Growing Sweet Potatoes in Cooler Climates

Boiled sweet potatoes mixed until smooth and baked with a delicious topping of brown sugar, cinnamon, and oats, –or– fresh sweet potatoes cubed and roasted with just a bit of olive oil and sea salt. Oh my goodness, fresh sweet potatoes are a treat! My whole family looks forward to harvesting and devouring the delicious root vegetable. My son says that harvesting sweet potatoes is like a treasure hunt. “You never know how many potatoes there will be, or what size they will be.” My daughter says that she just likes to “dig and dig and dig.”

Not only are sweet potatoes delicious, they are considered one of the most healthy foods as they are an excellent source of vitamin A (214% of your daily value), and very good sources of vitamin C, vitamin B6, manganese, copper, and pantothenic acid, among others. Anthocyanin (a color-related pigment) in sweet potatoes is equally valuable for it’s anti-inflammatory health benefits. Okay, you get the point. Sweet potatoes are delicious and healthy. But, can we grow them here in MN or in other cool climates?

Well, I’m so glad you asked! Yes, of course you can, but it takes just a little more work. Sweet potatoes are tropical plants, so they like hot weather, hot soil, and are very sensitive to frost. Here in MN, we have cooler weather with shorter growing seasons, so we need to do a little extra work to ensure that our sweet potato plants are happy enough to grow those delicious and healthy tubers. So, here it goes:

Starting Slips

Starting a sweet potato plant is probably one of the most easy and inexpensive vegetables to start. You do not start sweet potato plants from seed like most vegetables, you start them from what is called a seed potato (a firm sweet potato that is 1-2″ in diameter). Poke two to four toothpicks into the center of the sweet potato (on opposite sides), place the sweet potato halfway into a glass jar, and fill the jar with water until 1-2″ of the bottom of the sweet potato is submerged in water. Place the sweet potato under a fluorescent light (for up to 14 hours per day) or in a bright window. You will soon see roots starting to form on the submerged portion of the sweet potato followed by what are called sweet potato slips or shoots sprouting from the upper part of the sweet potato. Once the slips are 4-5″ long, gently twist or pinch the slip off of the sweet potato right where it connects to the sweet potato and place the slip into a jar of water until roots form (do not submerge the leaves as they will rot).

Preparing the Garden

As far as soil quality goes, sweet potatoes are very forgiving. They don’t need nutrient-dense soil. In fact, too much nitrogen will cause too much foliage growth which will inhibit tuber or root develop. So, if you do fertilize, choose a fertilizer with lower amounts of nitrogen. Also, sweet potatoes can have trouble in soil that contains a lot of clay (which is one of our problems here) by growing skinny and misshapen roots. If you have a lot of clay in your soil, a raised bed can do wonders for those sweet tubers.

Heat! Sweet potatoes love heat. The more 100F days, the better as far as sweet potatoes are concerned. The slips also love to be planted in warm soil (75F soil or warmer is best). To achieve that here in MN, we cover our raised bed with clear plastic sheeting and tuck the sides down between the bricks and the soil for a few weeks before we plant. Also, sweet potato plants do not like temperatures below 55F, so we typically plant 2-3 weeks after our average last frost date. June 1st is a good estimate of the time we typically plant sweet potatoes here in Minnesota. 

Once the sweet potatoes are ready to plant, you can make slits in the plastic sheeting (we like to make a cross with slits about 3″ in length), poke a couple of finger down into the soil, and gently lower the slip down into the hole.

Feel free to plant slips all the way to the top leaves as sweet potato slips will grow roots on the entire plant stem if plant in the ground. Fill the soil in around the plant and water (we find it easiest to lay a soaker hose under the plastic before planting). We started our slips pretty early this year, so we did plant some slips in 4″ pots indoors. Because of this, we planted these sweet potato plants before re-applying the plastic as large holes in the plastic would have been needed to plant, thus causing a lot of heat to escape throughout the season.

Once planted, the slips should be watered every day for a week. Once the plants are established they will need about 1″ of water per week until 2-3 weeks before harvest time.

Harvest Time

Sweet potatoes usually mature in 90-100 days. Harvest them when leaves start to turn yellow, and/or before the threat of frost occurs. Two weeks before harvesting, discontinue watering, as harvesting is much easier in dryer soil. When you are ready to harvest, gently dig around the sweet potato plant with a pitch fork or shovel trying not to pierce the tubers. Digging works best when you start on the outside of your garden and work inward toward the center. Sweet potatoes need to cure if you want the best sweet, rich, sweet-potato taste. Perfect curing conditions are 90% humidity with a temperature of 85F. After two weeks of curing, sweet potatoes are best stored in cool, dark areas with temperatures between 55F and 60F and humidity at 85-90%. Unheated basements work well for storage, and sweet potatoes should store for six months or more! We still have perfectly edible sweet potatoes in our basement that have been stored for nine months now. Talk about a long shelf life! 

Go ahead and try growing sweet potatoes, even if you live in cooler climates! The resulting harvest is one of delectable deliciousness and longevity (if you don’t eat them all right away). Happy planting!