Fall in zone 5aPhoto: Dietmar Rabich, CC BY-SA 4.0, via Wikimedia Commons
Frost dates for ZIP 51446 (Irwin, IA)
Average last spring and first fall frost dates with 10%, 50% and 90% odds, from NOAA's 1991–2020 climate normals for the nearest weather station, Harlan, IA (12 miles away).
- Average last spring frostApr 2750% chance of a 32°F freeze after this date
- Average first fall frostOct 950% chance of a 32°F freeze by this date
- Growing season (frost-free)165 daysmedian, 32°F threshold
- USDA hardiness zone5a-20 to -15°F avg. coldest night
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This week in your yard
As of Oct 18, 2026, from your frost dates, zone and soil temperature. Lawn timing · Vegetables · Pruning · Plants
Soil temperature now
2-inch depth
53°F 50–55°F
24-hour average · latest 53°F, as of Oct 18, 2:00 AM CDT
Estimate Not a measurement at this ZIP: distance-weighted average of Shagbark Hills (55 mi).
4-inch depth
53°F 50–55°F
24-hour average · latest 53°F, as of Oct 18, 2:00 AM CDT
Estimate Not a measurement at this ZIP: distance-weighted average of Shagbark Hills (55 mi).
Fall: Texas A&M AgriLife times fall pre-emergent (for winter weeds such as annual bluegrass) to soil cooling to about 70°F for several days. The 50–55°F band is the spring trigger. Always read and follow the product label.
7-day forecast and frost risk
The latest National Weather Service 7-day forecast and any Frost Advisory or Freeze Warning for 51446 load here. You can also open the NWS forecast for this ZIP.
Pre-emergent
Soil is 53°F at 4 inches (estimated soil temperature, not a station reading). Fall pre-emergent window is mostly past. Next: spring, when soil reaches 50°F (typically mid-April to early May).
Fertilizer window
Lawn region: North (cool-season). Timing only; always follow the product label.
- Kentucky bluegrass (cool-season): Early fall: Around Labor Day (late August–September); Late fall: Late October–November, before the ground freezes; Late spring: Optional around Memorial Day. Avoid: Early-spring and midsummer feeding (pushes top growth, favors crabgrass).
Plant now
No vegetable planting windows are open this week.
Prune now
No frost-timed pruning windows are open this week.
Spring-flowering shrubs are pruned right after their flowers fade, not by date. See the pruning guide.
Last spring frost odds
| Chance of a 32°F freeze after this date | Date |
|---|---|
| 90% (early: most years still freeze after this) | Apr 12 |
| 50% (the "average" last frost) | Apr 27 |
| 10% (cautious: only 1 year in 10 freezes later) | May 9 |
Most gardeners plant frost-tender crops (tomatoes, peppers, basil) after the 50% date, and closer to the 10% date for less risk.
First fall frost odds
| Chance the first 32°F freeze has happened by this date | Date |
|---|---|
| 10% (early: 1 year in 10 freezes this soon) | Sep 24 |
| 50% (the "average" first frost) | Oct 9 |
| 90% (late: most years have frozen by now) | Oct 22 |
Light frost, freeze and hard freeze
| Threshold | Last spring (50%) | First fall (50%) |
|---|---|---|
| 36°F (light frost possible on clear, calm nights) | May 5 | Sep 30 |
| 32°F (freeze) | Apr 27 | Oct 9 |
| 28°F (hard freeze) | Apr 17 | Oct 20 |
n/a means NOAA did not publish a date at that threshold for this station (usually because it rarely gets that cold).
