USDA zone 6a in Iowa
USDA plant hardiness zone 6a covers 8 locations across Iowa (average annual extreme minimum −10 to −5 °F): Donnellson, IA, Fort Madison, IA, Salem, IA, Silver, IA, Sperry, IA, Stockport, IA, and 2 more. Average last spring frost across this group ranges April 8 (Sperry) to April 30 (Salem), and growing seasons run 163–202 days (Salem to Sperry) — 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 Donnellson, this group's median-season location.
- USDA zone
- 6a−10 to −5 °F
- Last frost range
- Apr 8–Apr 30avg, 32°F
- First frost range
- Oct 10–Oct 29avg, 32°F
- Growing season range
- 163–202days
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) |
|---|---|---|---|---|---|
| Donnellson * | — | Donnellson | Apr 17 | Oct 22 | 187 |
| Fort Madison | Nauvoo, Denmark, Wever, Montrose, West Point, Niota | Ft Madison | Apr 12 | Oct 25 | 194 |
| Salem | Hillsboro, St. Paul, Houghton | Mt Pleasant 1 Ssw | Apr 30 | Oct 10 | 163 |
| Silver | — | Oakland | Apr 23 | Oct 14 | 174 |
| Sperry | Gladstone | Gladstone Dam 18 | Apr 8 | Oct 29 | 202 |
| Stockport | Cantril, Farmington, Bonaparte, Mount Sterling | Keosauqua | Apr 26 | Oct 13 | 169 |
| Tabor | — | Sidney 1sse | Apr 19 | Oct 19 | 182 |
| West Burlington | Beaverdale, Danville, Middletown | Burlington Muni Ap | Apr 15 | Oct 20 | 187 |
* Donnellson 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 (Donnellson, representative)
Computed from Donnellson's average frost dates — the median-season location in this Iowazone 6a 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 | Feb 20 – Mar 6 | Apr 24 – May 1 | Jun 23 – Jul 13 | — | matures comfortably |
| Pepper | Very tender | Feb 6 – Feb 20 | May 1 – May 8 | Jun 30 – Jul 30 | — | matures comfortably |
| Cucumber | Tender | Mar 20 – Mar 27 | Apr 24 – May 1 | Jun 13 – Jul 3 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | Apr 24 – May 1 | Jun 8 – Jun 23 | — | matures comfortably |
| Bush bean | Tender | — | Apr 24 – May 1 | Jun 13 – Jun 23 | Aug 23 – Sep 2 | matures comfortably |
| Sweet corn | Tender | — | Apr 17 – May 1 | Jun 16 – Jul 16 | — | matures comfortably |
| Basil | Very tender | Mar 6 – Mar 20 | Apr 24 – May 1 | May 24 – Jun 8 | — | matures comfortably |
| Lettuce | Half-hardy | Mar 6 – Mar 20 | Mar 20 – Apr 3 | May 4 – May 19 | Aug 9 – Aug 24 | matures comfortably |
| Pea | Hardy | — | Mar 6 – Mar 20 | Apr 30 – May 15 | Jul 30 – Aug 14 | matures comfortably |
| Spinach | Hardy | — | Mar 6 – Mar 20 | Apr 15 – Apr 25 | Aug 19 – Aug 29 | matures comfortably |
| Carrot | Half-hardy | — | Mar 27 – Apr 3 | May 26 – Jun 15 | Jul 20 – Aug 9 | matures comfortably |
| Broccoli | Half-hardy | Feb 20 – Mar 6 | Mar 20 – Apr 3 | May 14 – Jun 3 | Jul 25 – Aug 14 | matures comfortably |
Which crops fit this growing season
Using Donnellson's own 187-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 (Donnellson)
From NOAA's 1991–2020 Climate Normals at station USC00132299. 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 | Apr 28 | May 11 | Oct 12 | Oct 25 | 166 |
| 32°F (freeze) | Apr 17 | May 1 | Oct 22 | Nov 4 | 187 |
| 28°F | Apr 7 | Apr 21 | Nov 1 | Nov 15 | 208 |
| 24°F | Mar 29 | Apr 11 | Nov 10 | Nov 28 | 227 |
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 3,463; Donnellson's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 3,540 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 5,797 | cool-season crops (brassicas, greens) |
Hardiness zone 6a in Iowa
Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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 zones4b, 5a, 5b.
Explore zone 6a in other states at zone 6a, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 6a mean in Iowa?
- Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °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 6a is the growing season longest?
- Sperry runs the longest season on this page at about 202 days; Salem is shortest at about 163 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 6a?
- Using Donnellson's frost dates (this page's median-season location): start seeds indoors about Feb 20 – Mar 6, then transplant outside about Apr 24 – May 1. 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 6a 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.