Dogwood Garden Talk on Sweet Potatoes, July 2016, by Jennifer Krauss
I am going to talk about sweet potatoes. This is what I am taking about-cut. You might call them yams, but for tonight, I am calling these sweet potatoes.
Why would you want to grow sweet potatoes? Because sweet potatoes are a southern crop, so it's all about the challenge and the thrill of growing something that we don't expect to be able to grow in the north! My relationship with sweet potatoes is somewhat complex. But if I had to summarize it in a nutshell, I'd like to quote from the famous poet, Popeye the Sailor Man – "I yam what I yam".
I've been growing sweet potatoes for 16 years in my Coquitlam garden. The amount of harvest can vary dramatically from year to year, often for no discernible reason. Last year we had a hot summer and I got my best harvest ever – 88 pounds from 24 plants. I couldn't believe it. This year my plants had difficulty surviving the hardening off process, so I don't know if I'll get much harvest at all! Such is gardening!
I started off years ago by phoning Brian Minter's Country Garden Store in Chilliwack, probably in January, and asking them to order in some sweet potato plants for me. I made sure to say I didn't want the decorative plants, I wanted the vegetable plant. The variety to buy is Georgia Jet. This is the variety which does well in our northern gardens.
Then in April I drove out to Minter's store and picked them up. They were just plants in pots. I bought ten pots. Then I planted the plants in my garden. That was in 2001. By saving some tubers each year, I've been able to keep the process going year after year.
So let's get started. The most important thing to remember about sweet potatoes is that you cannot give them too much heat. So think heat, heat, heat. Hat on.
From last year's harvest, I saved some of the smaller tubers to plant for this year. The size of the tuber you plant does not affect the size of the crop. I picked small ones because they're too small to bother cooking, but also because they fit into a plant pot!
APRIL: So, sometime in the second week of April, take a pot, put some soil in it – say 2-3 inches – quality of soil doesn't matter because the tuber is going to provide all the nutrition for the plant – put the tuber on top and add another inch of soil. Water. The tuber is going to send up shoots which will become long vines. Pass around potatoes.
Now comes the important part. You must give the tuber some heat. If you don't, the tuber will just sit in the pot and take a vacation. Nothing will happen. So you have to put the tuber 'in the mood'! You must put the pot in a warm place, preferably between 27–32 degrees C (80–90 degrees F). I have a spare room which is heated by electricity separately from the rest of the house, so I crank up the heat in that room. In the past, though, for a few plants, I used a cooler with a 40 watt bulb in it, a thermometer inside as well, and a piece of wood to keep the lid open slightly so it did not overheat or melt. I kept this by my bed so I could keep a close eye on it as I considered it somewhat of a fire hazard!
With enough heat, sprouts should appear in 1–2 weeks.
Once the sprouts appear, move the pots to a sunny location. Warmth is now not as critical but should still be above 20 degree C (70 degrees F). Water occasionally when the soil dries out.
MAY: In May, prepare a warm garden bed for the plants. You can also experiment with containers. Find the hottest, sunniest spot in your garden. Mix in compost and manure and whatever else you put in, soak the soil, then cover with a sheet of clear plastic well tucked in. Show bowl with plastic. As the manure and compost rot and the sun shines through the plastic, the soil will heat up. That's the plan, anyway. The plastic I use is a vapour barrier which is 0.150 mm thick (.006 inches thick). It comes in a big roll and I cut it to fit my garden bed with overlap to tuck in. It can be reused. I bought it at Rona or Home Depot. The plastic is very important for gardens in the north as it really helps to warm up the soil. It must be clear.
So now it's the third week in May. The vines should be about three feet long or more. Show plant. Now is the time to start hardening off the plants.
JUNE: Aim to plant the vines on June 1st, and not before. A few days after is okay. The plants need as long a growing season as possible, but they also can't take the cold. June 1st seems to be a good date.
So on June 1st, mark out on your plastic covered bed where you are going to plant the plants. Plan to put them in one foot or more apart. Don't go less than this. I put down stones to mark the spots.
Choose the longest slips for planting. Hold the tuber down with one hand and grasp the slip at its base with the other hand and yank it out. It may or may not have any roots attached – probably not but it doesn't matter.
Take a pair of scissors and cut a slit about 8 inches long in the plastic where you want to plant the plant. Stir in the fertilizers you use. Put in the vine. Bury the stem vertically, no more than 6 inches deep, or horizontally, your choice. Remember about heat – the soil is cooler the deeper you go. Water. Try to keep the slit closed with sand or dirt or rocks to keep the heat in. Water frequently the first two weeks. The vines may look sick at first, but they put out roots quickly and soon you will see some new green growth. The plants can take drought very well, but to keep growth going, it is best to water once in a while. Warm water is best if possible.
OCTOBER: Harvest October first. Apparently, sweet potatoes don't mature, they just stop growing when the soil temperature drops below 16 degrees C (60 degrees F). They start rotting once the soil temperature goes below 10–13 degrees C (55–50 degrees F). Bearing this in mind, October first in general seems to be a good time to harvest the potatoes.
