in New Hampshire, the last spring frost typically falls between April 16 and April 27, and the first fall frost arrives between October 13 and October 26, based on NOAA’s 1991-2020 climate normals for the reference weather station. That gives a growing season window, but the actual dates for your yard depend heavily on where in the state you garden.
The frost dates for New Hampshire
Those numbers, April 16-27 for spring and October 13-26 for fall, come from a specific measurement method: NOAA’s National Centers for Environmental Information tracked decades of temperature readings at one official weather station and calculated the range of dates when there’s a 30% to 70% chance of a 32-degree freeze. That’s a statistical band built from real thermometer readings, which is why the range spans about ten days on each end rather than pinning down a single date.
The station behind these figures sits at an airport, which matters more than most gardeners realize. Airports are usually built on flat, open, often paved land that holds heat overnight, creating a small heat bubble that keeps the air near the sensor a degree or two warmer on clear, calm nights than a lawn ten miles away. The result: frost shows up later in spring and later in fall at that station than across most of the surrounding countryside.
New Hampshire’s geography amplifies this effect more than in flatter states. The Seacoast region near Portsmouth sees moderating ocean influence that can push frost dates later than inland areas. The Connecticut River Valley and the lakes region run cooler on calm nights because cold air drains downhill and pools in low spots. And anything in the White Mountains or at meaningful elevation faces a compressed season that can start a month later in spring and end a month earlier in fall than the airport station suggests.
This is why the April 16-27 and October 13-26 windows should be read as a reference point, not a promise. If you garden in a river valley, on a hilltop, or anywhere away from a large body of water or urban heat, your own last frost likely lands later than April 27 and your first fall frost likely arrives before October 13. The gap can run one to two weeks in typical suburban and rural settings, and more at elevation. Treat these dates as the starting line for your planning, then adjust based on your own site’s history.
How long the growing season actually is here
Subtract the spring date from the fall date and you get New Hampshire’s frost-free season at the reference station: roughly 170 to 190 days, depending on where within each range you land. That’s long enough for full-season tomatoes, pumpkins, and most warm-season crops to mature if planted promptly after the spring risk passes.
But that number shrinks fast once you leave the airport’s warmer microclimate. A garden in a frost pocket, a low-lying yard where cold air settles on still nights, can lose two to three weeks of growing season compared to the reference figures. Add elevation on top of that and a garden in the higher terrain of Coos or Grafton County might work with a season closer to 100 to 120 days rather than the 170-plus days the state-level normals suggest. That’s the difference between comfortably ripening a slicing tomato outdoors and needing row cover or a greenhouse to get there.
The USDA Plant Hardiness Zone Map places New Hampshire across a range of 3b to 6a, built from 1991-2020 climate normals. That system measures only the average annual extreme minimum winter temperature, which tells you what perennials and woody plants can survive the coldest nights of a typical winter. It says nothing about your frost-free window or growing season length, and it’s not built for timing vegetable planting. A property could sit in zone 5a for winter survival while still dealing with a short, late-arriving frost-free season because of local cold-air drainage. Zone and frost dates answer different questions, and conflating them leads to bad planting decisions.
For gardeners weighing whether their plot can handle a full-season crop like winter squash or a longer-maturing corn variety, the honest approach is to count backward and forward from your own observed frost dates, not the state’s zone number or the airport’s normals. If you don’t have several years of your own notes yet, your county’s UNH Cooperative Extension office keeps localized frost and growing-season guidance that reflects conditions closer to home than any statewide figure can.
Turning these dates into a sowing date
Seed packets and seedling tags almost always reference “days to maturity” and a recommended start date relative to your last frost. To use that information correctly, first settle on your own working last-frost date rather than the airport’s April 16-27 range wholesale. If you’re new to a property, a reasonable starting assumption is to add one to two weeks onto the later end of that window, landing closer to late April or even early May for outdoor transplanting of tender crops, especially if your yard sits away from open water or urban pavement.
