USDA zone 4a in Michigan
USDA plant hardiness zone 4a covers 8 locations across Michigan (average annual extreme minimum −30 to −25 °F): Amasa, MI, Covington, MI, Iron River, MI, Marenisco, MI, Michigamme, MI, Three Lakes, MI, and 2 more. Average last spring frost across this group ranges May 20 (Wakefield) to June 6 (Amasa), and growing seasons run 94–132 days (Amasa to Wakefield) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Michigan location's own frost dates; the printable planting calendar uses Watersmeet, this group's median-season location.
- USDA zone
- 4a−30 to −25 °F
- Last frost range
- May 20–Jun 6avg, 32°F
- First frost range
- Sep 9–Oct 1avg, 32°F
- Growing season range
- 94–132days
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) |
|---|---|---|---|---|---|
| Amasa | Republic, Crystal Falls | Amasa 1w | Jun 6 | Sep 9 | 94 |
| Covington | — | Alberta Ford For Ctr | May 23 | Oct 1 | 128 |
| Iron River | Gaastra, Caspian | Stambaugh 2sse | Jun 4 | Sep 10 | 95 |
| Marenisco | — | Bergland Dam | May 26 | Sep 27 | 123 |
| Michigamme | — | Clarksburg | Jun 5 | Sep 10 | 96 |
| Three Lakes | — | Herman | May 31 | Sep 22 | 115 |
| Wakefield | Ramsay | Ironwood | May 20 | Oct 1 | 132 |
| Watersmeet * | — | Eagle Rvr | May 25 | Sep 24 | 121 |
* Watersmeet 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 (Watersmeet, representative)
Computed from Watersmeet's average frost dates — the median-season location in this Michiganzone 4a 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 30 – Apr 13 | Jun 1 – Jun 8 | Jul 31 – Aug 20 | — | matures comfortably |
| Pepper | Very tender | Mar 16 – Mar 30 | Jun 8 – Jun 15 | Aug 7 – Sep 6 | — | matures comfortably |
| Cucumber | Tender | Apr 27 – May 4 | Jun 1 – Jun 8 | Jul 21 – Aug 10 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | Jun 1 – Jun 8 | Jul 16 – Jul 31 | — | matures comfortably |
| Bush bean | Tender | — | Jun 1 – Jun 8 | Jul 21 – Jul 31 | Jul 26 – Aug 5 | matures comfortably |
| Sweet corn | Tender | — | May 25 – Jun 8 | Jul 24 – Aug 23 | — | matures comfortably |
| Basil | Very tender | Apr 13 – Apr 27 | Jun 1 – Jun 8 | Jul 1 – Jul 16 | — | matures comfortably |
| Lettuce | Half-hardy | Apr 13 – Apr 27 | Apr 27 – May 11 | Jun 11 – Jun 26 | Jul 12 – Jul 27 | matures comfortably |
| Pea | Hardy | — | Apr 13 – Apr 27 | Jun 7 – Jun 22 | Jul 2 – Jul 17 | matures comfortably |
| Spinach | Hardy | — | Apr 13 – Apr 27 | May 23 – Jun 2 | Jul 22 – Aug 1 | matures comfortably |
| Carrot | Half-hardy | — | May 4 – May 11 | Jul 3 – Jul 23 | Jun 22 – Jul 12 | matures comfortably |
| Broccoli | Half-hardy | Mar 30 – Apr 13 | Apr 27 – May 11 | Jun 21 – Jul 11 | Jun 27 – Jul 17 | matures comfortably |
Which crops fit this growing season
Using Watersmeet's own 121-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 (Watersmeet)
From NOAA's 1991–2020 Climate Normals at station USC00472314. 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 | Jun 4 | Jun 20 | Sep 12 | Sep 27 | 97 |
| 32°F (freeze) | May 25 | Jun 6 | Sep 24 | Oct 7 | 121 |
| 28°F | May 13 | May 26 | Oct 5 | Oct 20 | 144 |
| 24°F | May 2 | May 15 | Oct 17 | Nov 3 | 166 |
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 1,596; Watersmeet's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 1,723 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,370 | cool-season crops (brassicas, greens) |
Hardiness zone 4a in Michigan
Zone 4a means an average annual extreme minimum winter temperature of about −30 to −25 °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. Michigan spans 7 hardiness zones in this dataset — see all Michigan locations for the full range, including zones4b, 5a, 5b, 6a, 6b, 7a.
Explore zone 4a in other states at zone 4a, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 4a mean in Michigan?
- Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 8 locations in Michigan fall in this zone. It does not set planting dates; see the frost calendar below for that.
- Where in Michigan's zone 4a is the growing season longest?
- Wakefield runs the longest season on this page at about 132 days; Amasa is shortest at about 94 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 Michigan's zone 4a?
- Using Watersmeet's frost dates (this page's median-season location): start seeds indoors about Mar 30 – Apr 13, then transplant outside about Jun 1 – Jun 8. 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 Michigan's zone 4a 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.