USDA zone 5a in Illinois
USDA plant hardiness zone 5a covers 8 locations across Illinois (average annual extreme minimum −20 to −15 °F): Chadwick, IL, East Dubuque, IL, Galena, IL, Lanark, IL, Lena, IL, Nachusa, IL, and 2 more. Average last spring frost across this group ranges April 21 (Nachusa) to May 2 (Savanna), and growing seasons run 154–180 days (Savanna to Nachusa) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Illinois location's own frost dates; the printable planting calendar uses Lena, this group's median-season location.
- USDA zone
- 5a−20 to −15 °F
- Last frost range
- Apr 21–May 2avg, 32°F
- First frost range
- Oct 7–Oct 19avg, 32°F
- Growing season range
- 154–180days
Locations in this group
Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.
| Location | Also covers | Nearest station | Last frost | First frost | Season (days) |
|---|---|---|---|---|---|
| Chadwick | — | Morrison | Apr 26 | Oct 13 | 167 |
| East Dubuque | Peosta, La Motte, St. Donatus, Zwingle | Dubuque Rgnl Ap | Apr 28 | Oct 11 | 165 |
| Galena | Hanover, Elizabeth, Scales Mound | Elizabeth | May 1 | Oct 9 | 160 |
| Lanark | Milledgeville | Polo 5nw | Apr 29 | Oct 7 | 161 |
| Lena * | Stockton, Warren, South Wayne, Nora, Apple River, Gratiot, +1 more | Stockton 3 Nne | Apr 27 | Oct 9 | 165 |
| Nachusa | — | Dixon 1w | Apr 21 | Oct 19 | 180 |
| Savanna | Mount Carroll | Mt Carroll | May 2 | Oct 7 | 154 |
| Willow Lake | Lena, Orangeville, Pearl, Shannon, Cedarville, Dakota, +2 more | Freeport Waste Wtp | Apr 26 | Oct 13 | 169 |
* Lena is this group's median-season location — the calendar below is computed from its data.
Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.
Planting calendar (Lena, representative)
Computed from Lena's average frost dates — the median-season location in this Illinoiszone 5a group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).
- Start indoors
- Plant out
- Fall sowing
- First harvest
| Crop | Frost tolerance | Start indoors | Plant out | First harvest | Fall planting | Season fit |
|---|---|---|---|---|---|---|
| Tomato | Tender | Mar 2 – Mar 16 | May 4 – May 11 | Jul 3 – Jul 23 | — | matures comfortably |
| Pepper | Very tender | Feb 16 – Mar 2 | May 11 – May 18 | Jul 10 – Aug 9 | — | matures comfortably |
| Cucumber | Tender | Mar 30 – Apr 6 | May 4 – May 11 | Jun 23 – Jul 13 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | May 4 – May 11 | Jun 18 – Jul 3 | — | matures comfortably |
| Bush bean | Tender | — | May 4 – May 11 | Jun 23 – Jul 3 | Aug 10 – Aug 20 | matures comfortably |
| Sweet corn | Tender | — | Apr 27 – May 11 | Jun 26 – Jul 26 | — | matures comfortably |
| Basil | Very tender | Mar 16 – Mar 30 | May 4 – May 11 | Jun 3 – Jun 18 | — | matures comfortably |
| Lettuce | Half-hardy | Mar 16 – Mar 30 | Mar 30 – Apr 13 | May 14 – May 29 | Jul 27 – Aug 11 | matures comfortably |
| Pea | Hardy | — | Mar 16 – Mar 30 | May 10 – May 25 | Jul 17 – Aug 1 | matures comfortably |
| Spinach | Hardy | — | Mar 16 – Mar 30 | Apr 25 – May 5 | Aug 6 – Aug 16 | matures comfortably |
| Carrot | Half-hardy | — | Apr 6 – Apr 13 | Jun 5 – Jun 25 | Jul 7 – Jul 27 | matures comfortably |
| Broccoli | Half-hardy | Mar 2 – Mar 16 | Mar 30 – Apr 13 | May 24 – Jun 13 | Jul 12 – Aug 1 | matures comfortably |
Which crops fit this growing season
Using Lena's own 165-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.
Frost & freeze dates (Lena)
From NOAA's 1991–2020 Climate Normals at station USC00118293. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.
| Threshold | Last spring — avg | Last spring — 90%-safe | First fall — avg | First fall — 90%-safe | Season (days) |
|---|---|---|---|---|---|
| 36°F | May 6 | May 21 | Sep 29 | Oct 12 | 144 |
| 32°F (freeze) | Apr 27 | May 12 | Oct 9 | Oct 23 | 165 |
| 28°F | Apr 17 | May 2 | Oct 21 | Nov 4 | 185 |
| 24°F | Apr 6 | Apr 22 | Nov 1 | Nov 17 | 208 |
Growing degree days
Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 8 locations is 2,844; Lena's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 2,807 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 4,870 | cool-season crops (brassicas, greens) |
Hardiness zone 5a in Illinois
Zone 5a means an average annual extreme minimum winter temperature of about −20 to −15 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. Illinois spans 5 hardiness zones in this dataset — see all Illinois locations for the full range, including zones5b, 6a, 6b, 7a.
Explore zone 5a in other states at zone 5a, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 5a mean in Illinois?
- Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 8 locations in Illinois fall in this zone. It does not set planting dates; see the frost calendar below for that.
- Where in Illinois's zone 5a is the growing season longest?
- Nachusa runs the longest season on this page at about 180 days; Savanna is shortest at about 154 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
- When should I plant tomatoes in Illinois's zone 5a?
- Using Lena's frost dates (this page's median-season location): start seeds indoors about Mar 2 – Mar 16, then transplant outside about May 4 – May 11. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
- Does every location in Illinois's zone 5a have the same frost dates?
- No — they do not all share a weather station, so frost dates vary by a week or more across the 8 locations listed (see the table above for each location's own dates).
Sources & method
Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.