What USDA hardiness zone is Jefferson County, Wisconsin?

Jefferson County, Wisconsin spans USDA hardiness zones 5a to 5b. 90% of its ZIP codes are zone 5b, so there is no single zone for the county.

When is the last spring frost in Jefferson County, Wisconsin?

The median last spring freeze runs April 25 – May 3 across the 7 NOAA weather stations serving Jefferson County, Wisconsin, from NOAA 1991-2020 Climate Normals. Half of years freeze later than the median.

Wisconsin

Planting zones in Jefferson County, Wisconsin

This county spans USDA zones 5a to 5b, so there is no single zone for Jefferson County and anyone who gives you one is rounding. 90% of its ZIP codes are zone 5b; the rest are not.

USDA zones 5a–5b 2 zones across 10 ZIP codes

ZIP codes 10 10 have a NOAA station within range

Weather stations 7 NOAA 1991–2020 normals, across 556 sq mi

Your ZIP code

Get the exact answer for your address

Everything above describes Jefferson County, Wisconsin as a whole. Hardiness zones and freeze dates are published per ZIP code, so yours is exact — enter it and this page will tell you your zone, which NOAA station it comes from and how far away that station is.

The growing season in Jefferson County is closing

Nothing in the 32-crop table still has time to mature here. The first fall freeze normally arrives October 9 – October 18 across the 7 NOAA stations serving this county .

Your next season already has a date

The first job of it is 🧅 Green Onions, start indoors, around February 28 – March 8 — counted back from a last spring freeze of April 25 – May 3 here.

Every spring date for your ZIP →

These are ranges across the whole county. The ZIP lookup above gives the dates from your own nearest station instead.

All 7 stations agreeAt least one station

Every one of the 7 NOAA stations serving this county counts May 3 – October 9 as growing season (159 days). Widening to any single station gives April 25 – October 18 (176 days).

Which zone you are in depends where you are

Jefferson County covers 2 hardiness zones. A guide that gives the county one zone has picked whichever is largest and rounded everyone else into it — fine for the 90% in zone 5b and wrong for the other 10%. Here is the actual split.

Zone	ZIP codes	Share
5b	9	90%
5a	1	10%

Zones are published per ZIP code, so yours is exact even though the county's is not. The hardiness tool will tell you which one you are in.

Your freeze dates

Last spring freeze April 25 – May 3

First fall freeze October 9 – October 18

Across the 7 NOAA stations serving this county. These are medians — half of years freeze later than the date shown — so treat the later end of each range as the cautious one.

The growing season runs 157–173 days depending which station you are nearest, as published by NOAA rather than computed from the dates above. Summer highs average 79.5–81.4 °F in June–August. Summer rain over the same months runs 12.9–14.4" across those 7 stations, against the rough inch a week — about 13 inches over the same 92 days — that a vegetable garden is usually reckoned to want. At the middle station that is covered, so watering in Jefferson County is for the gaps between storms rather than the whole season. The inches are NOAA's published normals; the inch a week is a rule of thumb, and it takes no account of a 80 °F summer afternoon drying the soil faster than a cool one.

Every ZIP code in Jefferson County

Zones are published per ZIP code, not per county, so this is the exact answer rather than the county's average. The station column is the nearest one to that ZIP code's centroid and how far away it is — the same figure the tools use, shown here so you can see which ZIP codes are well covered and which are reaching.

ZIP code	USDA zone	Nearest NOAA station	Distance
53036	5a	Oconomowoc WWTP, WI	10.6 km
53038	5b	Lake Mills WWTP, WI	8.6 km
53094	5b	Watertown WWTP, WI	3.4 km
53137	5b	Sullivan 3Se, WI	10.7 km
53156	5b	Sullivan 3Se, WI	9.2 km
53178	5b	Sullivan 3Se, WI	7.9 km
53538	5b	Ft Atkinson, WI	1.1 km
53549	5b	Jefferson WWTP, WI	3.3 km
53551	5b	Lake Mills WWTP, WI	1.6 km
53594	5b	Waterloo WWTP, WI	1.2 km

ZIP-code positions are area centroids, so a large rural ZIP code's listed distance is measured from the middle of its area rather than from any particular address in it.

When to start seeds in Jefferson County

Every date below is anchored on the last spring freeze at each station and reported as a range across all 7 of them, because no one station speaks for the county.