Vegetable planting calendar for 51446
Dates count from your average last spring frost (Apr 27) and first fall frost (Oct 9). Half of years have a frost after the spring date, so give tender crops (tomatoes, peppers, squash) some slack.
| Crop | Start indoors | Spring planting | Fall planting |
|---|---|---|---|
| Tomato | Mar 2–16 | May 4–18 (transplant) | – |
| Pepper (sweet and hot) | Feb 16–Mar 2 | May 11–25 (transplant) | – |
| Eggplant | Feb 16–Mar 2 | May 11–25 (transplant) | – |
| Tomatillo | Mar 2–16 | May 4–18 (transplant) | – |
| Lettuce | Mar 2–16 | Apr 6–27 (transplant) Mar 30–May 11 (sow outdoors) | Aug 14–Sep 4 (transplant) Jul 31–Aug 28 (sowing) |
| Spinach | – | Mar 16–Apr 6 (sow outdoors) | Aug 14–Sep 4 (sowing) |
| Kale | Mar 2–16 | Mar 30–Apr 13 (transplant) Mar 30–Apr 13 (sow outdoors) | Jul 17–Aug 14 (transplant) Jul 3–31 (sowing) |
| Collards | Mar 2–16 | Mar 30–Apr 27 (transplant) Mar 30–Apr 27 (sow outdoors) | Jul 17–Aug 14 (transplant) Jul 3–31 (sowing) |
| Broccoli | Feb 16–Mar 2 | Mar 30–Apr 13 (transplant) | Jul 3–31 (transplant) |
| Cabbage | Feb 16–Mar 2 | Mar 30–Apr 13 (transplant) | Jul 3–31 (transplant) |
| Cauliflower | Feb 16–Mar 2 | Apr 13–27 (transplant) | Jul 3–31 (transplant) |
| Brussels sprouts | Feb 2–16 | Mar 30–Apr 13 (transplant) | Jun 19–Jul 10 (transplant) |
| Kohlrabi | Mar 2–16 | Mar 30–Apr 13 (transplant) Mar 30–Apr 13 (sow outdoors) | Jul 31–Aug 14 (sowing) |
| Bok choy / Asian greens | – | Mar 30–Apr 13 (sow outdoors) | Aug 14–28 (sowing) |
| Arugula | – | Mar 30–Apr 27 (sow outdoors) | Aug 14–Sep 11 (sowing) |
| Mustard greens | – | Mar 30–Apr 20 (sow outdoors) | Aug 14–28 (sowing) |
| Swiss chard | Mar 16–30 | Apr 13–May 4 (transplant) Apr 13–May 11 (sow outdoors) | Jul 31–Aug 14 (sowing) |
| Beet | – | Mar 30–Apr 27 (sow outdoors) | Jul 31–Aug 14 (sowing) |
| Carrot | – | Apr 6–May 4 (sow outdoors) | Jul 17–31 (sowing) |
| Radish | – | Mar 23–May 4 (sow outdoors) | Aug 28–Sep 11 (sowing) |
| Turnip | – | Mar 30–Apr 20 (sow outdoors) | Jul 31–Aug 14 (sowing) |
| Parsnip | – | Apr 6–27 (sow outdoors) | – |
| Pea (garden, snap, snow) | – | Mar 2–30 (sow outdoors) | Jul 17–31 (sowing) |
| Onion (bulbing) | Feb 2–16 | Mar 16–30 (transplant) | – |
| Green onion / scallion | – | Mar 16–Apr 13 (sow outdoors) | Jul 31–Aug 14 (sowing) |
| Leek | Feb 2–16 | Mar 30–Apr 13 (transplant) | – |
| Garlic | – | – | Aug 28–Sep 25 (sowing) |
| Shallot | – | Mar 16–30 (transplant) | Aug 28–Sep 25 (sowing) |
| Potato | – | Mar 30–Apr 13 (transplant) | Jun 19–Jul 3 (transplant) |
| Sweet potato | – | May 18–Jun 1 (transplant) | – |
| Sweet corn | – | May 4–18 (sow outdoors) | – |
| Bean, bush (snap) | – | May 4–18 (sow outdoors) | – |
| Bean, pole | – | May 4–18 (sow outdoors) | – |
| Lima bean | – | May 11–25 (sow outdoors) | – |
| Southern pea (cowpea, black-eyed pea) | – | May 18–Jun 8 (sow outdoors) | – |
| Cucumber | Mar 30–Apr 6 | May 4–18 (transplant) May 4–18 (sow outdoors) | – |
| Summer squash / zucchini | Mar 30–Apr 6 | May 4–18 (transplant) May 4–18 (sow outdoors) | – |
| Winter squash | Mar 30–Apr 6 | May 4–18 (transplant) May 4–18 (sow outdoors) | – |
| Pumpkin | – | May 4–18 (sow outdoors) | – |
