What USDA hardiness zone is Williamson County, Illinois?

Every ZIP code in Williamson County, Illinois is USDA hardiness zone 7a, from the USDA 2023 Plant Hardiness Zone Map.

When is the last spring frost in Williamson County, Illinois?

The median last spring freeze runs April 4 – April 14 across the 6 NOAA weather stations serving Williamson County, Illinois, from NOAA 1991-2020 Climate Normals. Half of years freeze later than the median.

Illinois

Planting zones in Williamson County, Illinois

Every ZIP code in this county is USDA zone 7a — so unlike most of the places people look up, there is one right answer here and it holds wherever you are in the county.

USDA zone 7a The same in all 12 ZIP codes

ZIP codes 12 12 have a NOAA station within range

Weather stations 6 NOAA 1991–2020 normals, across 420 sq mi

Your ZIP code

Get the exact answer for your address

Everything above describes Williamson County, Illinois 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 Williamson County is closing

Nothing in the 32-crop table still has time to mature here. The first fall freeze normally arrives October 21 – October 28 across the 6 NOAA stations serving this county .

Your next season already has a date

The first job of it is 🧅 Green Onions, start indoors, around February 7 – February 17 — counted back from a last spring freeze of April 4 – April 14 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 6 stations agreeAt least one station

Every one of the 6 NOAA stations serving this county counts April 14 – October 21 as growing season (190 days). Widening to any single station gives April 4 – October 28 (207 days).

Your freeze dates

Last spring freeze April 4 – April 14

First fall freeze October 21 – October 28

Across the 6 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 189–206 days depending which station you are nearest, as published by NOAA rather than computed from the dates above. Summer highs average 86.8–87.7 °F in June–August. Summer rain over the same months runs 10.8–12.1" across those 5 stations, against the rough inch a week — about 13 inches over the same 92 days — that a vegetable garden is usually reckoned to want. The middle station covers about 90% of it, so expect to make up the rest. The inches are NOAA's published normals; the inch a week is a rule of thumb, and it takes no account of a 87 °F summer afternoon drying the soil faster than a cool one.

Every ZIP code in Williamson 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
62841	7a	Rend Lake Dam, IL	20.2 km
62902	7a	Carbondale Sewage Plt, IL	7.8 km
62915	7a	Carbondale Sewage Plt, IL	7.1 km
62918	7a	Carbondale Sewage Plt, IL	9.3 km
62921	7a	Carbondale Sewage Plt, IL	10.5 km
62922	7a	Dixon Springs Agric Ctr, IL	24.0 km
62933	7a	Carbondale Sewage Plt, IL	13.7 km
62948	7a	Carbondale Sewage Plt, IL	15.8 km
62949	7a	Carbondale Southern Il AP, IL	10.7 km
62951	7a	Benton 2 N, IL	23.1 km
62959	7a	Carbondale Sewage Plt, IL	22.2 km
62974	7a	Harrisburg, IL	24.3 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 Williamson County

Every date below is anchored on the last spring freeze at each station and reported as a range across all 6 of them, because no one station speaks for the county.