Crop	Job	When	Offset
🧅 Green Onions	Start indoors	February 28 – March 8	8 wk before
🌶️ Peppers	Start indoors	February 28 – March 8	8 wk before
🥦 Broccoli	Start indoors	March 14 – March 22	6 wk before
🥬 Cabbage	Start indoors	March 14 – March 22	6 wk before
🥬 Collards	Start indoors	March 14 – March 22	6 wk before
🥬 Kale	Start indoors	March 14 – March 22	6 wk before
🥬 Leaf Lettuce	Start indoors	March 14 – March 22	6 wk before
🫛 Peas	Direct sow	March 14 – March 22	6 wk before
🥬 Spinach	Direct sow	March 14 – March 22	6 wk before
🍠 Sweet Potato	Start indoors	March 14 – March 22	6 wk before
🍅 Tomatoes	Start indoors	March 14 – March 22	6 wk before
🥬 Bok Choy	Start indoors	March 21 – March 29	5 wk before
🥬 Kohlrabi	Start indoors	March 21 – March 29	5 wk before
🥬 Swiss Chard	Start indoors	March 21 – March 29	5 wk before
🥬 Arugula	Direct sow	March 28 – April 5	4 wk before
🥬 Baby Salad Greens	Direct sow	March 28 – April 5	4 wk before
🥕 Beets	Direct sow	March 28 – April 5	4 wk before
🧅 Green Onions	Direct sow	March 28 – April 5	4 wk before
🥬 Mustard Greens	Direct sow	March 28 – April 5	4 wk before
🌶️ Okra	Start indoors	March 28 – April 5	4 wk before
🌱 Radishes	Direct sow	March 28 – April 5	4 wk before
🥔 Turnips	Direct sow	March 28 – April 5	4 wk before
🥬 Cabbage	Set out	April 4 – April 12	3 wk before
🥕 Carrots	Direct sow	April 4 – April 12	3 wk before
🥬 Collards	Direct sow	April 4 – April 12	3 wk before
🥬 Collards	Set out	April 4 – April 12	3 wk before
🥒 Cucumbers	Start indoors	April 4 – April 12	3 wk before
🧅 Green Onions	Set out	April 4 – April 12	3 wk before
🥬 Kale	Direct sow	April 4 – April 12	3 wk before
🥬 Kale	Set out	April 4 – April 12	3 wk before
🥬 Kohlrabi	Direct sow	April 4 – April 12	3 wk before
🥬 Kohlrabi	Set out	April 4 – April 12	3 wk before
🥬 Leaf Lettuce	Direct sow	April 4 – April 12	3 wk before
🎃 Pumpkins	Start indoors	April 4 – April 12	3 wk before
🎃 Summer Squash	Start indoors	April 4 – April 12	3 wk before
🎃 Winter Squash	Start indoors	April 4 – April 12	3 wk before
🥒 Zucchini	Start indoors	April 4 – April 12	3 wk before
🥬 Bok Choy	Direct sow	April 11 – April 19	2 wk before
🥬 Bok Choy	Set out	April 11 – April 19	2 wk before
🥦 Broccoli	Set out	April 11 – April 19	2 wk before
🌿 Cilantro	Direct sow	April 11 – April 19	2 wk before
🥬 Leaf Lettuce	Set out	April 11 – April 19	2 wk before
🥬 Swiss Chard	Direct sow	April 11 – April 19	2 wk before
🥬 Swiss Chard	Set out	April 11 – April 19	2 wk before
🌿 Dill	Direct sow	April 18 – April 26	1 wk before
🫘 Bush Beans	Direct sow	May 2 – May 10	1 wk after
🥒 Cucumbers	Set out	May 2 – May 10	1 wk after
🎃 Pumpkins	Direct sow	May 2 – May 10	1 wk after
🎃 Pumpkins	Set out	May 2 – May 10	1 wk after
🎃 Summer Squash	Direct sow	May 2 – May 10	1 wk after
🎃 Summer Squash	Set out	May 2 – May 10	1 wk after
🌽 Sweet Corn	Direct sow	May 2 – May 10	1 wk after
🍅 Tomatoes	Set out	May 2 – May 10	1 wk after
🎃 Winter Squash	Direct sow	May 2 – May 10	1 wk after
🎃 Winter Squash	Set out	May 2 – May 10	1 wk after
🥒 Zucchini	Direct sow	May 2 – May 10	1 wk after
🥒 Zucchini	Set out	May 2 – May 10	1 wk after
🥒 Cucumbers	Direct sow	May 9 – May 17	2 wk after
🌶️ Okra	Direct sow	May 9 – May 17	2 wk after
🌶️ Okra	Set out	May 9 – May 17	2 wk after
🌶️ Peppers	Set out	May 9 – May 17	2 wk after
🫘 Southern Peas	Direct sow	May 9 – May 17	2 wk after
🍠 Sweet Potato	Set out	May 9 – May 17	2 wk after

Offsets are published guidance; the dates they land on are computed from measured NOAA normals. The seed-starting tool does this for your own ZIP code and counts the days.

