Plant sweet potato slips outdoors in the Northwest Territories after the average last spring frost of May 25, and in practice in early June under plastic or a low tunnel. Start the slips indoors in early to mid April. Harvest before the average first fall frost of September 18. The margin is thin, and it shapes everything below.
The short answer for Northwest Territories
Sweet potatoes are not sown outdoors. They are planted as rooted cuttings called slips, so the calendar has three dates: when you start the slips indoors, when they go into the ground, and when you must dig. The frost dates anchor the last two.
The figures come from Environment and Climate Change Canada’s 1981-2010 normals for the YELLOWKNIFE A station. Last spring frost falls on average on May 25. First fall frost falls on average on September 18. Both are mean dates, the midpoint of a spread. In one year out of two the last spring frost lands later than May 25, and in about half of years the fall frost arrives before September 18. Plant on May 25 and you are betting on a coin flip.
The two frost dates
Between those two average dates sit roughly 116 days. That is the entire frost-free stretch in a typical year at the reference station, and you cannot spend all of it, because the tubers need warm soil before they grow and the last weeks before frost are the riskiest. Sweet potatoes are damaged by cold well before a hard freeze, and a frost blackens the vines outright.
The page does not have a soil temperature record for the territory, so it cannot tell you the date the ground reaches the warmth sweet potatoes want. Gardeners generally look for soil of about 18 °C (65 °F) or warmer before planting out. In a cold northern spring that usually means a few weeks after the last frost, not on it.
A working calendar
Use the table as a starting frame, not a guarantee. The timings after the frost dates are gardening practice, not station data.
| Step | Timing | What it is tied to |
|---|---|---|
| Start slips indoors | Early to mid April | About 6 to 8 weeks before planting out |
| Plant out, covered | Early June, after the May 25 average has passed | Warm soil and a frost forecast that stays clear |
| Dig | Early September if possible | Ahead of the September 18 average first frost |
That leaves roughly three months of real growing. For a plant that usually wants a long, warm run, the choice of type matters as much as the date, and The Best Sweet Potatoes to Grow in Northwest Territories covers it. The broader picture is in Growing Sweet Potatoes in Northwest Territories.
What will not work here
Planting on the average last frost date fails first. A mean date is not a safe date, and a sweet potato slip that meets a late frost has nothing to fall back on. It has no tough crown like a potato, and a chilled slip can sit stalled for weeks. Wait for the date to pass, then wait for the soil.
Slow varieties and bare ground
A slow-maturing variety is the second casualty. With an average of roughly 116 frost-free days between the two Yellowknife A dates, and fewer once you wait for warm soil, a type that needs a long season to size up its roots will produce stringy, pencil-thin tubers or nothing. Early-maturing types are the only sensible bet, and even they depend on cover.
Bare ground in an open bed is the third. Gardeners further south can plant sweet potatoes into plain garden soil and let summer heat do the work. Here, black plastic laid over the bed for a couple of weeks beforehand, plus a low tunnel over the planted slips, buys the soil warmth the calendar does not provide.
Heat is not the problem, and dryness might be
Do not read too much into the heat count. Yellowknife A averages 0.45 days a year above 30 °C and 0.03 days a year above 32 °C. Those numbers count extreme days. They do not measure the growing season, and they do not mean a heat-loving crop is doomed. The frost-free window is the limit, not scorching afternoons.
Water is a quieter trap. Summer rainfall at Yellowknife A is about 109 mm (4.3 inches) across June, July and August, out of 290 mm (11.4 inches) for the year. June is the driest month at 29 mm (1.14 inches), then 41 mm (1.61 inches) in July and 39 mm (1.55 inches) in August. These are 1981-2010 normals, so they are not directly comparable with the 1991-2020 US figures. June is exactly when new slips are rooting in. On sandy soil, where rain drains within the day, a watering routine of once a week would leave them thirsty. On clay, the same rain can sit for a week. A rain gauge on your own plot settles it, and a finger pushed into the soil does nearly as well.
