In Yukon, plant sweet potato slips outdoors from mid-June, after the mean last spring frost of June 5 has passed, and keep them under cover. Start the slips indoors in April or early May so they are ready on time. The season is short, so early-maturing types and a warm, protected bed count for more than hitting an exact date. The full growing picture is in Growing Sweet Potatoes in Yukon.
The short answer for Yukon
Transplant sweet potato slips no earlier than June 5 and preferably in the second half of June, under row cover or a low tunnel. At Whitehorse A, the reference station, Environment and Climate Change Canada puts the mean last spring frost on June 5 and the mean first fall frost on August 25 (1981-2010 normals). Sweet potatoes are not sown from seed, so the window is for setting out rooted slips once the soil has warmed to about 18 °C (65 °F).
What does the June 5 frost date mean?
It is a midpoint. In one year out of two, the last spring frost falls later than June 5. The same logic applies in fall: in one year out of two, the first frost arrives before August 25. Treat both dates as the middle of a window, never as a safe line. The reference station has about 81 frost-free days between the mean last spring frost and the mean first fall frost.
Most sweet potato varieties need a long, warm season from transplanting. Eighty-one days on average is tighter than that, and in a cold year it is shorter still. The figures also come from one airport station, and northern or upland districts run weeks behind it. Canadian normals cover 1981-2010, so they will not line up exactly with the 1991-2020 numbers used for American states.
When should you start sweet potato slips in Yukon?
Count back from the day you plan to transplant. For a mid-June planting, that means setting sprouting roots in moist soil or water in a warm room from mid-April to early May. Harden the slips off outdoors before they go in the ground. If you buy slips instead, order them for delivery in the middle of June rather than early May, because slips that arrive too early sit in pots with nowhere warm to go.
Pick varieties by days to maturity first and flavor second. The shortest-season types are covered in The Best Sweet Potatoes to Grow in Yukon.
What will not work here
Which sweet potatoes fail in a short season like Yukon’s?
The slowest varieties are beaten by the frost-free window before they ever fill out. Planting uncovered slips in open ground on June 5 fails too, since half of all years still have frost after that date and a single night can blacken every vine. Cold, shaded or clay-heavy beds that hold winter chill into late June also fail, because slips set into soil below about 18 °C (65 °F) sit and stall instead of rooting.
Waiting for a heat wave to rescue the crop does not work either. Whitehorse A averages 0.92 days a year above 30 °C and 0.28 days a year above 32 °C. Those are counts of extreme days, so they do not prove the crop is hopeless. What decides the outcome is the length of the frost-free period and how warm the summer runs overall. But it does mean the warmth has to be built into the bed with black plastic, row cover, a south-facing slope or a raised mound, because the weather will rarely supply a spike.
Does Yukon’s dry summer mean you have to water?
Plan on it, especially early. Whitehorse A normally receives about 107 mm (4.2 inches) of rain from June through August and 262 mm (10.3 inches) over the whole year. June is the driest summer month at 33 mm (1.28 inches), with about 38 mm in July and 36 mm in August. That puts the thinnest rainfall right when newly set slips need steady moisture to root.
Soil changes the picture. A sandy bed lets a rain shower through within the day, so it can still need watering every few days. A clay bed holds water for a week, so overwatering becomes the risk. These are 1981-2010 normals for one airport, not a forecast for your plot. A straight-sided can set out in the garden is a cheap rain gauge, and a week of readings tells you more than the average does.
What changes across Yukon
Yukon is not one garden. Natural Resources Canada rates Whitehorse, the reference city, at hardiness zone 1b in the Canadian system (1991-2020 normals). That zone does not convert to a USDA zone, because the Canadian system weighs seven climate variables while the USDA system uses one. It also describes one city. The territory covers river valleys, high plateaus and country far north of Whitehorse, so there is no single answer for the whole territory.
Readers in the north or at altitude should expect the calendar to slip by weeks. Shift the transplant date later, and start the slips indoors later to match, so they do not outgrow their pots. Lean harder on the earliest-maturing varieties, and add more protection: double layers of cover on cold nights, black plastic laid a week or two before planting to preheat the soil, and a raised bed so the roots sit above the cold ground. Gardeners at the high end of that range may find that only a greenhouse gives sweet potatoes enough season.
Terrain matters even within a single yard. Cold air drains downhill on still nights, so valley bottoms and low corners frost first and a spot partway up a slope often stays a few degrees warmer. To find the zone for your own community, use the Canada’s Plant Hardiness Site map from Natural Resources Canada rather than borrowing Whitehorse’s number.
Where to check this for your own county
Yukon has no counties, so for a gardener here the equivalent is your own valley or community. The federal starting point is Agriculture and Agri-Food Canada, which is where to look for agricultural guidance and for contacts in territorial agriculture. Ask for two things: the frost dates and planting calendar for your area, and which short-season, warm-weather vegetables have been trialled near you. If sweet potatoes have been tried locally, the results will tell you more than any territory-wide window.
A local recommendation beats a territorial one every time. The window above is built from one airport station and sets the outer limits. A grower ten miles up a valley, or a few hundred feet higher, may be working from a calendar two weeks off. Use the territory-wide dates to plan your indoor start, then let local advice set the day the slips go outside.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs a gadget.
- Soil warmth by feel. Push your hand into the bed on a calm morning. If it feels cold, wait. A bed that feels warm early is a warm microclimate, and one that stays cool after its neighbors have warmed is shaded, low or heavy.
- New leaves on the slips. Slips that are rooting well push out fresh leaves after transplanting. If they sit unchanged, the soil is still too cold or too dry.
- Where frost shows first. After a cold night, note which corners have blackened leaves and which have none. Plant the sweet potatoes in the spots that stay green.
Keep notes for a season or two and you will have a local calendar sharper than any normal.
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 grow sweet potatoes outdoors in Yukon?
Yes, though it is a stretch. The frost-free window averages about 81 days at Whitehorse A, so success depends on early varieties, indoor-started slips, warmed soil and cover. Expect smaller roots than in warmer regions.
Can I plant sweet potato slips in mid-May?
Not outdoors. The mean last spring frost is June 5, which means frost still comes after that date in half of all years. Mid-May slips need to stay in pots indoors or in a heated greenhouse.
Do sweet potatoes need a greenhouse or row cover in Yukon?
Some protection is the safe plan. Row cover, a low tunnel or black plastic warms the soil and shields against late frost. A greenhouse gives the longest season, especially in the north or at altitude.
When should I harvest sweet potatoes in Yukon?
Before frost blackens the vines, if you can. The mean first fall frost is August 25, so in half of all years it comes sooner. Dig on a dry day, handle the roots gently, and cure them somewhere warm.