Janesville, IA 50647 Frost Dates
50647, near Janesville, IA: the frost window
The last spring frost here typically falls around 04/27, about 242 days from today. Fall's first freeze usually shows up around 10/10, roughly 165 days after the last spring frost.
Zone 5a (-20 to -15°F average annual minimum) comes from the USDA Plant Hardiness Zone Map; the frost dates above are the 1991-2020 normal at the nearest reporting NOAA station for Janesville, not a forecast for this year.
Stretching the season past the first frost
Janesville's growing season runs out in roughly 6 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
Why 04/27 isn't a guarantee
04/27 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 04/11 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/10 and only about 1 year in 10 sees frost after that.
Janesville against the rest of the dataset
Janesville's growing season runs about 165 days, ranking 30,643 of 40,283 US ZIP codes measured here, longest to shortest. Within IA, it ranks 515 of 1,042.
Learn more
Two guides worth reading on 50647: local microclimates, on what one yard in Janesville can differ from the 50647 normal, and zone vs. frost date, on why Zone 5a and Janesville's frost date answer different questions.