Sweet potato slips go into Colorado ground from late May to mid-June, once the soil has warmed and nights hold above 50°F. On the Denver plains the average last spring freeze falls between May 1 and May 11, yet sweet potatoes want warmth that arrives a couple of weeks later. Mountain towns and the western slope run on different clocks. For the full growing method, see Growing Sweet Potatoes in Colorado.
The short answer for Colorado
On the Front Range plains, set slips outdoors from about May 25 to June 10, and start them indoors in mid-March to early April. That timing puts the roots in warm soil and leaves them room to size up before fall frost.
The frost dates behind that window come from NOAA’s 1991-2020 Climate Normals for the Denver reference station. The last spring freeze is given as May 1 to May 11. That is a probability of a 32°F freeze: between those dates, the chance that one is still to come sits in a 30 to 70 percent band. The first fall freeze runs September 30 to October 12, and the same band applies to the chance that a freeze has already arrived. A freeze after May 11 or before September 30 can still happen in any given year.
When do you start sweet potato slips indoors in Colorado?
Slips are the leafy shoots that sprout from a mature sweet potato kept warm and damp. Growing your own takes time to reach a good transplant size with several leaves. Counting back from a late-May planting, that means mid-March to early April. Gardeners who buy slips by mail should order for delivery in the last week of May or the first week of June, so the shipment does not arrive while the ground is still cold.
When do you transplant sweet potatoes in Colorado?
Wait for two things: soil that feels warm to the touch, and nights that stay mild. On the plains that usually means two to three weeks after the last-freeze window closes. Black plastic or dark landscape fabric laid a week ahead warms a bed faster. Most sweet potatoes need a long season from transplanting. A slip set out around June 1 and a long-season variety are ready ahead of the fall freeze band. The short-season types are covered in The Best Sweet Potatoes to Grow in Colorado.
What will not work here
Planting on the average last-frost date fails more often than any other shortcut. May 1 to 11 only marks the point where a late freeze becomes less likely than not. Cold soil in early May stalls slips, and a night near freezing can blacken them. Sweet potatoes are a tropical root crop, and a slip that sits in cold soil loses ground it never recovers.
Which sweet potatoes are too slow for Colorado?
Varieties that need 120 days or more run out of season. Transplant one on June 10 and it needs until early October, which is exactly when the first fall freeze arrives (September 30 to October 12 at the Denver reference station). Frost kills the vines, and cold soil then damages the roots. A late start also leaves no room for a slow spring, since the plants put on most of their root weight in July and August heat. Pick types rated near 90 to 100 days and plant them as soon as the soil allows.
Does Colorado summer rain cover sweet potatoes?
It does not. June, July and August together bring about 5.7 inches of rain (June 1.94, July 2.14, August 1.58), out of 14.5 inches for the whole year. Meanwhile 39.6 days a year reach 90°F or more at the reference station. That heat pulls moisture out of a bed quickly, and rain that comes as a single thunderstorm tends to run off. A sandy bed drains within the day. A clay bed holds water longer but can crust over and shed it.
Sweet potatoes tolerate dry spells once their vines cover the ground. The danger is after transplanting and again when the roots swell. A bed watered lightly will give thin, stringy roots. Deep soaking, with mulch after the soil has warmed, works better. These numbers are normals for one station, so a cheap rain gauge on your own plot tells you what actually fell.
What changes across Colorado
Colorado has no single answer. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 data) runs from zone 3b to zone 7b across the state. That is a spread of several zones, from high mountain valleys to the warm lowlands. The dates in this article are anchored on Denver at about 5,280 feet. A reader at 8,000 feet or on the western slope should treat them as a starting point and no more.
Does hardiness zone tell you when to plant sweet potatoes?
Not directly. The USDA system rests on one variable, the average annual extreme minimum temperature, which describes winter cold. Sweet potatoes are grown as annuals and die at the first freeze, so what matters is summer warmth and the length of the frost-free stretch. Zone still hints at elevation and cold air, and a low zone number is a warning that the season will be short. To find your own zone, look up your ZIP code on the USDA map. Do not guess from a neighbor’s yard.
What should gardeners in northern Colorado or at altitude change?
Mountain towns have far shorter seasons and can frost in July. Outdoors in open ground, sweet potatoes there are a long shot. Gardeners at altitude have better odds with a greenhouse, a high tunnel, or grow bags set against a south-facing wall, where reflected heat adds weeks. Northern Front Range gardeners should plant later in the window, choose only the fastest varieties, and use black plastic and a low tunnel of row cover for the first three weeks. The western slope differs again, with warmer valley floors and frost dates that do not match Denver’s. Local data beats a statewide guess.
Where to check this for your own county
Colorado State University Extension runs county offices across the state, staffed by agents and Master Gardeners who know the local frost patterns. A county recommendation beats a state one. The dates above are statewide, anchored on the Denver plains, and they cannot account for a canyon, a river valley or a ridge.
What should you look for at Colorado State University Extension?
Two things matter most. First, a planting calendar for your county or the nearest town, with local last-frost and first-frost dates for warm-season vegetables. Second, the vegetable varieties trialled in your area, which are plants grown side by side in local conditions. A variety that finished well in a county trial is a better bet than one that looked good in a catalog. Search the site for sweet potatoes, then for warm-season crops, since not every office publishes a sweet potato sheet.
What should you ask the county office?
Call or email and ask when local gardeners usually set out peppers and tomatoes, because sweet potatoes follow about the same timing. Ask whether anyone nearby has grown sweet potatoes with success, and which varieties and tricks they used. Ask about local soil too. A heavy clay bed and a sandy bed behave differently, and the office can often tell you what is typical for your neighborhood.
Checking this against your own garden
Your own observations outrank every date here. Frost dates describe a weather station. Your yard has its own pockets of warmth and cold, and three signals show you which one you have.
Which three things should you watch?
- Soil warmth by feel. Push your hand into the bed to the depth of your knuckles in mid-morning. If it feels distinctly cool, wait. A bed that feels warm days before the next one tells you where your warmest microclimate is.
- New growth after transplanting. Fresh leaves unfurling mean the slip has rooted. If the plants sit unchanged, the bed is too cold or too dry.
- First frost damage on tender plants. Blackened basil or squash leaves show the freeze has arrived in your yard. Dig the sweet potatoes within a few days.
Low spots matter too. Cold air drains downhill and settles at the bottom of a slope, so a bed there can freeze after a hillside bed stays safe. Over a season or two, the same beds will warm first and frost first, and that pattern is worth more than any average.
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 you plant sweet potatoes in Colorado in July?
It is too late on the plains. A July 1 transplant with a 90-day variety lands near the end of September, right as the first fall freeze band begins. Roots would stay small. Aim for June.
How long do sweet potatoes take to mature in Colorado?
Most take a long season from transplanting. Short-season varieties sit at the low end and suit the plains and cooler spots best. Choose by days to maturity first.
When do you harvest sweet potatoes in Colorado?
Dig before the first hard freeze, which at the Denver reference station falls between September 30 and October 12. If a freeze is forecast earlier, harvest ahead of it. Cure the roots in a warm, humid place before storing.
Can you grow sweet potatoes in the Colorado mountains?
Open ground is a gamble, since frost can come in July. Greenhouses, high tunnels and containers in the warmest corner of a yard offer the best chance. Your county extension office can tell you what has worked locally.