In New Hampshire, sow cover crops from the time beds empty in late summer until a few weeks before the first fall frost, with a smaller window in spring once the soil can be worked. Winter rye forgives late seeding better than anything else. The frost dates below come from Manchester’s airport weather station. Gardens elsewhere in the state will run on a different clock.
The short answer for New Hampshire
The two frost dates that frame the calendar are probabilities, not predictions. Using NOAA’s 1991-2020 U.S. Climate Normals, the last spring freeze at the reference station falls between April 16 and April 27. That is the band where the chance of a 32°F freeze sits between 30 and 70 percent. The first fall freeze falls between October 13 and October 26, the same probability band.
| Event | Reference-city window | What it measures |
|---|---|---|
| Last spring frost | April 16 to April 27 | 30-70% probability band for a 32°F freeze |
| First fall frost | October 13 to October 26 | 30-70% probability band for a 32°F freeze |
Those figures describe an airport, not your yard. Suburban and rural gardens usually see the last spring frost later and the first fall frost earlier, often by one to two weeks, and by more at elevation. Treat the table as a starting point and shift it toward the shorter season.
Why the fall window matters most
A fall cover crop has to put on leaf and root before growth stops. Seed that goes in after the first freeze band mostly sits there. The Manchester station’s coldest month has a mean daily minimum of 17.1°F, so winter is hard enough to kill frost-tender covers outright. That is useful if you want oats to die back on their own. It is a problem if you wanted a clover to survive.
The spring window
Spring sowing is the shorter, less forgiving option. Hardy species can go in as soon as the soil is workable, which in practice means around the April frost band at the reference city and later inland. Tender species have to wait until the last freeze has passed. Which species fit which window is covered in The Best Cover Crops for New Hampshire. The broader case for growing them at all is in Cover Crops for New Hampshire Gardens.
What will not work here
Waiting until the leaves have dropped. By then the first fall freeze band (October 13 to October 26 at the reference city, earlier for many gardens) has usually passed. Seed broadcast in November often germinates weakly, if it germinates at all, and what does come up may be too small to hold soil or smother weeds through winter. If the beds are not clear by early October, the better choice is a hardy species, sown anyway, over a tender one.
Heat-loving covers are the second casualty. Species that want a long, warm season, sorghum-sudangrass being the usual example, are built for places where summer lasts well into fall. Here the gap between the last spring freeze and the first fall one is too short for them to finish the job. A frost-tender cover sown after a late-summer harvest will be killed by the first freeze before it has made much biomass.
Early spring seeding goes wrong in a quieter way. A warm week in April tempts people to sow, but a 30 to 70 percent chance of another 32°F freeze in the second half of that month (and later inland) means tender seedlings can be knocked back, and cold soil slows even the hardy ones. Sowing a tender cover at the airport’s frost date is a gamble in a suburban or rural yard.
The fourth failure is borrowing a calendar from somewhere else. Planting guides written for the Mid-Atlantic or the South assume frost dates months apart from New Hampshire’s. A sowing date that is safe in Virginia can leave you a month late here. This page does not have a New Hampshire cover crop calendar from the extension service in its facts, so it gives you frost-based logic rather than one.
Last, a winter rye stand left too long in spring gets tall and tough before you plant. That is a timing failure, not a species one, and it is why the end date of a cover crop matters as much as the start.
What changes across New Hampshire
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) puts New Hampshire at 3b to 6a. That spans four zone numbers and has no single answer. The map is built on one variable only, the average annual extreme minimum temperature, so it tells you about winter survival and says nothing directly about how long your growing season is.
In practice, the colder end of that range means a shorter season. A garden in the northern part of the state, or high on a hillside, will see its first fall frost earlier and its last spring frost later than the Manchester figures. The fall sowing window closes sooner, sometimes before the garden is even finished producing. Sow earlier, and plan on the hardiest species. Spring sowing slides later, and the workable-soil date can arrive weeks after it does at the reference city.
Gardens at the warmer end, nearer the southern tier and valley floors, will track the reference figures more closely. Even there, cold air pooling in a low spot can bring frost earlier than the airport shows, so the reference dates are not a guarantee.
Which zone applies to your garden is for you to look up. Check the USDA Plant Hardiness Zone Map for your address rather than guessing from a neighbor’s yard or a seed packet. Then use the zone to decide how much earlier to sow in fall and how much later in spring, not to choose a date outright.
Where to check this for your own county
University of New Hampshire Cooperative Extension is the state’s land-grant extension service and the right next stop. Two things are worth finding there. The first is a local planting calendar for your county, which will give frost dates and sowing guidance closer to your elevation and soil than any state average. The second is any variety trial results from your area, which show what actually grew rather than what grew in a catalog photo.
Be plain with yourself about this: a county recommendation beats a state one. This page is the state one. It rests on one reference station, and its dates are probabilities from that station. The extension office can tell you how far your county’s season sits from them.
If you cannot find a cover crop calendar by name, ask for a vegetable or garden planting calendar for your county and work backward from its fall frost date. Staff can also tell you which cover crops local growers have had success with, and which ones have struggled on local soils. That kind of knowledge is rarely written down on a state-wide page.
Checking this against your own garden
Your own observations outrank every date above. Three things are worth watching, and none needs equipment.
- Soil warmth by feel. Push your hand a few inches into the bed in the morning. If it feels cold and clammy, seed will sit and may rot. If it feels cool but crumbly, hardy covers can go in. A bed that warms sooner than your neighbor’s is telling you it sits in a sunnier, more sheltered spot.
- The first male flowers on squash. Squash and pumpkin vines produce male flowers first. When they appear tells you how fast your garden warms. Early flowers point to a warm microclimate, late ones to a cooler or shadier plot, which should push your fall sowing earlier.
- The first true leaves. After the first seed leaves, a seedling produces its true leaves. In a small test row of oats or peas, the day they appear shows how fast things grow in your soil. Slow growth in fall means you are sowing too late for that spot.
Write the dates down for a season or two. A notebook of when each of these happened will give you a calendar tuned to your yard, which no state page can match.
Some gardeners also time this by the Moon: when to plant cover crops by the moon, as almanac tradition rather than fact.
Common questions
What is the latest I can sow winter rye in New Hampshire?
This page does not have a published cutoff date for New Hampshire. The reference city’s first fall freeze runs October 13 to October 26, and many gardens see it earlier. Rye tolerates later sowing than most covers, but earlier is better. Ask your county extension office for a local date.
Can I sow cover crops in spring instead of fall?
Yes, with hardy species once the soil is workable, and tender ones only after the last freeze. The reference-city band for that is April 16 to April 27, later for many inland and higher gardens. Fall sowing gives covers more time to establish, so spring is the backup option.
Do the Manchester frost dates apply to my town?
Only roughly. They come from an airport station. Suburban and rural gardens typically see the last spring frost later and the first fall frost earlier, often by one to two weeks, and more at elevation.
How cold does it get in winter here?
The mean daily minimum of the coldest month at the Manchester reference station is 17.1°F. That is an average night, not a record low. It is cold enough to kill frost-tender covers, and the USDA zones run from 3b to 6a, so colder parts of the state will be harsher still.