Wyoming’s average last spring frost falls between May 7 and May 18, and the first fall frost typically arrives between September 24 and October 5, based on 1991-2020 NOAA data from a single reporting station. That window gives roughly 130 frost-free days at elevations near 6,000 feet, but frost can strike in any month once you climb higher. Check your local University of Wyoming Extension office before betting a season on these numbers alone.
The frost dates for Wyoming
The dates above come from NOAA’s National Centers for Environmental Information, using 1991-2020 Climate Normals for freeze probability at 32°F, recorded at station USW00024018. That’s a real weather station, not a statewide average pulled from thin air. The May 7 to May 18 range and the September 24 to October 5 range represent the 30% to 70% probability band, meaning there’s a meaningful chance of frost on either side of those dates, not a hard line you can circle on a calendar and trust completely.
Wyoming’s USDA Plant Hardiness Zone rating sits between 3a and 6a, according to the USDA Plant Hardiness Zone Map built from 1991-2020 normals. That’s a wide spread for one state, and it matters here more than almost anywhere else in the country. The USDA system is built on a single variable: the average annual extreme minimum temperature. It tells you what a plant might survive in the coldest stretch of winter. It says nothing about when your last spring frost hits or how long your growing season runs. Two gardeners in the same zone rating can have frost-free windows that differ by a month or more, simply because one lives in a valley and the other on a bench above it.
What makes Wyoming different from flatter states is the sheer range of elevation packed into a single set of borders. A gardener in a low-lying river valley might be gambling with a very different calendar than someone thirty miles away on a ridge. The station-based figures above are a starting point, not a verdict. They tell you what one location, tracked over three decades, typically experiences. Your own frost dates could easily run two or three weeks earlier or later depending on where you sit relative to that station, your elevation, and how cold air pools or drains through your particular patch of ground.
Treat May 7 to May 18 and September 24 to October 5 as the middle of a bell curve, not the edges. Plenty of springs will bring a damaging frost after May 18. Plenty of falls will stay clear well past October 5. Planning around the probability band, rather than a single fixed date, is the difference between a garden that survives a cold snap and one that doesn’t.
How long the growing season actually is here
Do the math on the two frost windows and you land on a growing season of roughly four and a half months at elevations around 6,000 feet, call it 130 frost-free days when conditions run average. That’s a workable stretch for a lot of vegetables, but it’s short compared to states at lower elevation, and it can shrink fast once you move upward. Wyoming doesn’t have a single growing season. It has dozens, stacked by altitude.
Elevation is the variable doing most of the work here, more than latitude, more than which side of the state you’re on. Climb from a valley floor to a mountain bench and you can lose weeks of frost-free time in a matter of a few thousand feet of gain. At higher elevations, frost isn’t confined to spring and fall. It’s possible in any month of the year, which changes how you think about planting entirely. A tomato plant that would cruise through a July night in a low valley can get nipped at altitude on the same date.
Why 130 days is a starting estimate, not a guarantee
That 130-day figure applies to the station elevation and location behind the NOAA data cited above, not to every backyard in the state. If you garden at a higher elevation, your realistic frost-free window is almost certainly shorter, sometimes by weeks. If you’re lower and more sheltered, you might get more usable days than the average suggests. Cold air is heavy. It sinks into low spots and settles there overnight, which means a valley bottom can actually see more frost risk than a slope just above it, even at similar elevation. This is the opposite of what most people assume, and it trips up a lot of new gardeners who figure “lower elevation” automatically means “safer.”
The practical takeaway is to treat the 130-day estimate as a reference point you adjust against your own site, not a number you plant by directly. Local extension staff track how these patterns play out in specific counties, and they’ve seen enough seasons to know where the average breaks down.
