Growing Season Length by Region

Growing season length is the gap between the last spring freeze and the first fall freeze, and the threshold you pick changes the answer. Understanding how it is calculated keeps you from borrowing a number that does not fit your farm.

Key points

Growing season length depends on the freeze temperature threshold and the years of data used.
Frost-free days do not equal heat: check growing degree days for long-season crops.
Use station data close to your elevation and terrain, and compare risk levels instead of one average.
Work backward from a fall risk date using days to maturity plus a harvest buffer.

How is growing season length defined?

The simplest definition is the span between the last spring freeze and the first fall freeze at a chosen temperature. The Midwestern Regional Climate Center freeze date tool, for example, reports growing season length this way and lets users view it at several thresholds, including 28, 30 and 32 degrees F and colder or warmer values. A 32 degree season is shorter than a 28 degree season in most places, because it ends at the first light frost.

So when you read that a region has a certain number of frost-free days, ask which temperature it used and which years it covered.

Why does season length vary so much by region?

Latitude, elevation, nearness to large water or an ocean, and local terrain all play a part. Coastal areas often have mild winters but cool summers that keep soil cold well into the season. Inland valleys can swing hot and cold. Mountain sites have short seasons even at modest latitudes.

Season length is only part of the story. Two regions with the same number of frost-free days can still differ in heat accumulation, which determines whether a long-season crop can finish. That is where growing degree days come in.

Elevation changes the picture in a simple way: higher ground usually has a shorter frost-free period, and the same county can contain several different seasons. When you read a regional figure, ask which part of the region it describes before you apply it to your own land.

Is a long season the same as a warm season?

No. A cool coastal climate may have a long frost-free period but too little heat for crops that need warm nights. Oregon State Extension notes that cold rain, wind and cool nights can keep coastal soils cold well into summer, and recommends varieties with shorter days to maturity for heat-loving crops there.

Match season length to the crop's days to maturity, then check heat accumulation. Seed catalogs and extension variety guides usually list days to maturity, but check how each source counts them, from direct seeding or from transplanting.

How do you find reliable season length for your location?

Use NOAA climate normals, which are 30-year averages that NCEI currently publishes for 1991 to 2020 and updates every ten years. NCEI lists growing season length among its available statistics. Regional climate centers and state climatologists often offer county or station summaries, and the freeze date tool covers counties in the north-central and northeastern states.

Pick the station closest to your farm in elevation and terrain, not just in distance. A valley airport station and a hillside field a few miles away can have noticeably different freeze dates.

Note the threshold temperature used.
Note the years of data.
Compare 10, 50 and 90 percent dates, not just the average.

How should you use season length when choosing crops?

Count backward from your chosen fall freeze date using days to maturity, then add a buffer for slower growth in cool weather. For example, if a crop needs a given number of days from transplant and your fall risk date allows fewer than that, either pick an earlier variety, start transplants earlier, or choose a different crop.

Add time for harvest, too. A crop that reaches maturity the week of your first freeze may still need days to pick, which is a real risk for large acreages.

What are common mistakes when comparing regions?

The big ones are comparing seasons measured at different thresholds, treating a 30-year average as a promise, and forgetting that recent years may differ from the full record. The freeze date tool shows decade-by-decade comparisons so you can see whether recent seasons differ from the long record, though one decade alone does not establish a trend.

Use season length as a planning guide, then confirm with soil temperature and the forecast each year.

How can you track season length on your own farm?

Write down the date of your last spring freeze and first fall freeze each year, along with the temperature you used to define it and where your thermometer sat. A few years of notes will show how your fields compare with the nearest official station, and they let you test whether a published figure fits your ground.

Keep the notes next to your planting log. When a crop struggles in a short season, you will be able to see whether the cause was a late spring, an early fall freeze or something unrelated to frost altogether.

When to get local help: Your state climatologist or county extension office can tell you which station best represents your farm and which threshold local growers use.

Sources and further reading

Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.

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