Plant sweet potato slips in Alaska from late May to mid-June, once the soil has warmed, and start the slips indoors earlier. At the Anchorage reference station the mean last spring frost falls between May 3 and May 12. Sweet potatoes want warm ground more than a frost-free night, so cover the bed with plastic and plan to dig before the first fall frost, which arrives between September 22 and October 1.
The short answer for Alaska
The planting window runs from late May into mid-June. The frost dates behind it are a probability of a freeze: between May 3 and May 12, the chance of a 32°F freeze on or after that date sits in a band of 30 to 70 percent. In the fall, the same band covers September 22 to October 1. These come from the NOAA 1991-2020 Climate Normals for Anchorage, and they describe typical odds over thirty years. Any single spring can freeze a week later.
The reference station has about 143 frost-free days between the mean last spring frost and the mean first fall frost. A sweet potato needs most of that time after it goes into the ground, and it does nothing useful while the soil is cold. That is why the planting date is set by soil warmth, and the frost date only sets the earliest you could try.
| Task | Timing in the Anchorage area |
|---|---|
| Start slips indoors | Early to mid-April |
| Mean last spring frost (30-70% freeze odds) | May 3 to May 12 |
| Lay black plastic over the bed | Mid to late May |
| Set slips out under cover | Late May to mid-June, when the soil feels warm |
| Mean first fall frost (30-70% freeze odds) | September 22 to October 1 |
| Dig the roots | Early to mid-September, ahead of the frost band |
How early should you start sweet potato slips?
Slips take weeks to grow from a sprouting root, so start them well before a late May or early June planting. Keep them in a bright, warm room. Harden them off before they go outside, because a slip moved straight from a windowsill to a breezy bed loses leaves fast. Sweet potato slips count their days from transplanting, so the clock only starts when they are in the bed.
When do you dig sweet potatoes in Alaska?
Aim to dig in early to mid-September, ahead of the fall frost band. Frost blackens the vines, and cold, wet soil can start rot in the roots. Choosing a quick type matters here, and The Best Sweet Potatoes to Grow in Alaska goes through which ones finish in a short season. For the full growing picture, start with Growing Sweet Potatoes in Alaska.
What will not work here
Anchorage averages 0.0 days a year at or above 90°F. Sweet potatoes come from warm country, and a summer with no hot days gives them little to work with. The roots swell best in warm soil and warm nights, and an open Alaska bed rarely supplies either. Planting slips bare in the ground and hoping for the best is the most common way to end up with pencil-thin roots in September.
Which sweet potatoes fail in a short, cool summer?
Slow varieties. Many sweet potatoes need a long season from transplanting, and the 143 frost-free days at the reference station include weeks when the soil is too cold to grow anything. A late type set out in mid-June can run into the first fall frost before the roots reach a useful size. Choose varieties sold as early or short-season, and expect modest roots even then. Long summer daylight helps the vines grow leaf, but root bulking still depends on soil warmth.
Why does planting at the last frost date go wrong?
The May 3 to May 12 window marks the point where a freeze is still a coin flip, and soil lags well behind the air. Slips set into cold ground stop growing and may rot. Wait until a hand pressed into the bed feels warm, and use black plastic or a low tunnel to speed it along.
Rain adds another trap. Summer brings about 5.8 inches across June, July and August, out of 16.4 inches for the whole year. June is the driest month at 1.02 inches, and that is exactly when new slips are rooting in, so they need a deep soak if the bed is dry a few inches down. August is the wettest at 2.93 inches, just as the roots are sizing up and the soil is cooling. A heavy clay bed holds that water for days, and roots sitting in cold, soggy soil can rot. Raised rows or mounds drain better. A rain gauge on your own plot tells you more than a regional average, because slope, shade and soil texture change what actually reaches the roots.
What changes across Alaska
Alaska spans USDA hardiness zones 1a through 8a on the 2023 map, which is built on the average annual extreme minimum temperature. That is one of the widest spreads of any state, so there is no single planting date for the whole state. Anchorage is one of the mildest inhabited parts of Alaska. The Interior and the north have far shorter frost-free seasons than the 143 days quoted for the reference station.
How should you shift the dates in the north or at altitude?
Plant later and dig earlier, and treat the Anchorage window as the best case. A garden in the Interior or on a hillside has fewer frost-free days, so a slow variety that barely finishes near Anchorage will not finish there. A greenhouse, a high tunnel or large containers moved under cover at night turn a marginal season into a workable one. Even then, the first fall frost sets a hard end date, so plant as soon as the soil warms.
Does the hardiness zone decide whether sweet potatoes work?
Not much. Sweet potatoes are grown as an annual, so winter lows do not threaten them. Summer warmth and the length of the frost-free season matter more, and a zone number measures neither. A coastal garden with mild winters can still have a cool, cloudy July. To see where your own garden sits, check the USDA Plant Hardiness Zone Map by ZIP code, then compare it with how early the soil warms in your yard.
Where to check this for your own county
The University of Alaska Fairbanks Cooperative Extension Service is the place to confirm these dates for your area. Look for a local planting calendar and for any warm-season vegetable trials run near you. Sweet potatoes are a stretch crop in Alaska, so a trial result from a garden with your kind of soil and elevation is worth more than any general guideline.
Ask the local office two things. First, what dates do gardeners in your area use to set out frost-tender crops such as tomatoes and squash? Sweet potatoes should go in no earlier than those. Second, which varieties or season-extension methods have been tried there, and how did the roots size up? An agent who has watched local gardens through a full season can tell you things a regional average cannot.
A county or borough recommendation beats a state one. This article is the state-level view, built on one reference station, and it works as a starting point. Local frost pockets, river valleys, hillsides and wind exposure can move your dates by a week or more in either direction.
Checking this against your own garden
Your own observations outrank every date above. Three things are worth watching, and none needs a tool.
- Soil warmth by feel. Push your hand into the bed. If it feels cool or clammy, hold off. A bed under black plastic that feels warm a week before a bare one tells you how much the plastic is gaining you.
- New leaves on the slips. After planting, a healthy slip should unfurl new leaves. If it sits still or the leaves turn purple or yellow, the soil or the nights are still too cold. Slow growth in one corner also flags a cooler, shadier spot.
- Where frost shows up first. In late summer, note which part of the garden gets the first white patch on the leaves. That low or open spot is your frost pocket, and it tells you where to plant earlier crops and where the sweet potatoes should not go.
Write the dates down each year. Notes on when the soil felt warm, when the slips took off and when frost arrived will give you a calendar that fits your own ground.
If you follow the almanac, here is when to plant sweet potatoes by the moon. It is a tradition, not a proven method.
Common questions
Can you plant sweet potatoes in Alaska in May?
Late May works only if the soil is warm and the bed is covered with plastic or a low tunnel. The mean last spring frost at the Anchorage reference station falls between May 3 and May 12, but cold soil in May stalls slips. If a hand pressed into the bed feels cool, wait.
Can you grow sweet potatoes outdoors in Alaska without a greenhouse?
It is possible in the mildest areas with plastic mulch, a low tunnel and an early variety. The result is usually small roots. A greenhouse or high tunnel gives better odds, especially away from the Anchorage area.
When should you harvest sweet potatoes before frost?
Dig in early to mid-September, ahead of the first fall frost window of September 22 to October 1. If frost blackens the vines before you dig, harvest within a few days so the roots do not sit in cold, wet soil.
How do you grow sweet potato slips in Alaska?
Set a sprouting sweet potato in water or damp soil in a warm, bright room before the last spring frost. When the shoots are several inches long, twist them off and root them in water or moist soil. Harden them off before transplanting.