Plant sweet potato slips in New Hampshire after the last spring frost has passed and the soil has warmed, which means late spring, not April. The frost tables show April 16 to April 27 for the reference airport station, but sweet potatoes come from the tropics and stall in cool ground long after the last 32F night. The first fall frost, October 13 to October 26, is the other end of the clock.
The short answer for New Hampshire
Put slips in the ground in late spring, once the frost window has closed and the soil feels warm, never on the frost date itself. In NOAA’s 1991-2020 normals for the reference station, the last spring frost falls between April 16 and April 27, and the first fall frost between October 13 and October 26.
What those dates are a probability of
They are not promises. Each range is built from 32F freeze probabilities: the stretch of days where the chance of a freeze sits in the 30 to 70 percent band. Inside that stretch, a freeze is still a live possibility. A gardener who plants on April 27 because the table says so is betting against odds that have not gone to zero.
| Measure | Reference-station figure | What it means for sweet potatoes |
|---|---|---|
| Last spring frost (32F, 30-70% band) | April 16 to April 27 | Earliest point at which frost risk starts to fade, not a planting date |
| First fall frost (32F, 30-70% band) | October 13 to October 26 | The deadline the crop has to beat |
| Days at or above 90F per year | 10.9 | A modest summer heat load |
These numbers come from an airport weather station, and the NOAA NCEI normals file describes that one location. Suburban and rural gardens typically see the last spring frost later and the first fall frost earlier, often by one to two weeks, and by more at elevation. So treat the figures as the reference-city picture, not a state-wide guarantee.
From frost date to planting date
On paper, the gap between the two frost ranges is a bit over five months. The planting window does not cover all of it. A sweet potato slip is a rooted sprout with no tolerance for cold, so the real sequence runs frost risk fading, then soil warming, then planting. In a garden where the last frost comes one to two weeks after the airport’s, frost risk can reach well into May before warm soil even enters the picture.
Work backward from the fall range as well. Whatever day the slips go in, the crop has to size up before the vines are hit in October. That is why late planting costs more here than it would in a longer-season state. The full crop plan is laid out in Growing Sweet Potatoes in New Hampshire.
What will not work here
Planting on the frost date
Setting slips out the week the airport table says frost is fading is the most common way to lose the crop’s first month. June is the wettest summer month at the reference station, at 4.04 inches, and cool, wet soil is where a freshly set slip sulks, yellows or rots instead of rooting. A slip that sits still for three weeks has burned part of a season that was not long to begin with.
The slowest varieties
With a first fall frost as early as October 13 at the reference station, and earlier in many rural gardens, the longest-maturing sweet potatoes are a gamble. The slips go in late, the frost comes early, and the gardener digs small roots or none. Heat does not rescue them either. The reference station averages just 10.9 days a year at or above 90F, a couple of hot weeks scattered across the whole summer, not a long run of scorching nights that warms the ground for months. Varieties bred for long southern summers are working against the climate here. Shorter-season types are the better match, and The Best Sweet Potatoes to Grow in New Hampshire covers which ones.
Bare, flat, unwarmed beds
Because heat is modest, a cold-season habit of planting into flat, unmulched, shaded ground works against you. Sweet potatoes do better where the soil has been given every advantage: a sunny spot, a raised bed or mound that drains and warms faster, and black plastic laid ahead of planting to preheat the soil. None of this is fancy, but skipping it is how a New Hampshire crop ends up with pencil-thin roots.
Watering by the calendar
Summer rain at the reference station totals about 10.7 inches across June, July and August (40.4 inches over the year), with July the driest month at 3.3 inches and August close behind at 3.35. That is not a drought climate, so a fixed weekly soaking on heavy soil can drown the roots. On sandy ground, though, the same rain drains within a day, and a dry July stretch can leave vines thirsty. Rain that arrives as one summer thunderstorm mostly runs off rather than soaking in. These are normals for one station, not a forecast, and a rain gauge on your own plot is what settles the question.
