Growing sweet potatoes north of the border is fun, easy and very rewarding when you start with some helpful information and a little understanding.
Sweet potatoes need to be warm and love to grow in hot conditions. Even their edible roots can suffer chill damage at temperature below 10 degrees. Never store your sweets in the fridge, a cool room temperature allows them to last for up to one year.
The sweet potato plant it self is able to withstand chilly temps right down to plus one, more so then it's edible roots. Vines cannot withstand any frost and should be covered in the event of frost. The roots will be fully protected by warm soil.
Warm soil is a crucial factor for sweets, although they can be planted in moderate soil temperatures they will only start to grow when they feel some heat. When soil temperatures rise the sweet potatoes will thrive.
To increase soil temperature in the beginning of the season you might consider covering your soil with black plastic. Even clear plastic can be used to warm the soil but then must be removed before planting. Black plastic can be applied to the surface a few weeks in advance, then holes poked and slips planted directly in the plastic that stays all season.
Using a black plastic mulch method is credited with greatly boosting size of roots harvested in cool climates. The plastic helps pre-warm and then regulate soil temperature. It also suppresses weeds and helps to retain moisture.
Similarly the use of any sort of cold frame or low tunnel can help to create a warmer micro climate so that sweets can get right to growing. This need not be sophisticated and one friend uses old sheer curtains to keep plants cozy for the first 2 weeks after planting. This is also helpful in fall when there is a cold snap and your sweets need a little protection from cool nights.
Figuring out when to harvest is quite possibly the biggest challenge while learning to grow your own sweet taters in Canada. They do almost all of their bulking up in the last month of the growing season and if they are not allowed to finish then they will not fully develop.
Manitoba in particular has just the right amount of summer to pull off a nice sweet potato crop and to be fare, we have no time to waste. Even though the maturity date is 90-100 days, I still recommend leaving them in the ground as long as possible. Over the past 4 years this has been around in Winnipeg October 7th.
Less than spoiled
The special treatments that are discussed above are fantastic methods that you may chose to experiment with but I must stress that I have not YET provided such luxuries to my sweet potatoes.
My largest tater to date was 1 lb 2oz and was thrilled to hear of a 2.2 pounder raised by one of my customers. He did provide a cold frame for the first 2 weeks and then claimed no additional pampering. You will most likely end up with a variety of sizes from one plant and can expect to harvest 2-4 pounds.