I had always assumed that sweet potatoes were something that only grew as far north as Georgia. But I love sweet potatoes, so last winter I started some research on-line. I was pleasantly surprised when I stumbled upon an article in Mother Earth News written by Ken Allan from Kingston, Ontario. Ken assured readers that anyone could grow a great crop as long as they understood what sweet potatoes need to grow in cold climates. His writing was very thorough and easy to understand (unlike many internet sources which are vague or incomplete), so I ordered his book “Sweet Potatoes for the Home Garden (with special Techniques for Northern Growers)” As soon as Ken’s book arrived I read it cover to cover. It was the best $20.00 (including shipping!) I had ever spent. Could it really be that easy to grow sweet potatoes? When I got to the section where Ken wrote that there’s no reason on earth to add brown sugar or molasses (or marshmallows!) to properly grown, harvested and cured sweet potatoes, I was hooked. (I had never understood why anyone would do such a thing.) I decided to follow his instructions to the letter. Mike had already identified Mad Dog Farm as a Canadian source for sweet potato slips, so we placed an order. Unfortunately, due to health issues, they were not able to fulfil the order and refunded our payment. I was crestfallen – so close and yet so far. Luckily, Mike found another source – Ottawa Gardener who generously offered some of her seedlings. We were going on a road trip to Ottawa in the middle of May and stopped by to pick up her precious slips, leaving behind some Apios americana (something she had been searching for) and a few surprises – Autumn olive and a viciously thorny but prolifically fruiting blackberry.
I now had slips from four varieties: Fraser White, Georgia Jet, Ginseng Red and Tainung 65. I decided to grow one more to be on the safe side and grew a slip from a supermarket sweet potato. It was easy – support a section of sweet potato with three toothpicks and submerge the bottom in a glass of water – just like you used to do with an avocado pit when you were a kid. Keep in a warm place (surprisingly, Ken advises not to put them in a sunny window – although it will be warm during the day, the window will be the coldest part of the room at night) and wait for slips to sprout from the eye(s), just like a regular potato. Once mine sprouted and were about 3″ tall, I held the slips at their base and snapped it off the sweet potato. Then I put the slips in a glass of water, just like I had with I had picked up from Ottawa Gardener. In a few weeks, all the slips were at least 6″ tall and had good roots. I was excited, but it was still too cold to put them outside. Following Ken Allan’s detailed instructions, I had prepared one of our raised beds (3″x 12″) by double digging in last year’s grass clippings and compost, breaking up clumps and making sure the soil was light and friable. I had grown regular potatoes last year in compacted soil, ending up with small, deformed potatoes (including one with an uncanny resemblance to a rubber ducky) and was determined not have the same thing happen to the sweet potatoes. I marked where the sweet potatoes would be (12″ between each plant) and dug shallow indentations, which would collect water at the base of the plants. I then covered the entire surface of the soil with 6ml vapour barrier, weighing down the edges with scrap 2″ by 3″ wood and marking the location of each indentation. This increased the temperature of the soil as well as blocked weeds. In fact, the only weeds I had to deal with were at the edges where there were gaps in the wood. I put a soil thermometer in the centre of the bed and waited for the soil to warm up, which was actually the hardest part of the whole exercise!
Finally, by the second week in June, the soil had reached 74°F and I could plant the slips. I cut X’s in the plastic, planted nine slips, leaving only two to three leaves above ground, which would encourage tubers to form along the underground stem. I weighed down the area around each plant with sand.
Throughout the summer I hand-watered each plant every two days. This was to supplement the measly two days of rain at the end of July and another two days’ worth mid-August. I emptied water from our rain barrels (which caught water from our roof) into a large bucket near the sweet potato bed, which warmed up the water – no use chilling the roots after all the time that went into warming them up! As predicted, nothing happened above ground for the first few weeks as the roots settled in and grew. By July 1st, the above-ground slips started growing
and eventually the leaves covered the bed by mid-August.
One day while watering, I pulled the leaves back and discovered beautiful white and purple flowers on the Georgia Jet plants. There were never more than a dozen of them and they remained hidden under the foliage, which was a shame because we were the only ones who got to see them.
In August, I decided to put a trellis at the end of the bed to encourage more growth in the southern-most plant – the more leaves you have, the more the tubers grow. Throughout the summer, I monitored the soil temperature every two days. The highest temperature reading was 90°F in August (it never went below 80°F). Sweet potatoes don’t begin to suffer until the soil reaches 115°F, but I didn’t have to worry about that! By the end of August, the plants were overflowing the bed. I continued watering every two days. Happily, the plants weren’t bothered by any pests, although something was chewing small holes in the leaves. Apparently we’re too far north – so there is an advantage to growing in Canadian zone 5a!
Once September arrived, I kept a close eye on the soil temperature – sweet potatoes take about 90 days to mature, but the tubers stop growing in soil temperatures lower than 60°F and can be damaged by lower temperatures. Growing sweet potatoes is an act of supreme hope – you have no idea of how good your crop is until you dig the tubers. And unlike other root vegetables such as carrots or garlic, it’s difficult to dig one or two to check on their progress without disturbing the rest of the plant. Unfortunately, size and number of leaves don’t give you a clue as to how the tubers are growing underground. By the third week in September, the temperature was dropping and I was covering the bed with an old sleeping bag on chilly nights.
Eventually, though, the temperature dropped below 65°F and it was time to harvest, which we did on September 24th. Afraid of spearing unseen tubers (I had no idea what their spreading pattern would be), I started digging by hand with my Japanese hori knife. Mike soon rescued me and began gingerly digging with a fork to loosen the soil and unearthed the tubers.
I couldn’t have been happier with the results. The nine plants produced thirty pounds of sweet potatoes. The largest one dwarfed my hand and weighed 1-3/4 pounds. The last – and most important – task was the cure the tubers. I had followed Ken Allan’s growing directions and saw no reason to abandon his counsel at this stage, especially as proper curing allows a layer of suberin – like a second skin – to form, which Ken likens to an organic plastic baggy. This protects them in storage as well as improves the flavour. (I finally discovered the reason that so many recipes call for added sweetener is because many store-bought sweet potatoes are not properly cured, so their starch is not converted to sugar.) Ken recommends curing them at 85°-90°F and 80% to 90% humidity for a week. I carefully washed the tubers and placed them in stacking boxes I rescued from a supermarket, covered them with large blue recycling bags, and put them in a closet in our downstairs bathroom with a space heater set on maximum temperature. I could only reach about 80°F to 85°F, so I extended the curing time to two weeks. Despite a pan of water in the closet and draping wrung-out towels over the boxes, I never could get the humidity to more than 75%. But the tubers neither dried out nor got mouldy, so I’m hoping that was sufficient.
After two weeks, I moved the boxes to a spare bedroom in the basement, where the temperature was about 70°F. After a month, I noticed that some tubers were sprouting, so rubbed off the sprouts and moved them into an unheated closet which is about 65°F. As of the end of November, the tubers are no longer sprouting and are still firm. I continue to monitor them weekly. Once properly cured, sweet potatoes should last in storage for six months, with the flavour fully developing by Christmas. I’ll have to be patient a bit longer to enjoy the results of the careful planting, harvesting and curing of my sweet potatoes. I certainly don’t want to waste all this time and effort by rushing things at the very end.
More pictures here ……………