USDA zone 4b in Iowa
USDA plant hardiness zone 4b covers 8 locations across Iowa (average annual extreme minimum −25 to −20 °F): Arlington, IA, Cresco, IA, Delaware, IA, Elma, IA, Riceville, IA, Sibley, IA, and 2 more. Average last spring frost across this group ranges April 30 (Riceville) to May 8 (Strawberry Point), and growing seasons run 146–163 days (Strawberry Point to Riceville) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Iowa location's own frost dates; the printable planting calendar uses Delaware, this group's median-season location.
- USDA zone
- 4b−25 to −20 °F
- Last frost range
- Apr 30–May 8avg, 32°F
- First frost range
- Oct 1–Oct 9avg, 32°F
- Growing season range
- 146–163days
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) |
|---|---|---|---|---|---|
| Arlington | — | Fayette | May 4 | Oct 4 | 151 |
| Cresco | Chester, Lime Springs, Ridgeway, Protivin | Cresco 1 Ne | May 8 | Oct 1 | 147 |
| Delaware * | Greeley | Manchester #2 | May 3 | Oct 4 | 154 |
| Elma | — | New Hampton | May 2 | Oct 5 | 155 |
| Riceville | — | Osage | Apr 30 | Oct 9 | 163 |
| Sibley | Bigelow | Sibley | May 5 | Oct 3 | 152 |
| Spillville | — | Decorah | May 3 | Oct 7 | 157 |
| Strawberry Point | Elkader, Edgewood, Volga, Elkport | Elkader 6 Ssw | May 8 | Oct 1 | 146 |
* Delaware 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 (Delaware, representative)
Computed from Delaware's average frost dates — the median-season location in this Iowazone 4b 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 8 – Mar 22 | May 10 – May 17 | Jul 9 – Jul 29 | — | matures comfortably |
| Pepper | Very tender | Feb 22 – Mar 8 | May 17 – May 24 | Jul 16 – Aug 15 | — | matures comfortably |
| Cucumber | Tender | Apr 5 – Apr 12 | May 10 – May 17 | Jun 29 – Jul 19 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | May 10 – May 17 | Jun 24 – Jul 9 | — | matures comfortably |
| Bush bean | Tender | — | May 10 – May 17 | Jun 29 – Jul 9 | Aug 5 – Aug 15 | matures comfortably |
| Sweet corn | Tender | — | May 3 – May 17 | Jul 2 – Aug 1 | — | matures comfortably |
| Basil | Very tender | Mar 22 – Apr 5 | May 10 – May 17 | Jun 9 – Jun 24 | — | matures comfortably |
| Lettuce | Half-hardy | Mar 22 – Apr 5 | Apr 5 – Apr 19 | May 20 – Jun 4 | Jul 22 – Aug 6 | matures comfortably |
| Pea | Hardy | — | Mar 22 – Apr 5 | May 16 – May 31 | Jul 12 – Jul 27 | matures comfortably |
| Spinach | Hardy | — | Mar 22 – Apr 5 | May 1 – May 11 | Aug 1 – Aug 11 | matures comfortably |
| Carrot | Half-hardy | — | Apr 12 – Apr 19 | Jun 11 – Jul 1 | Jul 2 – Jul 22 | matures comfortably |
| Broccoli | Half-hardy | Mar 8 – Mar 22 | Apr 5 – Apr 19 | May 30 – Jun 19 | Jul 7 – Jul 27 | matures comfortably |
Which crops fit this growing season
Using Delaware's own 154-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 (Delaware)
From NOAA's 1991–2020 Climate Normals at station USC00135086. 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 12 | May 25 | Sep 26 | Oct 7 | 135 |
| 32°F (freeze) | May 3 | May 16 | Oct 4 | Oct 17 | 154 |
| 28°F | Apr 23 | May 7 | Oct 15 | Oct 28 | 174 |
| 24°F | Apr 11 | Apr 29 | Oct 26 | Nov 6 | 197 |
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 2,581; Delaware's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 2,660 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 4,639 | cool-season crops (brassicas, greens) |
Hardiness zone 4b in Iowa
Zone 4b means an average annual extreme minimum winter temperature of about −25 to −20 °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. Iowa spans 4 hardiness zones in this dataset — see all Iowa locations for the full range, including zones5a, 5b, 6a.
Explore zone 4b in other states at zone 4b, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 4b mean in Iowa?
- Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 8 locations in Iowa fall in this zone. It does not set planting dates; see the frost calendar below for that.
- Where in Iowa's zone 4b is the growing season longest?
- Riceville runs the longest season on this page at about 163 days; Strawberry Point is shortest at about 146 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 Iowa's zone 4b?
- Using Delaware's frost dates (this page's median-season location): start seeds indoors about Mar 8 – Mar 22, then transplant outside about May 10 – May 17. 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 Iowa's zone 4b 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.