What survives the winter changes across Jefferson County

Zone 5a carries 48 of 74 perennial edibles; zone 5a carries 48.

Hardiness ranges are a judgement, published by growers rather than measured; the zone they are compared against is measured. On a warm-end verdict, check with your local extension service — a coastal zone 8 and an inland zone 8 are not the same summer.

Cautious — shortens the seasonMedian across stationsOptimistic

Last spring freeze: Plan to May 21 to be careful. Median April 25 – May 3 across 7 stations; April 12 is the optimistic end.
First fall freeze: Plan to September 25 to be careful. Median October 9 – October 18 across 7 stations; October 31 is the optimistic end.

NOAA publishes these as probabilities, not averages — the middle figure is a median, so half of years fall either side of it. The cautious end is the 10% tail: in roughly nine years in ten the freeze falls on the safe side of it.

The 7 stations behind these numbers

All 7 NOAA stations whose normals this page quotes, spanning about 31 km north to south. Every one is named with its identifier in the station list below — this page does not single one out, because no one station speaks for a whole county.

A relative sketch, not a map with a projection: longitude is adjusted for latitude so the shape is not stretched, and there is no basemap. For the one station nearest a specific address, with its real distance, use the ZIP lookup above.

These are the stations this county's ZIP codes actually cite as nearest — not a claim that each one sits inside its boundary, since a rural ZIP legitimately reaches over it. Every figure above comes from them, and each is named so you can check it.

Station	NOAA ID	Last spring freeze	First fall freeze	Season
Ft Atkinson, WI	USC00472869	April 28	October 13	166 days
Jefferson WWTP, WI	USC00474108	April 25	October 18	173 days
Lake Mills WWTP, WI	USC00474482	April 29	October 14	167 days
Oconomowoc WWTP, WI	USC00476200	April 30	October 12	164 days
Sullivan 3Se, WI	USC00478316	May 3	October 9	157 days
Waterloo WWTP, WI	USC00478910	April 30	October 9	163 days
Watertown WWTP, WI	USC00478919	April 28	October 12	167 days

These are medians, not averages — NOAA publishes freeze dates as a probability distribution, and half of years freeze later than the date shown.

Your own ZIP

Work out your own dates

This page describes a whole county. Every tool below answers for one ZIP code, which is the grain the underlying data is published at.

Other places to compare

Jefferson County sits in Wisconsin. That page carries the whole state's zone spread, its metro areas, and every other county in it with a page here.

More of Wisconsin

Counties with the same zones — 5a–5b

Matched on the exact zone set, not a dominant zone, so the winter answer on these pages really is the same one as here.

Gear

If you want to act on this

Each link opens an Amazon search rather than a product we picked. We have not tested these and do not rank them — the search is the honest version, and you choose.

Trunk wrapZone 5a is cold enough that winter sun on a young trunk does more damage than the cold does.
Row cover and frost clothThe season here closes at the first fall freeze dated above — a cover buys back some of those days at the end.
MulchFor the perennials nearest their cold limit in zone 5a, where an insulated crown is what decides whether they come back.

As an Amazon Associate we earn from qualifying purchases. That costs you nothing extra, and it buys no influence over anything on this site: the dates come from NOAA normals and the zones from the USDA, and no product affects either.

Sources

Where these numbers come from

Hardiness zones are the USDA 2023 Plant Hardiness Zone Map, published per ZIP code by PRISM at Oregon State. Freeze dates and growing-season lengths are NOAA 1991–2020 Climate Normals, taken per station and cited above by name and NOAA ID.

County boundaries come from the Census ZCTA-to-county relation, which is what places each ZIP code — and which treats parishes, boroughs and census areas as county equivalents. No station is chosen to represent this county: every range above is taken across all of them and the count is stated, because picking one would be a judgement dressed as a measurement.