Turning these dates into a sowing date
Frost dates are only useful once you turn them into an actual planting decision, and that starts with knowing whether what you’re growing tolerates cold or gets wiped out by it. Frost-tolerant crops (spinach, peas, kale) can go in the ground before the last spring frost date with reasonable confidence, since a light freeze won’t kill them outright. Frost-sensitive crops (tomatoes, peppers, squash, most beans) need to wait until real frost risk has passed, which in Wyoming means treating May 18, the later end of the probability band, as your safer benchmark rather than May 7.
Work backward from there using the “days to maturity” or “days to transplant” listed on a seed packet. If a pepper variety needs eight weeks indoors before it goes outside, and you’re aiming for a May 18 transplant date, you’re starting seeds indoors in mid-March. That backward math is the same method gardeners use everywhere, but in Wyoming the stakes are higher because the season is shorter and the margin for error is thinner.
Building in a buffer for the fall side
On the fall end, work forward from September 24, the earlier edge of the fall frost band, not October 5. If a squash variety needs 95 days to mature and you’re counting on a full season, planting late risks losing the crop to an early frost that arrives well within the historical range. Subtracting a week or two of buffer from both ends of the season, rather than planting right up to the edges of the average dates, is the difference between a harvest and a loss in a year that runs colder than normal.
Season-extension tools aren’t optional here the way they might be in a milder state. Row cover, cold frames, and even simple cloches can shift your effective planting window by two to three weeks on either end, which in a 130-day season is a meaningful gain. Given how tight the frost-free window already runs at higher elevations, most Wyoming gardeners end up leaning on some form of protection every year, not just in a rough one.
For crop-specific guidance tied to your county, county-level programs under the University of Wyoming Extension track local variety performance and planting windows than a statewide average ever could.
Why your own garden differs from this figure
The frost dates cited here come from one station, USW00024018, which means they describe conditions at one specific location tracked over thirty years. Your yard is not that station. Elevation is the biggest factor pulling your actual dates away from the average: gain a few thousand feet and you can lose weeks of frost-free time, sometimes with frost possible in a summer month that would otherwise be safe at lower ground.
Slope and drainage matter almost as much. Cold air flows downhill and pools in low spots, so a garden at the base of a slope can see frost on nights when a garden partway up stays clear. Gardeners sometimes call these low spots frost pockets, and in hilly or mountainous terrain they’re common enough to throw off a seasonal plan built purely on regional averages.
Surfaces close to a planting bed also shift the local temperature. A south-facing wall, a paved driveway, or a stretch of exposed rock can hold daytime heat and release it overnight, softening a frost event that would otherwise hit hard in an open field. Windbreaks, dense plantings, and even the mass of a nearby building all nudge the numbers in ways no station-based dataset can capture.
The USDA hardiness range of 3a to 6a for Wyoming makes this variability explicit rather than hiding it. That’s not a single zone, it’s a five-step range covering very different winter minimums, and it exists because the terrain genuinely varies that much across relatively short distances. Whichever end of that range your property falls closer to, use the station-based frost dates as your baseline, then adjust based on your elevation relative to the reporting station, your slope, and how sheltered or exposed your particular growing space is. That adjustment, more than the raw NOAA figures, is what actually determines your planting calendar.
Common questions
Can I trust the same frost date for my whole county?
No. The dates published here come from one station (USW00024018), and Wyoming’s elevation changes enough within a single county to shift frost timing by weeks. Treat the county as a rough zone, not a guarantee.
Is it safe to plant frost-sensitive vegetables right on the average last frost date?
The average date sits in the middle of a probability range, meaning frost still has a real chance of occurring after it. Waiting until the later end of the May 7 to May 18 window, or using row cover as insurance, reduces that risk.
Does a higher USDA hardiness zone number mean a longer growing season?
Not directly. The USDA rating measures average extreme minimum winter temperature only. It doesn’t track spring or fall frost timing, so two locations in the same zone can have very different frost-free periods.
What should I do if I garden well above 6,000 feet?
Expect a shorter frost-free window than the roughly 130-day figure tied to the reporting station, and plan for the possibility of frost outside the typical spring and fall months. Local county extension staff can offer guidance specific to your elevation band.