Growing Sweet Potatoes

If you are looking to expand your garden repertoire and grow a vegetable that is delicious and can be stored for months, look no further than the sweet potato. While many associate this vegetable with South America, a gardener in Kingston can grow a satisfying crop of these veggies with a little work and some cooperation from Mother Nature. A little forethought about location is time well spent though: these plants absolutely love sunshine and heat and, when mature, the vines will spread over a 6’ wide area. 

Spreading sweet potato vines
Spreading sweet potato vines

Sweet potatoes are easily grown from a section of a sweet potato vine, called a slip. Some local greenhouses sell sweet potato slips in the spring which are ready to plant when the soil is warm, usually by the end of May in Kingston. On the other hand, if you are lucky enough to know someone who grows sweet potatoes, ask them for 2 or 3 in mid-April and you can grow your own slips.

Starting sweet potatoes: 

  • If you only want a few plants, then all you need to do is partially submerge a tuber in some water and wait for roots and sprouts to grow. 
  • If you would like to grow many plants, then an easier method is to plant tubers in moist potting soil in a 2-3” high flat. The flats need to be kept warm – between 20-30°C – in order for the sprouting to occur. 
  • Once the tubers sprout, place the flat in a sunny area to encourage healthy, green, sprout growth. Once the sprouts are about 8”, pull them out of the flat. Some sprouts will just break off the parent tuber, while others will have attached roots. Not a problem: place all of the sprouts, with or without roots, in a jar of water and roots will grow quickly. Keep the water topped up.
  • At the end of May or beginning of June, when the garden soil 6” below the soil surface feels warm to the touch, it’s safe to plant your slips. (If the spring has been cold, then you may need to pre-warm your soil with plastic that has been sealed tightly around your garden bed and plant your slips through slits in the plastic. A comprehensive explanation of all things sweet potato can be found in Kingstonian Ken Allen’s book Sweet Potatoes for the Home Garden). The slips should now be about 12” long with many nodes along the length of the slip. 
  • Plant the slips horizontally in a trench about 3-6” deep with 1/2 of the slip under the soil and 1/2 above the soil with at least 2 nodes underground. Plant each slip 12-24” apart in a row 3’ wide, mulch with straw and water well. 

How to harvest:

  • Sweet potatoes need a long, hot growing season, so, to maximize the season, I harvest immediately after the first frost kills the vines. However, sweet potatoes are sensitive to cold, so harvesting must happen before the soil temperature drops below 10°C.
Sweet potato vines make an attractive ground cover
A carpet of lush, green leaves
  • Remove the vines to expose the soil and carefully dig up the sweet potatoes with a broad fork or shovel. You will find that most of the tubers are within 2 feet of the main stem. Harvesting is a lot of fun since you never know what you will find: thumb-sized delicacies or football-sized behemoths?
Harvesting sweet potatoes can be a lot of fun!
Harvesting sweet potatoes is fun!
  • Even though the sweet potatoes are firm, treat them like eggs, as any damage done now will result in bruising and rot weeks later.
  • Rinse the sweet potatoes gently in warm water and air dry indoors.

Curing and storage:

  • Your harvest now needs to be cured in order to reduce moisture loss, heal cuts and scrapes, and convert starch to sugars. The recommended curing environment is one of high temperature (30-32°C) and high humidity (80-90%) for 5-7 days. 
  • Here is where you can get creative! For a small number of potatoes, you can use a paper bag to put the sweet potatoes in and put the bag in the oven with the pilot light turned on. For a large amount, a bathroom with a shower and independent thermostat provides a nice, warm, humid environment for curing. I layer my sweet potatoes on metal racks in a large, plastic tote with a lid. I put about an inch of water at the bottom of the tote and pop the tote in a bathroom with the thermostat turned up. 
  • After the curing period, the sweet potatoes can be stored in a loosely covered tote for months at room temperature. Be patient though; the best flavour develops after at least a month of storage. I frequently have sweet potatoes left over in May that are still firm and tasty with only a few sprouts.

Sweet potatoes add interest to my garden by covering a part of it in a gorgeous carpet of green, heart-shaped leaves that look lovely all summer long. The plants spread quickly and outcompete any weeds. The real magic happens underground where tubers busily grow for months, creating a rich harvest that feeds my family for a year. Try them once and you’ll be hooked forever!