What changes across New Hampshire
There is no single answer for the state. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places New Hampshire across zones 3b to 6a, a spread of several zones inside one set of borders. That is a wide range, and it tells you the state is not one garden.
The zone system rests on one variable, the average annual extreme minimum temperature, so it measures winter cold. Sweet potatoes are grown as annuals and never face that winter. What the spread still tells you is how differently the north and the hills behave compared with the valley floor, and the frost dates move with it.
A reader in the north, in the hills or at altitude should shift in the same direction on every count. Expect the last spring frost to arrive later than the airport figures, and the first fall frost earlier, by more than the usual one to two weeks. Plant later in the spring, choose the shortest-season varieties on offer, and use every warming trick: black plastic, raised beds, the sunniest south-facing ground. In the coldest corners, containers on a sunny deck are a fair option, since they warm faster than garden soil and can be moved under cover when an early frost is forecast.
A reader in a sheltered, lower-elevation town will sit closer to the reference figures, though even there one to two weeks of drift is typical for suburban and rural plots. Nobody should assume they know their own zone from this page. The USDA map lets you enter a ZIP code and read it off directly, and that is the place to start.
Where to check this for your own county
Your state extension service holds the local answer. University of New Hampshire Extension is the land-grant extension for the state, and its county offices and publications are the next stop after this page.
Two things are worth looking for there:
- A local planting calendar, ideally with frost dates for your county or nearby town rather than for an airport.
- Sweet potato varieties trialled in your county, meaning grown and compared in New Hampshire conditions, not just sold by a catalog.
This page does not have a New Hampshire sweet potato calendar from the extension, so it does not offer one. Extension calendars are often written one crop or one species at a time, so look for the sweet potato or warm-season vegetable one specifically and not a general garden schedule.
A county recommendation beats a state one. This page is the state one, built from a single reference station and a hardiness map that spans several zones. Where the extension’s county guidance and this article disagree, go with the extension.
Checking this against your own garden
Your own observations outrank every date above, and three are easy to make without any instrument.
Start with soil warmth by feel. Push your bare hand a few inches into the bed in the morning. If it still feels cool and clammy, the slips will sit and sulk, whatever the calendar says. A bed under black plastic, or a raised one, will tell you it is ready days before open ground does, and that gap is your microclimate speaking.
Next, watch for the first new leaves on the slips after planting. New growth in the first couple of weeks says the roots have taken hold and the site is warm enough. Slips that stay flat and pale point to cold soil or a shaded, late-warming spot, and that is a signal to mulch with plastic or choose sunnier ground next year.
Last, keep track of where frost shows up first in the fall. Tender plants in a low corner or near the field edge will blacken while those by the house wall carry on, because cold air pools in hollows. If your low spot gets hit first, your real first fall frost is earlier than the airport’s October 13 to October 26, and your digging date should move up with it.
Common questions
Can I plant sweet potato slips on the last frost date?
Not safely. The April 16 to April 27 range marks where freeze odds sit in the 30 to 70 percent band at the reference station, and the soil is usually still too cool for slips. Wait until frost risk has clearly passed and the ground has warmed.
Are sweet potatoes grown from seed in New Hampshire?
Gardeners here grow them from slips, which are rooted sprouts, not from seed. Buy slips for planting time or grow your own from a sweet potato indoors, and time them to go out after the frost window.
Is New Hampshire too cool for sweet potatoes?
No, but it rewards shorter-season varieties. The reference station averages 10.9 days a year at or above 90F, so summer heat is modest, and the first fall frost can arrive as early as October 13. Variety choice and warm soil make up the difference.
When should I dig them relative to fall frost?
Aim to harvest around the first fall frost. The reference range is October 13 to October 26, but a cold pocket or higher elevation can bring frost one to two weeks sooner, so watch your own garden and dig promptly once the vines are hit.