Plant sweet potato slips in Wyoming in early to mid-June, once the last spring frost has passed and the soil has warmed. The window closes fast, because the first fall frost arrives between September 24 and October 5. Start slips indoors, harden them off, and plan on black plastic or row cover to make up for the heat the season lacks.
The short answer for Wyoming
Those dates come from NOAA’s 1991-2020 Climate Normals for the Cheyenne reference station, and they are probabilities of a 32°F freeze. The last spring frost falls between May 7 and May 18, the stretch where the chance of another freeze at 32°F sits in the 30 to 70 percent band. The first fall frost, September 24 to October 5, is measured the same way. A freeze outside those dates can still happen.
Roughly 130 frost-free days at 6,000 feet separate the two ranges. Sweet potatoes are tropical vines, so a freeze blackens the leaves and cold soil stops the roots from growing. That is why the planting date follows soil temperature more than the frost date. A bed that has just cleared the frost range is usually still cold, and early to mid-June is when it typically warms up.
When should you start sweet potato slips in Wyoming?
Slips are the leafy sprouts that grow from a mature sweet potato, and they take several weeks to reach planting size. Starting them indoors, in a warm and bright spot, gives you transplants ready for the garden. If you buy slips instead, order them to ship for planting time. Give either kind time outdoors in a sheltered spot before planting so the leaves toughen up.
Can you plant sweet potatoes in May in Wyoming?
Only with help. Black plastic spread over the bed ahead of planting warms the soil, and a row cover or low tunnel over the slips holds that warmth at night. Even then, keep the cover handy. Since May 18 is the far edge of a probability band, a late freeze is a real possibility. For picking varieties that finish in a short run, see The Best Sweet Potatoes to Grow in Wyoming.
What will not work here
Why do slow sweet potato varieties fail in Wyoming?
A slip set out in early to mid-June has the season before the first fall frost range opens. Varieties bred for long Southern seasons need more time than that to size up their roots. They put out plenty of vine and then get caught by cold weather with small, thin roots underneath. Southern calendars that say “plant in May, dig in October” assume a growing season Wyoming does not offer.
Heat is the other half of the problem. The Cheyenne reference station averages only 10.4 days a year at 90°F or more. Sweet potatoes put on root weight in sustained warmth, so a garden with few hot days grows them slowly. That is also why bare, unprotected soil in an open bed does poorly even in a June planting. Plastic mulch, a south-facing spot, or a sheltered bed against a wall adds some of the warmth the climate leaves out.
Planting by the calendar alone also fails. At higher elevations frost is possible in any month, so a mild week in late May means little. A gardener who plants on May 20 because the Cheyenne range has ended can lose the whole bed to one clear night.
Does Wyoming rain cover a sweet potato bed?
No. Cheyenne gets about 5.8 inches of rain across June, July and August, out of 15.4 inches for the whole year. June brings 2.16 inches, July 2.11, and August only 1.52, the driest of the three. That falls well short of what a bed of sweet potatoes wants, and much of it arrives in thunderstorms that run off dry ground instead of soaking in.
Newly set slips need steady moisture to root, and the vines need it again while the roots swell. Sweet potatoes tolerate some dryness in between, but a gardener who waits on the weather will end up with small, uneven roots. How often you need to water depends on the soil. A sandy bed drains within the day, while clay holds a rain for a week. A rain gauge on your own plot tells you what the sky actually delivered.
One more failure is leaving the crop in too long. Digging should happen as the first fall frost range nears. Waiting through October on the hope of a few more warm weeks bets against the odds the frost dates describe.
What changes across Wyoming
Wyoming has no single answer. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places the state anywhere from zone 3a to 6a. That gap is wide, and it comes mostly from elevation. The Cheyenne dates describe one city at roughly 6,000 feet, so treat them as a reference point and not as a rule for the whole state.
The zone itself rests on one thing, the average annual extreme minimum temperature. For a crop grown as an annual, winter lows matter less than how many warm days there are between the last and first frost. So the zone number is a rough guide to how harsh a location is, and the local frost dates carry the real information. To find your own zone, enter your ZIP code on the USDA map.
If you garden in the north or at altitude, shift everything toward caution. High valleys can see frost in any month, so plant later, keep covers on for longer, and choose the fastest-maturing types. Where a bed will not warm reliably, large fabric containers set against a south-facing wall heat up faster than open ground. Lower basins generally give a longer, warmer season than mountain valleys, but even there the Cheyenne dates are a starting guess. The full picture for the state is in Growing Sweet Potatoes in Wyoming.
Where to check this for your own county
Your state’s extension service is the next stop. University of Wyoming Extension runs county offices across the state, staffed by people who know the local frost pockets and which gardens warm first. A county recommendation beats a statewide one every time, and the dates here are statewide, built from one station.
What should you ask the county extension office?
A few specific questions will get you further than a general one:
- The typical last spring and first fall frost dates for your town, not the nearest big city.
- Whether the county has trialed sweet potato varieties, and which ones finished a crop.
- If local gardeners rely on high tunnels, plastic mulch or row covers, and how they set them up.
- When local growers usually plant warm-season crops, since sweet potatoes follow the same pattern as peppers and tomatoes.
Trial results from a county at your elevation are worth more than results from a county at a different one. If your own county has none, ask for results from the closest place with a similar growing season. Extension staff can also tell you which nearby weather station best matches your garden.
Checking this against your own garden
Dates on paper only go so far. Three things you can see yourself will outrank any calendar, and each one tells you something about your own microclimate.
Soil warmth by feel. Push your hand into the bed at planting depth in the morning. If the soil feels cool or damp-cold against your skin, the slips will sit and wait. Compare a spot beside a south-facing wall with open ground. A bed that feels warm days earlier than the rest is where your earliest slips belong.
New leaves on the slips. After transplanting, a slip that pushes out fresh leaves has rooted. Leaves that yellow, turn purple-tinged, or stay flat suggest the bed is too cold or too dry. If this happens to you, your garden runs cooler than the state dates assume, and next year’s planting should wait longer or use more cover.
The first frost damage in the fall. Watch which tender plants blacken first, such as basil or squash leaves. If one corner of your garden gets nipped before another, you have a cold pocket. Dig the sweet potatoes there first, and keep next year’s slips in the warmer corner.
Some gardeners also time this by the Moon: when to plant sweet potatoes by the moon, as almanac tradition rather than fact.
Common questions
Can I plant sweet potatoes in Wyoming in May?
Only under plastic mulch, row cover or a low tunnel, and with the cover ready for a late freeze. The last spring frost range runs May 7 to May 18 at the Cheyenne reference station, and at higher elevations frost can come later. For most gardens, early to mid-June is safer.
How long is the growing season for sweet potatoes in Wyoming?
Roughly 130 frost-free days at 6,000 feet separate the last spring frost from the first fall frost. Slips set out in June get less than that, and higher elevations get fewer days still. This is why varieties that mature quickly make the difference between a real harvest and small roots.
Do sweet potatoes need a lot of water in Wyoming?
They need steady moisture when the slips are rooting and when the roots are swelling. Summer rainfall is about 5.8 inches across June, July and August, and August is the driest at 1.52 inches. Most gardens will need to irrigate, and a rain gauge shows how much your plot actually received.
Should I start my own slips or buy them?
Either works. Starting your own from an organic sweet potato costs little, though grocery-store roots are sometimes treated to stop sprouting. Mail-order slips arrive bare-root and should go into the ground soon after arrival, so time the order for planting time.