a frosty night or two.
Often, our first killing frosts are followed by above-freezing spells, meaning possible weeks more of growth if plants can survive that first night or two of trouble.
The most tender plants — so-called “houseplants” such as monsteras, bromeliads, and philodendrons — start to suffer even when nights dip into the low 40s. So if you give them a summer vacation outside, forget the fabric protection and get them back inside ASAP. (Hose them off first to short-circuit bugs trying to hitch a ride inside, too.)
Keep in mind that even annuals, summer vegetables, and borderline-hardy tropicals don’t all bite the dust at 32 degrees.
Sally McCabe, the Pennsylvania Horticulture Society’s associate director of community education, describes the different types of “frosts” this way in a PHS blog post:
“There’s the just-too-cold-for-the-basil frost (around 40 degrees); the nipping frost, which nicks the tops of tomatoes and the edges of annuals; the really-heavy-dew-on-the-low-spots-on-the-lawn frost, which kills most of the annuals and any tomatoes you haven’t covered at night with a blanket, and the white-stuff-on-the-windows frost, which makes all the leaves fall off the ginkgo trees in one day and leaves frost on your fall pumpkins.
“After this comes the killing frost, which dips below 28 degrees and wipes out all the plants you’ve been trying to protect. Beyond that is the H***-freezes-over frost, where the ground freezes solid, hoses, rain barrels, and water pipes burst, figs are lost, and even spinach dies.”
Wind, humidity, and each plant’s cold-tolerating genetics play a role in the differing tolerances to temperatures, which explains why some plants die even when overnight temperatures didn’t go below, say, 35 or 36 degrees.
Complicating that is the curveball that the National Weather Service uses temperatures monitored at five feet above the ground in its official record-keeping and forecasts.
This explains why you’ll sometimes see NWS issue “frost advisories” even when the forecasted overnight temperatures are between 33 and 36 degrees.
Temperatures can be a degree or two warmer five feet up than at ground level where the plants are growing. Plus, killing frost is more likely on a night that’s calm and clear than a night that’s cloudy and/or breezy.
If you want to drill down as specifically as possible to when frosts are likely to hit your garden, the National Gardening Association has an online tool that not only zeroes in on frost data by ZIP code but also gives the percentages that frosts are likely to occur by given dates.
If your ZIP code is 17050 on the West Shore, for example, NGA’s tool says there’s only a 10 percent chance that frost is going to get your zinnias by Oct. 2. The odds go up to 30 percent by Oct. 12, to 60 percent by Oct. 23, and to 90 percent by Nov. 5.
Similar percentages are listed if you’re trying to figure out when to plant tender plants in spring. The tool says, for instance, that you have a 50-50 shot of avoiding frost in the 17050 ZIP code if you wait until April 23 to plant.
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