Late May to early June is the planting window for sweet potato slips in most of Minnesota, and the soil matters more than the calendar. Frost closes the season at both ends. The Twin Cities see their last spring freeze between April 17 and April 30, and the first fall freeze between October 11 and October 23. Sweet potatoes need warmth between those bookends, and Minnesota supplies it with little to spare.
The short answer for Minnesota
Set slips in the ground about two weeks after the last-frost window closes. In the Twin Cities area that means mid-May at the very earliest. Late May to early June is the safer bet for most gardens. That timing is a judgment built on the frost data, not a measured date, and the data itself deserves a careful reading.
The frost dates here are probabilities. They come from NOAA’s 1991-2020 Climate Normals for the Minneapolis reference station, and each window marks the stretch of days when the odds of a 32°F freeze sit between 30 and 70 percent. April 17 to April 30 is the spring band. October 11 to October 23 is the fall band. Neither is a guarantee. A freeze on May 8 is unusual, but it happens, and a frost on the night of October 5 is not unheard of either.
Waiting past the last frost date has a mechanical reason. A sweet potato slip is a tropical cutting. It survives a frost about as well as a basil plant does, and it also sulks in cold ground. Soil that has merely stopped freezing is not warm enough. Roots stall, leaves yellow, and the plant loses weeks it cannot spare.
Do the arithmetic and the squeeze shows. From April 30, the end of the spring window, to October 11, the start of the fall one, is 164 days. A slip planted on June 1 has 132 days before the earliest date in the fall window. That is the real growing budget, and it is why variety choice matters so much here. The The Best Sweet Potatoes to Grow in Minnesota page sorts out which types fit inside it.
Slips started indoors should go into pots about six to eight weeks before transplanting, which means early to mid April for a late-May planting. For the broader picture of how this crop behaves in Minnesota, see Growing Sweet Potatoes in Minnesota. The rest of this page covers timing only.
What will not work here
Planting on the last-frost date fails first. It looks logical (the danger has passed, go), but a date with 30 to 70 percent freeze odds is the middle of the risk, not the end of it. Even when the air stays above freezing, late April and early May soil in Minnesota is cold. Slips set out then tend to sit for weeks doing nothing, and the ones that rot are the lucky ones compared with the ones that limp along and produce pencil-thin roots in October.
Waiting too long fails just as reliably. A slip planted June 25 has 108 days before the earliest date in the fall window, and that is before counting the stall that comes with transplant shock. Slow varieties, the ones that need a long stretch to size up, simply run out of calendar. A southern-style sweet potato that thrives in a hot, long Georgia summer will be caught by frost in Minnesota with roots half the size of a lunch sandwich. Matching the variety to the season is most of the game, as the variety page explains.
Heat is the next misjudgment. The reference station averages 10.6 days a year at 90°F or more. That is a modest heat load, nothing like the Deep South, and it means Minnesota summers give sweet potatoes warm days but not the weeks of baking heat they enjoy elsewhere. Bare, cool clay does not help. Gardeners here often lean on black plastic mulch, raised beds, or grow bags to add the few degrees the weather will not. Skip those tricks and the season gets shorter in practice than the frost dates suggest.
Watering habits fail in a less obvious way. Rainfall across June, July and August adds up to about 13.0 inches (June 4.58, July 4.06, August 4.34), out of 31.6 inches for the whole year. July is the driest of the three, yet it still gets over four inches. On heavy clay, that is enough to keep sweet potatoes from needing a hose most weeks, and overwatering there invites rot. On sandy ground, which Minnesota has in many places, the same rain drains within the day, and the plants may want water far more often. These are normals for one station over 1991-2020, not a forecast. A cheap rain gauge on your own plot settles the question better than any statewide average.
What changes across Minnesota
Minnesota is not one garden. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state across zones 3a to 5a. That is five half-zone steps, and since each full zone is 10°F, the average coldest winter night differs by about 20°F between the mildest and the coldest corners. No single answer exists for the state. The dates above describe the Twin Cities, the mild south of Minnesota, and they do not describe Duluth, the Iron Range, or the border with Canada.
Northern Minnesota runs two to three weeks tighter at both ends. Shift the Twin Cities windows by that much and the spring band lands in May, while the fall band lands in late September or early October. A northern gardener’s planting date slides later, the harvest date slides earlier, and the season that already felt short gets shorter still. Those shifts are rough arithmetic on the reference dates, not measured values for any northern town.
A reader in the north should lean on the fastest-maturing types, start the bed warming with black plastic a couple of weeks before planting, and think about grow bags or raised beds, which heat up faster than open ground. Low hoops with row cover can buy days in both spring and fall. Near Lake Superior and other big lakes, cool breezes can hold the ground back further, so even a neighbor’s dates 20 miles inland may not apply.
Where to check this for your own county
The University of Minnesota Extension is the place to take this page and test it against local reality. As the state’s land-grant extension service, it publishes garden guidance written for Minnesota conditions and staffs county offices that field questions from home gardeners.
Look for three things there. First, local frost information for your county, since the spread between the Twin Cities and a northern county is wide enough to move your planting date by weeks. Second, any vegetable planting calendar that lists a sweet potato window, which will be more specific than the one here. Third, variety trials run in or near your county. Sweet potatoes have a short track record in the state compared with tomatoes or potatoes, so a variety that ripened in a trial plot three counties away is far better evidence than a seed catalog description.
A county recommendation beats a state one, every time. This page is the state one. It rests on a single reference station in the Twin Cities and on normals averaged over thirty years, so treat it as a starting frame and let the extension office narrow it down.
Checking this against your own garden
Your own observations outrank every date here, and three of them cost nothing.
The first is soil warmth by feel. Push your hand into the bed, wrist deep, in mid-to-late May. If the soil feels cool and slightly clammy, wait. If it feels distinctly warm, like a room left on low heat, you can plant. A south-facing bed beside a wall will reach that point days before a shaded or low-lying one, and the gap tells you how much your microclimate differs from the regional average.
The second is the first true leaves. After transplanting, slips that are happy push out fresh, full-size leaves within a week or two. If the plant holds still, or the existing leaves turn pale, the ground is still too cold or too wet. Growth that starts quickly suggests your spot runs warmer than the state picture.
The third is frost in your own low spot. Note the dates of your last spring frost and your first fall frost for a few years running, on the lowest, flattest part of the garden and on the highest. Cold air pools like water, and a hollow can freeze while the slope above stays green. That difference tells you where to plant first, where to plant last, and where to put the crop you can least afford to lose.
Common questions
Can I plant sweet potato slips before the last frost date?
It is a gamble, and cold soil makes it a poor one even if the air stays mild. The April 17 to April 30 band for the Twin Cities only marks when a 32°F freeze has 30 to 70 percent odds. A covered, plastic-warmed bed can start a little sooner, but the slips still need warm ground.
When should I start slips indoors?
Roughly six to eight weeks before you plan to transplant. For a late-May or early-June planting, that points to early or mid April, on a warm windowsill or under a grow light.
How late is too late to plant?
Counting from the earliest date in the fall window, a June 25 planting has 108 days and a June 1 planting has 132. The later you go, the faster the variety needs to be. In northern Minnesota, where the fall window comes sooner, the cutoff arrives earlier.
When do I dig them up?
Before frost, ideally, or soon after a first frost blackens the vines. In the Twin Cities that means watching the October 11 to October 23 window. Dig on a dry day and handle the roots gently, because the skin bruises easily and bruised roots store poorly.