When is the last frost in Janesville, WI?

The average last spring frost (50% probability of 32°F) in Janesville is around April 26, based on NOAA/NCEI 1991–2020 normals at the Afton - Janesville Wwtp, WI station (7.1 mi away). Half of years it falls outside that date, so wait 1–2 weeks past it before setting out frost-tender plants.

When is the first fall frost in Janesville?

The average first fall frost in Janesville, WI is around October 15, giving roughly a 170-day frost-free growing season.

What planting zone is Janesville, WI?

Janesville is in USDA plant hardiness zone 5b (average annual extreme minimum temperature -15 to -10°F), from the 2023 USDA Plant Hardiness Zone Map.

What is the weather like in Janesville?

In Janesville, WI the warmest month is July (avg high 83.4°F) and the coldest is January (avg low 11.6°F). It averages about 37.4″ of precipitation a year and 36.6″ of snow.

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Janesville, WI Planting Calendar & Frost Dates

USDA hardiness zone 5b · ZIP 53545 · elevation 232′ · NOAA & USDA data

Janesville is in USDA hardiness zone 5b (winters can dip to -15 to -10°F), with a growing season of about 170 days between the average last spring frost (April 26) and first fall frost (October 15). The warmest month is July (avg high 83.4°F); the coldest is January (avg low 11.6°F). It averages 37.4″ of precipitation a year and 36.6″ of snow, mostly mid-winter. Start cool-season crops several weeks before the last frost; wait until after it for tomatoes, peppers and other tender crops.

Hardiness zone

min temp -15 to -10°F

Last spring frost

April 26

avg (50% prob, 32°F)

First fall frost

October 15

avg (50% prob, 32°F)

Growing season

170 days

frost-free window

Warmest month

July

avg high 83.4°F

Month-by-month typical weather in Janesville

Average conditions from the NOAA/NCEI 1991–2020 U.S. Climate Normals at Afton - Janesville Wwtp, WI (7.1 mi away). Temperatures °F, precipitation & snow in inches.

Month	Avg high	Avg low	Mean	Precip (in)	Snow (in)
January	28.3	11.6	19.9	1.6	10.8
February	32.4	14.7	23.5	1.6	10.1
March	44.6	25.4	35	2.1	4.1
April	57.9	36.1	47	3.6	1.3
May	69.9	48	58.9	4.1	0
June	79.9	58	68.9	5	0
July	83.4	61.7	72.6	4.2	0
August	81.7	59.5	70.6	4.3	0
September	75.1	51	63	3.7	0
October	61.7	39.5	50.6	3	0.2
November	46.3	28.5	37.4	2.3	2.4
December	33.5	18.1	25.8	1.9	7.7

What to plant when in Janesville

Crop	Season	What to do & when
Tomatoes	warm	Start seeds indoors: March 15 · Transplant out: May 3
Peppers	warm	Start seeds indoors: March 1 · Transplant out: May 10
Cucumbers	warm	Direct sow: May 3
Zucchini / squash	warm	Direct sow: May 3
Bush beans	warm	Direct sow: May 3
Sweet corn	warm	Direct sow: May 3
Basil	warm	Transplant out: May 3
Peas	cool	Sow (spring): March 22 · Sow (fall): August 20
Spinach	cool	Sow (spring): March 15 · Sow (fall): August 20
Lettuce	cool	Sow (spring): March 29 · Sow (fall): August 20
Kale	cool	Transplant (spring): April 5 · Sow (fall): August 6
Broccoli	cool	Transplant (spring): April 12
Carrots	cool	Sow (spring): April 5 · Sow (fall): August 6
Onions	cool	Transplant (spring): March 29
Potatoes	cool	Plant seed potatoes: April 12
Radishes	cool	Sow (spring): March 29 · Sow (fall): September 3

Dates are computed from Janesville's average frost dates using standard cool-/warm-season timing. They're a starting point — a cold spring, a warm microclimate, or a fast-maturing variety can shift them by a week or two.

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Gear that helps you plant on time

Some links below are Amazon affiliate links (tag theboringthin-20). If you buy through them we may earn a small commission at no extra cost to you — it keeps this almanac free. We only list generic, genuinely-useful categories.

Soil thermometer →Know when the ground is warm enough to sow — the honest signal, not the calendar.Seed-starting trays + dome →Start tomatoes & peppers indoors on the schedule below.Seedling heat mat →Warms the soil for faster, more even germination indoors.Floating row cover / frost cloth →Buys you 4–8°F to beat an early or late frost.Raised garden bed kit →Warms and drains faster in spring than in-ground soil.Garden gloves →Basic kit for transplant day.

Nearby cities in Wisconsin

Beloit, WI Madison, WI Waukesha, WI New Berlin, WI Brookfield, WI Milwaukee, WI Kenosha, WI Racine, WI

Sources & method. Hardiness zone: 2023 USDA Plant Hardiness Zone Map (PRISM Climate Group, Oregon State University). Frost dates & growing season: NOAA/NCEI 1991–2020 U.S. Climate Normals, nearest station with published freeze normals — Afton - Janesville Wwtp, WI, 7.1 mi (confidence: high). Monthly weather normals: Afton - Janesville Wwtp, WI, 7.1 mi (confidence: high). ZIP centroid from U.S. Census 2023 ZCTA data. Frost dates are the 50%-probability date — half of years frost occurs outside the stated date. Nothing here is invented; where the nearest station is far, we say so. Last updated 2026-07-22.