What changes across Northwest Territories
The territory is large, and every figure on this page comes from one airport station at Yellowknife. Airport stations sit outside town and read differently from the neighborhoods around them, so even the reference city is an approximation of your own street.
Natural Resources Canada lists Yellowknife at hardiness zone 1b, in the Canadian system, using 1991-2020 normals. That is not a USDA zone, and it does not convert to one. The Canadian system is built on seven climate variables, the USDA on one, and Natural Resources Canada itself says there are no simple conversions between them. Zone 1b is the zone of the reference city, not of the whole territory. A territory this long has no single answer, and the page does not have the full spread of zones across it, so it will not guess at one.
What you can do is shift the dates the right way. Northern and upland districts run weeks behind the Yellowknife A frost dates: later spring frost, earlier fall frost, a shorter window than the table above. If you garden north of the reference station or at elevation, treat the May 25 and September 18 averages as optimistic. Start slips a little later so they do not outgrow their pots, plant out later, and plan to dig earlier.
To find your own zone, use the map on Natural Resources Canada’s Plant Hardiness Site. Look up your own community there rather than assuming it matches Yellowknife.
Where to check this for your own county
The Northwest Territories has no counties, so the local unit is your community or region. The best source is the Government of the Northwest Territories’ agriculture services from the Department of Industry, Tourism and Investment. Treat it as the territorial counterpart to a state extension service.
When you get there, look for two things. First, a local planting calendar for your community, with frost dates from a station near you rather than from Yellowknife A. Second, any record of the varieties trialled in your area, especially short-season types and any trials that used plastic or tunnels. A variety that has been grown through a real northern summer is worth more than any general recommendation. If sweet potatoes are not covered, ask whether warm-season crops like melons or peppers have been trialled nearby, since they need similar conditions.
A local recommendation beats a territorial one, and this page is the territorial one. Everything above rests on a single reference station and 1981-2010 normals, a different period from the 1991-2020 figures used for US states elsewhere on this site. Where an agriculture office, a nearby grower or a community garden group has real local experience, believe them over the averages here.
Checking this against your own garden
Your own observation outranks every date on this page. Three things are worth watching, and none needs equipment beyond your hands and eyes.
- Soil warmth by feel. Push your hand into the bed at about 4 inches deep in mid-morning. If it feels cool and slightly clammy, it is too early. If it feels like a mild room, you are close. Compare a spot under plastic with an uncovered one to see how much your covers actually gain.
- The first flush of male flowers. Watch the native wildflowers and weeds near you, such as dandelions and willow catkins, and note when they open. A late year for them predicts a late spring for the garden. Keep a note from year to year, because your own local pattern will beat the regional average within a few seasons.
- The first true leaves on your slips. Once the slips have rooted and put out new growth, they tell you the plant is ready for the ground, and how quickly they grow outdoors tells you how warm your bed really is. Slow, purple-tinged growth after planting out signals cold soil or wind. Move the tunnel closer to the plants and wait.
If you keep a notebook of these three observations alongside your planting date, one season will tell you more about your microclimate than any normal can.
Common questions
Can you plant sweet potatoes in the Northwest Territories at all?
It is possible, but with little margin. The average dates at the Yellowknife A station leave about 116 frost-free days, and you need to start slips indoors, use warm soil tricks like black plastic and tunnels, and choose early-maturing types. Expect smaller harvests than in warmer places.
Is May 25 a safe planting date?
No. It is the average last spring frost at Yellowknife A, which means that in one year out of two the last frost comes later. Plant after it passes, and after the soil has warmed, which for sweet potatoes usually means early June.
Do I need to worry about summer heat?
Rarely. The station averages 0.45 days a year above 30 °C and 0.03 days a year above 32 °C. Cold nights and a short frost-free season are the real constraints.
How often should I water new slips?
It depends on your soil. June is the driest summer month at Yellowknife A, with 29 mm (1.14 inches) of rain on average, so new slips often need help. Sandy soil may need watering every couple of days, clay much less often. Check with a finger or a rain gauge instead of following a fixed schedule.