| Watermelon | Mar 30–Apr 6 | May 11–25 (transplant) May 11–25 (sow outdoors) | – |
| Cantaloupe / muskmelon | Mar 30–Apr 6 | May 11–25 (transplant) May 11–25 (sow outdoors) | – |
| Okra | Mar 16–30 | May 11–25 (transplant) May 11–25 (sow outdoors) | – |
| Celery | Feb 2–16 | May 4–11 (transplant) | – |
| Basil | Mar 16–30 | May 4–18 (transplant) May 4–18 (sow outdoors) | – |
| Cilantro | – | Apr 13–May 11 (sow outdoors) | Aug 14–28 (sowing) |
| Parsley | Feb 16–Mar 2 | Apr 13–27 (transplant) Apr 6–27 (sow outdoors) | – |
| Dill | – | Apr 27–May 11 (sow outdoors) | Aug 14–28 (sowing) |
| Sunflower | – | Apr 27–May 11 (sow outdoors) | – |
| Asparagus (crowns) | – | Mar 30–Apr 13 (transplant) | – |
| Rhubarb (crowns) | – | Mar 30–Apr 13 (transplant) | – |
| Strawberry (plants) | – | Mar 30–Apr 13 (transplant) | Jul 31–Aug 28 (transplant) |
Typical ranges from university extension planting calendars, adjusted to your frost dates. Probabilities, not guarantees. Fall windows that would overlap spring planting are left out, as are fall crops of frost-tender plants where the frost-free season is under 210 days.
Where these numbers come from
Station: Harlan, IA (NOAA GHCN-Daily ID USC00133632), elevation 1,270 ft, 11.6 miles from the ZIP's center point. NOAA completeness flag: R (representative, 10+ years).
Nearby stations
| Station | Distance | Last frost (50%) | First frost (50%) |
|---|---|---|---|
| Audubon, IA | 15 mi | May 2 | Oct 3 |
| Denison, IA | 19 mi | Apr 27 | Oct 11 |
| Carroll, IA | 27 mi | Apr 27 | Oct 10 |
The average last-frost dates of these stations span 6 days, a rough guide to local variation. See data sources & methods.
Frost questions for 51446
When is the last frost in 51446?
The average (50% chance) last spring freeze of 32°F for ZIP 51446 is April 27. For a more cautious planting date, use May 9, when only 1 year in 10 still sees a later freeze.
When is the first frost in 51446?
The average first fall freeze is October 9. In 1 year in 10 it comes by September 24.
What hardiness zone is 51446?
ZIP 51446 is in USDA zone 5a (-20 to -15°F average coldest winter night), per the 2023 USDA Plant Hardiness Zone Map ZIP listing.
What is the soil temperature in 51446 right now?
This page shows the latest 2-inch and 4-inch soil temperature from the nearest public soil station (USDA SCAN, NOAA USCRN or TexMesonet), or a labeled estimate when no station is close, against the 50–55°F spring pre-emergent band.
Nearby ZIP codes
51447 Kirkman 51454 Manilla 51527 Defiance 51432 Aspinwall 51543 Kimballton 51537 Harlan 51578 Westphalia 51455 Manning
Other ZIP codes in Shelby County
51447 51527 51530 51531 51537 51562 51565 51570 51578 All Shelby County frost dates · Iowa frost dates
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Weekly yard timing, plus frost and soil-temperature alerts for your ZIP code.
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Free. One weekly email plus alerts when your ZIP has a frost or a soil-temperature change worth acting on. After you submit, Kit (our email provider) sends a confirmation email: check your email and click the link to confirm. You aren't subscribed until you do. We collect only your email address and ZIP, sent to Kit to deliver The Dial. Unsubscribe anytime.