So where in Ontario can you grow sweet potatoes, anyways?

The sweet potato is a tropical plant native to Central and South America, with most commercial production occurring in warm regions with very long frost free periods. Given this, many people are surprised to learn that sweet potatoes are commercially grown in parts of Ontario, mostly along the northern shore of Lake Erie. Over the past few years we have received numerous calls from people in other parts of the province wondering whether sweet potatoes can be commercially produced in their area. It is difficult to give a set answer to this question. The best thing to do is to review the growing requirements of the crop and determine whether conditions in your area are appropriate. In this article, we will provide answers to some of the most common questions we receive on this topic.

What are the environmental requirements of a sweet potato?

With apologies to Anne Murray, you can think of sweet potatoes as being somewhat like snowbirds - happy to spend time in Canada as long as they're out before it gets too cold. Sweet potatoes require extended frost-free growing periods with long days and warm nights. They cannot tolerate freezing temperatures, and should not be planted until all danger of frost has passed. Sweet potatoes are also easily damaged by exposure to chilling temperatures below 10-12°C for more than a few hours, and roots need to be harvested before soil temperatures drop below these levels. These factors combine to make the growing season in Ontario run typically from early June to September. Other factors are also important, including soil type. In Ontario, sweet potatoes seem to grow better on deep, fine sandy soils with relatively low levels of organic matter and clay content. These soils warm rapidly, which may contribute to the successful production of sweet potatoes in Ontario.

The key questions are: can you produce sweet potatoes of marketable size within the frost-free season in your area and can you get your desired acreage harvested before soil temperatures drop to chilling levels?

My plant hardiness zone is the same as that of the north shore of Lake Erie - can I grow sweet potatoes commercially?

An area's climate can be mapped in several ways, including hardiness zones, heat units and length of growing season, among others. Hardiness zones are based on winter hardiness, reflecting an area's minimum temperatures for perennial species such as trees. They are not appropriate for determining whether sweet potatoes, an annual crop produced only in the summer, can be grown in an area. Crop heat units (CHUs), which use a mathematical calculation to describe daily accumulations of heat throughout the growing season, are more appropriate for use on annual vegetables. Originally developed for corn, heat units are calculated daily from planting to harvest using the daily maximum and minimum temperatures for a given area. A crop heat unit map for corn and other warm season crops in Ontario can be found in OMAFRA's Agronomy Guide for Field Crops (Publication 811) or on the OMAFRA website (www.omafra.gov.on.ca). Our best estimate is that commercial production of sweet potatoes requires 3000 CHUs or more. It may be possible to grow sweet potatoes with less heat but yields will be lower and more variable from year to year.

Does variety matter?

Absolutely. Most sweet potato cultivars available for commercial production are developed in the southern United States, where growing seasons are much longer, and consequently the majority require a minimum frost-free period of 100-125 days. However, there is considerable variation among cultivars. Short season cultivars such as Beauregard, do better in northern regions. While you can experiment with longer season cultivars (e.g. 120 days), there is a much higher risk of climate-induced crop failures.

Would plastic row covers help?

Research from the United States and elsewhere has shown that season extension tools such as black plastic, clear plastic and row covers can be an effective means of increasing soil temperature, allowing sweet potatoes to mature faster in cooler areas. However there are costs associated with buying and disposing of the plastic, as well as any specialized planting equipment needed for such a system. Furthermore, removing soil-applied plastic mulch prior to harvest is problematic and expensive. You will have to factor in these associated costs to determine whether producing sweet potatoes under plastic or row covers is economical.

If you are interested in trying to grow sweet potatoes in a more northerly climate, we recommend that you start small. Grow an acre or two to determine whether roots reach marketable size in a reasonable time period in your area, and whether you can harvest the crop before the soil reaches chilling temperatures in the fall. Remember, temperature can fluctuate considerably from year to year. This can dramatically impact yield and, in some regions, you may be able to achieve economically viable yields only in certain years.