Janesville, WI planting zone & frost dates
Janesville is in USDA hardiness zone 5b. The last spring frost usually comes around April 26 and the first fall frost around October 15, for a growing season of about 170 days.
Planting calendar for Janesville
Dates are counted from the average last spring frost (April 26) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Mar 1 – Mar 15 Transplant outdoors: May 3 – May 10 |
| Peppers | Start seeds indoors: Feb 15 – Mar 1 Transplant outdoors: May 10 – May 17 |
| Eggplant | Start seeds indoors: Feb 15 – Mar 1 Transplant outdoors: May 10 – May 17 |
| Broccoli | Start seeds indoors: Feb 15 – Mar 1 Transplant outdoors: Mar 29 – Apr 12 |
| Cabbage | Start seeds indoors: Feb 15 – Mar 1 Transplant outdoors: Mar 29 – Apr 12 |
| Onions (sets) | Plant outdoors: Mar 29 – Apr 12 |
| Peas | Sow outdoors: Mar 15 – Mar 29 |
| Spinach | Sow outdoors: Mar 15 – Mar 29 |
| Lettuce | Sow outdoors: Mar 29 – Apr 12 |
| Carrots | Sow outdoors: Mar 29 – Apr 12 |
| Potatoes | Plant outdoors: Mar 29 – Apr 12 |
| Beans | Sow outdoors: May 3 – May 10 |
| Sweet corn | Sow outdoors: May 3 – May 10 |
| Cucumbers | Sow outdoors: May 3 – May 10 |
| Squash & zucchini | Sow outdoors: May 3 – May 10 |
| Basil | Start seeds indoors: Mar 15 – Mar 29 Transplant outdoors: May 3 – May 10 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 20 (8 weeks before the average first fall frost on Oct 15).
Useful for zone 5b gardens
- Seed starting kit with dome — to start seeds indoors before the last frost
- LED grow light for seedlings — stops leggy seedlings
- Frost cloth / plant covers — for late spring and early fall frosts
- Soil thermometer — warm-season crops need soil above 60°F
- Cold frame / mini greenhouse — adds weeks to a short season
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Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Apr 13 | Apr 26 | May 11 |
| First 32°F frost in fall is before | Oct 27 | Oct 15 | Oct 1 |
How to read it: in spring there is a 10% chance of a frost after May 11, so that is a safe date for tender plants. In fall there is a 10% chance of frost before October 1. A hard freeze (28°F) usually ends around April 15.
Hardiness zone by ZIP code in Janesville
- 53545 zone 5b
- 53546 zone 5b
- 53547 zone 5b
- 53548 zone 5b
Nearby cities
- Beloit zone 5b, last frost May 1
- Fort Atkinson zone 5b, last frost Apr 28
- Stoughton zone 5b, last frost Apr 30
- Whitewater zone 5b, last frost May 2
- Oregon zone 5a, last frost May 1
- Fitchburg zone 5b, last frost May 1
- Sun Prairie zone 5a, last frost May 1
- Madison zone 5a, last frost May 1
Questions
What growing zone is Janesville, WI?
Janesville is in USDA Plant Hardiness Zone 5b (average extreme winter low -15°F to -10°F), according to the USDA 2023 map.
When is the last frost in Janesville?
The average last spring frost (32°F) in Janesville is April 26. There is still a 10% chance of frost until May 11.
When can I plant tomatoes in Janesville?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 3 – May 10. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Afton - Janesville Wwtp, WI (USC00470045), 6.3 km (3.9 mi) from the city center, elevation 232 m, based on 29 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.