We have been growing sweet potatoes in Saskatoon for about seven years, harvesting enough tubers to store and eat some each week until the New Year. We were introduced to the idea of growing sweet potatoes by our gardening friends, Cedric and Anne Gillott, who started growing them in 2015, and shared their experience with us.
Sweet potatoes (Ipomoea batatas) are native to tropical America and are related to the morning glory flowers that grow in many gardens. They are not closely related to our common potatoes (Solanum tuberosum) or yams (Dioscorea spp). Although the white fleshed sweet potatoes may be called yams in the grocery store, they are mis-labelled. True yams are usually grown in Africa and Asia, not in North America.
Sweet potatoes require a longer frost-free period than is normally experienced in Saskatchewan and extra care must be taken to give them warm soil and adequate water during the summer, and to protect them from frost in late spring or early fall. They are planted as shoots (slips) taken from parent plants, that have been placed in water for 5-6 days, until they have produced roots. We used to buy rooted slips from seed companies in Eastern Canada, but for the past few years we have kept plants growing indoors through the winter, and then produced slips from them in the spring. In winter 2023/24 our tubers produced small shoots, and I also started plants from these. Then, I produced slips from the plants, ready to put out in the garden in early June.
Originally, we tried about 8 of the cultivars available, but for the past few years we have grown only Covington and Japanese Yam (really a sweet potato). Covington has a brown skinned tuber with orange flesh, which I prefer to the red skinned, white fleshed Japanese Yam. This year we tried a new cultivar, Radiance, which is about 3 weeks earlier to mature than other available varieties. Radiance was bred in the Vineland Research and Innovation Centre, Ontario, and sufficient numbers of slips were produced by an Alberta company, for sale to Canadian growers.
We dig our soil bed in mid May, adding in compost, watering the soil well, laying down a soaker hose, and then covering the bed with plastic (either clear or black). This allows the soil to warm up adequately before planting. In past years we planted the slips in early June in a rectangular bed, 3.5m long and 1m wide, in a staggered pattern (about 45cm apart). A circle or slit was cut out of the plastic at regular spacing and the rooted slips were planted in the soil, through the holes in the plastic. A saucer-like depression was left around the slip, to catch water, and the edges of the plastic were held down by a layer of earth. The bed was watered frequently, every 2-3 days depending on rainfall and temperature. When there was a risk of frost the beds were covered, at the beginning and end of the season. This extends the growing season for the plants, as they require a temperature above 10°C to grow, and will be killed by frost.
This year we tried a new method, by making a raised bed, the width of 1 row, like a hilled potato row, with a flat top and slanted sides. We ran 2 soaker hoses on each side of the top of the row, covered it with a strip of plastic that was anchored with soil in the depressions each side of the row. We planted the slips in the centre of the row, through slits made in the plastic, at about 30cm apart (Photo 1). The weather was dreadful, cold and windy for about 2 weeks, and the new plants were very slow to develop. Luckily the season was long, with some hot spells, and eventually the plant tops grew vigorously kept well watered with the drip irrigation (Photo 2).
Harvest is exciting, as you really don’t know what to expect. The green leaves and stems are cut off at ground level so the plastic can be rolled back, and the soil is dug away carefully. Most plants have a cluster of tubers hanging down near the main stem. Occasionally the roots go searching for water, and then a tuber will be found away from the main plant. This year our biggest tubers were produced on the Radiance plants. These tubers were 20-30 cm and weighed 600-700 gm each (Photo 3), but usually tubers range from about 300g to unusable thin roots, about 20- 30cm long. We keep a few plant tops attached to tubers initially, so we can identify the cultivar, and save a few stems to grow on for winter plants.
Harvested tubers are placed in crates with good air circulation, but initially the crates are enclosed in large, plastic bags, for about 3 weeks at high room temperature, being careful not to allow high humidity, and moisture build-up. The bags are then removed, and the cured tubers can be kept in the crates at room temperature for the remainder of the winter. We usually eat any damaged tubers and the smaller tubers first. Some of the smaller tubers can be kept till spring when they will often sprout and can be used to produce new slips.
Mice (or voles) like the tubers and in our first year several of the biggest Japanese Yam tubers were completely hollowed out. This year the rodents again ate tubers, and interestingly they again selected Japanese Yams and did not eat the orange fleshed tubers. This may have been a chance selection, I have no data on mice preferences! We have only noticed insect pests one year when there were small green caterpillars (identified as caterpillars of the morning glory plum moth) eating new leaves. One spray of Bt insecticide was sufficient to reduce the caterpillar population significantly. We did not notice an impact on yield.
The book “Sweet Potatoes for the Home Garden” by Ken Allan, gives excellent information on growing this crop, and describes most of the older cultivars. There is also an article in The Gardener magazine that provides information on cultivation of sweet potatoes in the prairies (Summer 2017, Vol 23 Issue 2).
Jill Thomson is a plant disease specialist (retired) who enjoys gardening in Saskatoon with her family, including the dogs.
This column is provided courtesy of the Saskatchewan Perennial Society (SPS; [email protected]). Check our website () or Facebook page () for a list of upcoming gardening events.
