Melanie Filotas – Horticulture IPM Specialist, Ontario Ministry of Agriculture, Food and Rural Affairs
The sweet potato is a heat-loving tropical plant native to Central and South America, with most commercial production occurring in areas with very long frost-free periods, such as the southern US. Despite its tropical origin, sweet potatoes have been commercially grown in Ontario since the late 1980s, with annual acreage fluctuating between 1500-2000 over the last ten years. Large scale commercial production of sweet potatoes is mainly concentrated in the sandy soils along the north shore of Lake Erie, particularly in Norfolk County.
Sweet potatoes are often confused with yams and white potatoes, however they are completely unrelated to both of these. True yams are dry and starchy with a tough, brown skin that looks like bark, and cannot easily be grown in Ontario. White potatoes are Solanaceous plants in the same family as tomatoes while sweet potatoes are in the morning glory family.
This distinction is important because they have very different production requirements and cannot be handled the same way. Many US extension workers have adopted the one-word term “sweet potatoes” to emphasize that they are not just a sweet version of a white potato.
Sweet potatoes take 90-120 days to reach maturity, depending on variety, and grow best with hot days (ideally 25°C or higher) and warm nights. They cannot tolerate freezing temperatures, which means they must be planted after all danger of frost has passed (typically early June), and they must be harvested before soil temperatures drop below 10-12°C to avoid chilling injury to the roots. Sweet potatoes also grow better on deep, fine sandy soils with relatively low levels of organic matter, which warm up more rapidly in the spring, and lead to more ideal root shapes. Norfolk County’s sandy soils and warmer springs are ideal for sweet potatoes.
Sweet potatoes are not planted as seed pieces, as is done with potatoes. Instead growers plant “slips” which are basically unrooted vine cuttings. These are typically brought in from propagators in the southern US and planted into pre-formed hills. If you’ve driven around Norfolk County in early June, you’ve likely seen sweet potatoes being planted. Although they don’t look very good immediately after planting, they will grow quickly with onset of hot weather, and those fields will be completely covered with vines by the end of July.
Aboveground growth doesn’t necessarily tell you what the crop will be like, however. If environmental conditions are ideal, sweet potato slips will produce several storage roots – the marketable root – per plant. However, if plants are exposed to low temperature or soil moisture, or other stresses before or after planting, they tend to produce more fibrous roots or pencil roots, which are not marketable.
Sweet potatoes harvest typically begins sometime in September. Special harvesting equipment is required to avoid damaging the delicate skin of the roots. The roots need to be cured shortly after harvesting, which involves placing them at 26-29°C and 90-95% humidity for 4-7 days. Curing is essential for healing wounds and converting the starches in the roots to sugars. Many people are not aware that a freshly harvested sweet potato is actually not sweet at harvest – it is the curing process that leads to their characteristic sweet flavour. If properly cured and stored, sweet potatoes can be stored for several months.
All of these factors mean sweet potatoes are not the easiest crop to grow. Sweet potatoes require significant investments in specialised planting and harvesting equipment, and facilities for curing and storage. They are also quite labour intensive, especially during planting, harvest and packing. Pests, particularly wireworms and grubs, can also be a challenge.
Insecticides that were previously used by growers to manage these pests have been lost as of this year due to re-evaluations and marketing decisions, although it is hoped that new ones will be registered in the future. It can also take considerable trial and error to determine fertility, irrigation and other requirements, which vary with cultivar.
So when you see sweet potatoes growing in local fields, be aware that a lot of time, effort and experience has gone into making this a successful specialty crop in Ontario.
For more information related to growing sweet potatoes and other vegetables in Ontario, consult the OMAFRA blog, ONspecialtycrops (onspecialtycrops.ca) and ONvegetables (onvegetables.com).