A growing season is the period of the year when weather conditions are warm enough for plants to actively grow and produce crops. In most of the United States, it’s defined as the number of frost-free days between the last freeze of spring and the first freeze of fall. Depending on where you live, that window can be as short as 90 days or stretch beyond 300.
How the Growing Season Is Measured
The most common way to define a growing season is by frost dates. Your local growing season starts on the date of the last spring freeze (when temperatures drop to 32°F or below) and ends on the date of the first fall freeze. Everything in between is your frost-free window. A location in northern Minnesota might have a growing season of only 100 days, while southern Florida’s stretches nearly year-round.
These frost dates aren’t fixed. They’re calculated as probabilities based on decades of weather station data. When a seed packet or gardening guide says “plant after your last frost date,” it’s referring to the date by which there’s roughly a 50% chance that freezing temperatures are done for the season. More conservative gardeners wait until the date with a 90% probability of no more frost, which falls a week or two later.
Temperature thresholds matter too. Some resources use 32°F as the benchmark, while others use 28°F (a “hard freeze”) because many plants can tolerate a light frost but not a deep one. The threshold you care about depends on what you’re growing. Tomatoes suffer at 32°F. Kale and spinach shrug it off and keep producing until temperatures drop well below that.
What Triggers Plants to Grow
Temperature is the headline factor, but daylight hours play an equally important role. Plants have internal systems that measure how long each day’s light period lasts, a process called photoperiodism. These light-measuring systems influence when a plant flowers, when it shifts energy into storing starch in its roots, and how fast it grows. A warm spell in February won’t trick most perennials into leafing out if the days are still short, because the plant is reading the light signal alongside the temperature signal.
This is why the growing season isn’t just about avoiding frost. Even in a heated greenhouse, winter’s short days slow down growth for many species. Conversely, the long summer days at high latitudes can partially compensate for cooler temperatures, allowing crops to mature surprisingly fast in places like Alaska, where some summer days deliver nearly 20 hours of sunlight.
Growing Degree Days: A More Precise Tool
Farmers and agronomists track the growing season with a measurement called growing degree days (GDD). Instead of simply counting frost-free days, GDD measures how much warmth accumulates over time. The basic calculation takes the average of each day’s high and low temperatures, then subtracts a base temperature below which a specific crop doesn’t grow. For corn, that base is 50°F. For wheat, it’s 32°F.
If today’s high is 80°F and the low is 60°F, the average is 70°F. Subtract corn’s base of 50°F, and that day contributed 20 growing degree days. These accumulate over the season, and each crop needs a specific total to reach maturity. Corn typically needs around 2,500 GDD to produce a harvestable ear. This system is far more useful than counting calendar days because a cool, cloudy week contributes almost nothing to a crop’s development even though the days still tick by.
How Location Shapes Your Season
Latitude is the biggest factor. The closer you are to the equator, the longer and warmer your growing season. But elevation matters nearly as much. Temperature drops roughly 3.5°F for every 1,000 feet of altitude gain, so a mountain town at 7,000 feet can have a dramatically shorter season than a valley 30 miles away. Urban areas also tend to have slightly longer growing seasons than surrounding rural land, because pavement, buildings, and human activity generate extra heat.
Proximity to large bodies of water makes a difference too. The Great Lakes, for example, moderate temperatures along their shores. Water warms up slowly in spring (delaying the season slightly) but releases stored heat in fall, pushing the first frost later and extending the growing window by a few weeks compared to inland areas at the same latitude.
In Tropical Climates, Water Replaces Frost
The frost-based definition of a growing season only works in places that actually experience frost. In tropical and subtropical regions, the growing season is defined by rainfall rather than temperature. The key question shifts from “is it warm enough?” to “is it wet enough?”
Researchers identify the start of the wet season as the point when monthly rainfall exceeds the amount of water lost through evaporation and plant transpiration for two consecutive months. When rainfall drops back below that threshold, the growing season ends. In practice, this means a tropical region might have two distinct growing seasons per year, separated by dry spells, or one long wet season and one long dry season. Farmers in these climates plan their planting around monsoon timing rather than frost dates.
The Season Is Getting Longer
Across the contiguous United States, the average growing season has lengthened by nearly two weeks since the beginning of the 20th century. Spring’s last freeze is arriving earlier, and fall’s first freeze is arriving later. This might sound like good news for gardeners, and in some cases it is. A longer season means more flexibility in what you can plant and more time for warm-season crops to mature.
But the shift comes with complications. Earlier springs can trigger fruit trees to bloom before the risk of a late frost is truly gone, leading to devastating crop losses when a cold snap hits blossoms. Warmer, longer seasons also extend the active period for agricultural pests and plant diseases. And in regions where water is already scarce, a longer warm season means more evaporation and greater irrigation demand.
USDA Hardiness Zones and What They Tell You
You’ve probably seen hardiness zones referenced on plant tags at the nursery. The USDA’s Plant Hardiness Zone Map, most recently updated in 2023 using weather data from 1991 to 2020, divides the country into 13 zones based on the average annual extreme minimum winter temperature. Each zone covers a 10°F range and is split into two half-zones (like 7a and 7b) covering 5°F each.
Hardiness zones tell you whether a perennial plant can survive your winter, but they don’t directly describe your growing season length. A zone 6 location in Colorado and a zone 6 location in Virginia might have the same winter lows but very different growing season lengths due to altitude, humidity, and summer rainfall patterns. Use your hardiness zone for choosing perennials, shrubs, and trees. Use your frost dates and growing season length for planning annual vegetables and flowers.
Matching Crops to Your Season
Every vegetable variety lists a “days to maturity” number, and comparing that number to your frost-free days is the most practical way to use growing season information. If your growing season is 120 frost-free days, you need crops that mature well within that window. Here’s a rough sense of what fits where:
- Short-season crops (25 to 70 days): Radishes (25 to 40 days), leaf lettuce (45 to 60 days), green beans (50 to 70 days), and zucchini (50 to 60 days). These work in almost any growing season.
- Mid-season crops (70 to 100 days): Carrots (60 to 80 days), cucumbers (55 to 65 days), and potatoes (90 to 120 days). Most temperate climates accommodate these comfortably.
- Long-season crops (100+ days): Leeks (120 to 150 days), dry onions (100 to 120 days), parsnips (110 to 130 days), and sweet potatoes (100 to 125 days). These need a genuinely long, warm season or a head start indoors.
If your season is too short for a crop you want to grow, starting seeds indoors four to eight weeks before your last frost date effectively extends the season on the front end. Season-extension tools like row covers, cold frames, and high tunnels push both ends, protecting young plants from late spring frosts and mature plants from early fall ones.
Finding Your Local Frost Dates
Your growing season is hyper-local. Two towns 20 miles apart can have frost dates that differ by two weeks or more due to elevation, terrain, and proximity to water. The Midwestern Regional Climate Center at Purdue University offers a freeze date tool that provides probability-based frost dates by county, including whether your local season has gotten statistically longer over time. The Old Farmer’s Almanac and many cooperative extension services also offer searchable frost date databases by ZIP code. Checking with experienced gardeners in your neighborhood is another reliable approach, since microclimates can vary even within a single town.