Crop	Job	When	Offset
🧅 Green Onions	Start indoors	February 7 – February 17	8 wk before
🌶️ Peppers	Start indoors	February 7 – February 17	8 wk before
🥦 Broccoli	Start indoors	February 21 – March 3	6 wk before
🥬 Cabbage	Start indoors	February 21 – March 3	6 wk before
🥬 Collards	Start indoors	February 21 – March 3	6 wk before
🥬 Kale	Start indoors	February 21 – March 3	6 wk before
🥬 Leaf Lettuce	Start indoors	February 21 – March 3	6 wk before
🫛 Peas	Direct sow	February 21 – March 3	6 wk before
🥬 Spinach	Direct sow	February 21 – March 3	6 wk before
🍠 Sweet Potato	Start indoors	February 21 – March 3	6 wk before
🍅 Tomatoes	Start indoors	February 21 – March 3	6 wk before
🥬 Bok Choy	Start indoors	February 28 – March 10	5 wk before
🥬 Kohlrabi	Start indoors	February 28 – March 10	5 wk before
🥬 Swiss Chard	Start indoors	February 28 – March 10	5 wk before
🥬 Arugula	Direct sow	March 7 – March 17	4 wk before
🥬 Baby Salad Greens	Direct sow	March 7 – March 17	4 wk before
🥕 Beets	Direct sow	March 7 – March 17	4 wk before
🧅 Green Onions	Direct sow	March 7 – March 17	4 wk before
🥬 Mustard Greens	Direct sow	March 7 – March 17	4 wk before
🌶️ Okra	Start indoors	March 7 – March 17	4 wk before
🌱 Radishes	Direct sow	March 7 – March 17	4 wk before
🥔 Turnips	Direct sow	March 7 – March 17	4 wk before
🥬 Cabbage	Set out	March 14 – March 24	3 wk before
🥕 Carrots	Direct sow	March 14 – March 24	3 wk before
🥬 Collards	Direct sow	March 14 – March 24	3 wk before
🥬 Collards	Set out	March 14 – March 24	3 wk before
🥒 Cucumbers	Start indoors	March 14 – March 24	3 wk before
🧅 Green Onions	Set out	March 14 – March 24	3 wk before
🥬 Kale	Direct sow	March 14 – March 24	3 wk before
🥬 Kale	Set out	March 14 – March 24	3 wk before
🥬 Kohlrabi	Direct sow	March 14 – March 24	3 wk before
🥬 Kohlrabi	Set out	March 14 – March 24	3 wk before
🥬 Leaf Lettuce	Direct sow	March 14 – March 24	3 wk before
🎃 Pumpkins	Start indoors	March 14 – March 24	3 wk before
🎃 Summer Squash	Start indoors	March 14 – March 24	3 wk before
🎃 Winter Squash	Start indoors	March 14 – March 24	3 wk before
🥒 Zucchini	Start indoors	March 14 – March 24	3 wk before
🥬 Bok Choy	Direct sow	March 21 – March 31	2 wk before
🥬 Bok Choy	Set out	March 21 – March 31	2 wk before
🥦 Broccoli	Set out	March 21 – March 31	2 wk before
🌿 Cilantro	Direct sow	March 21 – March 31	2 wk before
🥬 Leaf Lettuce	Set out	March 21 – March 31	2 wk before
🥬 Swiss Chard	Direct sow	March 21 – March 31	2 wk before
🥬 Swiss Chard	Set out	March 21 – March 31	2 wk before
🌿 Dill	Direct sow	March 28 – April 7	1 wk before
🫘 Bush Beans	Direct sow	April 11 – April 21	1 wk after
🥒 Cucumbers	Set out	April 11 – April 21	1 wk after
🎃 Pumpkins	Direct sow	April 11 – April 21	1 wk after
🎃 Pumpkins	Set out	April 11 – April 21	1 wk after
🎃 Summer Squash	Direct sow	April 11 – April 21	1 wk after
🎃 Summer Squash	Set out	April 11 – April 21	1 wk after
🌽 Sweet Corn	Direct sow	April 11 – April 21	1 wk after
🍅 Tomatoes	Set out	April 11 – April 21	1 wk after
🎃 Winter Squash	Direct sow	April 11 – April 21	1 wk after
🎃 Winter Squash	Set out	April 11 – April 21	1 wk after
🥒 Zucchini	Direct sow	April 11 – April 21	1 wk after
🥒 Zucchini	Set out	April 11 – April 21	1 wk after
🥒 Cucumbers	Direct sow	April 18 – April 28	2 wk after
🌶️ Okra	Direct sow	April 18 – April 28	2 wk after
🌶️ Okra	Set out	April 18 – April 28	2 wk after
🌶️ Peppers	Set out	April 18 – April 28	2 wk after
🫘 Southern Peas	Direct sow	April 18 – April 28	2 wk after
🍠 Sweet Potato	Set out	April 18 – April 28	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 lives through a zone 7a winter

Of 74 perennial edibles, 58 come through a zone 7a winter — and 1 fail here for being too warm rather than too cold, needing more winter chill than this county gets or more heat than they tolerate. Among the ones that work: Cranberry, Honeyberry, Apple, Aronia, Black Currant, Elderberry, Gooseberry, Summer Raspberry.

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 April 30 to be careful. Median April 4 – April 14 across 6 stations; March 19 is the optimistic end.
First fall freeze: Plan to October 6 to be careful. Median October 21 – October 28 across 6 stations; November 10 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 6 stations behind these numbers

All 6 NOAA stations whose normals this page quotes, spanning about 67 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
Benton 2 N, IL	USC00110608	April 4	October 28	206 days
Carbondale Sewage Plt, IL	USC00111265	April 9	October 22	195 days
Carbondale Southern Il AP, IL	USW00093810	April 9	October 22	195 days
Dixon Springs Agric Ctr, IL	USC00112353	April 14	October 21	189 days
Harrisburg, IL	USC00113879	April 7	October 26	202 days
Rend Lake Dam, IL	USC00117187	April 4	October 27	204 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

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

More of Illinois

Counties with the same zones — 7a

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.

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 7a, 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.