Finding the perfect spot
Sweet potatoes prefer light, sandy, somewhat acidic soil (pH 5 to 6.5), but New Brunswick grower Greg Wingate of Mapple Farm notes they do just fine in his clay soil. Plant them in the warmest, sunniest spot possible. You can help speed soil warming (highly recommended, given the short summers throughout most of Canada) by planting in wide raised beds or rows about 20 centimetres high, and by covering the soil with sheets of clear or black plastic mulch several weeks before you plant; the soil needs to be at least 13°C.
Transplanting your slips
On transplanting day, thoroughly weed the area. Wingate sometimes adds a side dressing of potassium-rich wood ash—which promotes stronger, more disease-resistant root growth (avoid excess nitrogen, which will produce lush, leafy growth at the expense of the tubers)—working it into the soil as he weeds.
Cut holes in the plastic about 23 centimetres in diameter, and 45 to 60 centimetres apart; anchor the slips using soil, tucking them into the centres of the holes, leaving about two or three leaves above ground. Water well, and provide some shade for the first few days if there's a lot of sun.
Growing sweet potatoes in pots
Sweet potatoes can also be grown in containers, according to Ken Allan, a grower in Kingston, Ontario. At a minimum, you'll need a 30-centimetre pot—bushel baskets and half-barrels work well, too. The advantages are that the soil in containers will heat up quickly and you can move them to a warm, sheltered spot as needed.
The main drawback is that the soil will cool faster than a bed in the ground, so be vigilant—a soil temperature of 10°C or lower will ruin your crop (below 10°C, harmful rot spots may form, turning your sweets into brown mush during storage). You'll also have to water more often, as the roots can't reach deep into the ground for moisture as they do in regular beds.
Regardless of your planting location, growth may seem slow at first as the slips take root. Then, toward midsummer, the vines will grow like crazy.
Weed thoroughly (but carefully, so as not to damage the young, shallow roots) as the vines begin to spread. The vines may try to root into the bare ground. If you wish, you can lift them to slightly dislodge any roots that may have formed, but it's not critical to do so. Come August, your sweet potato tubers will be growing and swelling.