Cold-hardy crops like peas, spinach, lettuce, and brassicas tolerate light frost and can go into the ground several weeks before your last frost date. Warm-season crops, tomatoes, peppers, squash, beans, and cucumbers, need soil that’s warmed up and air that’s reliably past frost risk, so those should wait until after your adjusted last-frost estimate, not the calendar date printed on a seed rack grown for a milder region.
Working backward from transplant date gives you your indoor seed-starting timeline. A tomato variety that needs six to eight weeks of indoor growth before transplanting, started against a late-April outdoor date, means starting seeds indoors in early to mid-March. Push that transplant date to mid-May because your property runs cooler, and your indoor start shifts later too, which works in your favor: less time babysitting leggy seedlings under grow lights.
Fall planning runs the same math in reverse. Count backward from your expected first fall frost, using October 13 as a cautious starting point for gardens that tend to run cooler than the reference station, and subtract the days-to-maturity for whatever you’re planting. A fall crop of bush beans needing 55 days, planted with an October 13 frost target, needs to go into the ground by roughly mid-August. Cutting it closer only works if you’re comfortable covering plants on a cold snap or accepting some risk.
None of this replaces watching your own weather. Frost forecasts from your local National Weather Service office, issued days ahead during the shoulder seasons, are far more precise for a given week than any historical normal. Use the seasonal dates for planning your calendar months out, then switch to short-range forecasts once you’re within two weeks of your estimated frost window.
Why your own garden differs from this figure
Microclimate is the word that explains most of the gap between the reference dates and what actually happens in your yard. A south-facing garden bed against a house foundation absorbs heat during the day and releases it slowly overnight, often buying a grower a week or more of extra frost protection compared to an open field. Raised beds drain and warm faster in spring than ground-level plots, which can nudge safe planting earlier for cold-sensitive seedlings.
Low spots do the opposite. Cold air is denser than warm air, so it flows downhill and collects in depressions and dips on calm, clear nights, the exact conditions that produce frost. A garden at the base of a hill can see frost weeks before a garden of the same size sitting fifty feet higher on the same property. This single factor explains more variation between neighboring New Hampshire gardens than any statewide zone map ever could.
Tree cover, wind exposure, and proximity to pavement or structures all shift the numbers further. Open lawns lose heat fast to a clear night sky and frost first; areas under tree canopy or near a heated building often stay a few degrees warmer. Coastal towns and gardens near large lakes benefit from water’s slow temperature changes, which can delay both the first fall frost and the last spring frost compared to inland sites just a short drive away.
Given all that variability, the single best move for a New Hampshire gardener is building a personal frost log: the date of the last spring frost and first fall frost, observed at your own site, for a few years running. Layered against the reference dates here, that log turns a statewide estimate into a number you can actually plant by. Your county office of the UNH Cooperative Extension can help interpret local patterns if you’re trying to shortcut that multi-year process.
Common questions
Is it safe to plant tomatoes right after the last frost date passes?
Technically the frost risk has dropped below 30% at that point per the historical normals, but soil temperature matters just as much as air temperature for tomatoes. Many gardeners wait an extra week or two past their last frost date to let soil warm, since cold soil stunts transplants even without a frost event.
Do these frost dates apply the same way across the whole state?
No. The April 16-27 and October 13-26 windows come from one reference weather station, typically an airport site with a mild microclimate. Higher-elevation towns, valley floors, and rural properties away from pavement commonly see later spring frosts and earlier fall frosts than this range.
What’s the difference between my hardiness zone and my frost dates?
The USDA hardiness zone (3b to 6a across New Hampshire) is based solely on average annual extreme minimum winter temperature and tells you what perennial plants can survive winter there. Frost dates measure a completely different thing: the window when your growing-season crops face freeze risk. A garden can sit in a relatively mild hardiness zone and still have a short, late-starting frost-free season.
How do I find more accurate frost dates for my specific town?
Your county’s UNH Cooperative Extension office can offer more localized guidance than statewide normals, and keeping your own multi-year frost log for your exact property is the most reliable method available to any home gardener.