Sweet potatoes should be cured starting on the day they are harvested. Curing develops the flavour and ensures good storage. The best conditions for curing are 30–32 degrees C (85–90 degrees F) for 5 to 7 days. If the temperature is reduced to 27–30 degrees C (80–85 degrees F) then it takes 10-–14 days. Again, you'll have to figure out a way to create these conditions in your home.
I cut off the vines, remove the plastic, and dig the tubers out carefully by hand. At this stage they are quite 'crisp' like a cucumber so it is easy to accidentally break one, that's why I do it by hand. I brush most of the dirt off and let the potatoes dry out in the sun a little. Just for fun, I weigh my harvest. Then I put them into paper bags, fold the bags up and put them in cardboard boxes, put a blanket over the box and put them in the warm area I've prepared for curing. You want them loosely wrapped to provide some air circulation. After the curing is over you can move the potatoes to a cool area of the house but not below 13 degrees C (55 degrees F). Keep them in the bags and boxes that you cured them in. Do not store in the refrigerator! I store mine in the basement but off the floor. Now you can start eating them. If properly cured, they will keep for months until the next planting season!
My information comes from a wonderful book called Sweet Potatoes for the Home Garden by Ken Allan. Ken Allan is a grower in Kingston, Ontario. He wrote this book specifically for growing sweet potatoes in the north. This is the sweet potato bible as far as I'm concerned. This book is a treasure trove of information and I am not going to lend it out! I keep going back and back to this book, and every time I learn something new. I bought it at Brian Minter's Country Garden Store in Chilliwack. But – the book came out in 1998 and so is now 18 years old and appears to be out of print. But it is available as a used copy on Amazon.com. I've done some searching and can't find anything more recent on growing sweet potatoes in the north so my opinion is that Ken Allan's book still stands as an authority on the subject. I did find that Ken Allan had written an article for the magazine Mother Earth News in 2011, so I've made some copies and if anyone is really interested in trying sweet potatoes you can take a copy. I've also got copies of what I've said tonight.
Why would you want to grow sweet potatoes? Because sweet potatoes are a southern crop, so it's all about the challenge and the thrill of growing something that we don't expect to be able to grow in the north! My relationship with sweet potatoes is somewhat complex. But if I had to summarize it in a nutshell, I'd like to quote from the famous poet, Popeye the Sailor Man – "I yam what I yam".
I've been growing sweet potatoes for 16 years in my Coquitlam garden. The amount of harvest can vary dramatically from year to year, often for no discernible reason. Last year we had a hot summer and I got my best harvest ever – 88 pounds from 24 plants. I couldn't believe it. This year my plants had difficulty surviving the hardening off process, so I don't know if I'll get much harvest at all! Such is gardening!
I started off years ago by phoning Brian Minter's Country Garden Store in Chilliwack, probably in January, and asking them to order in some sweet potato plants for me. I made sure to say I didn't want the decorative plants, I wanted the vegetable plant. The variety to buy is Georgia Jet. This is the variety which does well in our northern gardens.
Then in April I drove out to Minter's store and picked them up. They were just plants in pots. I bought ten pots. Then I planted the plants in my garden. That was in 2001. By saving some tubers each year, I've been able to keep the process going year after year.
So let's get started. The most important thing to remember about sweet potatoes is that you cannot give them too much heat. So think heat, heat, heat. Hat on.
From last year's harvest, I saved some of the smaller tubers to plant for this year. The size of the tuber you plant does not affect the size of the crop. I picked small ones because they're too small to bother cooking, but also because they fit into a plant pot!
APRIL: So, sometime in the second week of April, take a pot, put some soil in it – say 2-3 inches – quality of soil doesn't matter because the tuber is going to provide all the nutrition for the plant – put the tuber on top and add another inch of soil. Water. The tuber is going to send up shoots which will become long vines. Pass around potatoes.
Now comes the important part. You must give the tuber some heat. If you don't, the tuber will just sit in the pot and take a vacation. Nothing will happen. So you have to put the tuber 'in the mood'! You must put the pot in a warm place, preferably between 27–32 degrees C (80–90 degrees F). I have a spare room which is heated by electricity separately from the rest of the house, so I crank up the heat in that room. In the past, though, for a few plants, I used a cooler with a 40 watt bulb in it, a thermometer inside as well, and a piece of wood to keep the lid open slightly so it did not overheat or melt. I kept this by my bed so I could keep a close eye on it as I considered it somewhat of a fire hazard!
With enough heat, sprouts should appear in 1–2 weeks.
Once the sprouts appear, move the pots to a sunny location. Warmth is now not as critical but should still be above 20 degree C (70 degrees F). Water occasionally when the soil dries out.
MAY: In May, prepare a warm garden bed for the plants. You can also experiment with containers. Find the hottest, sunniest spot in your garden. Mix in compost and manure and whatever else you put in, soak the soil, then cover with a sheet of clear plastic well tucked in. Show bowl with plastic. As the manure and compost rot and the sun shines through the plastic, the soil will heat up. That's the plan, anyway. The plastic I use is a vapour barrier which is 0.150 mm thick (.006 inches thick). It comes in a big roll and I cut it to fit my garden bed with overlap to tuck in. It can be reused. I bought it at Rona or Home Depot. The plastic is very important for gardens in the north as it really helps to warm up the soil. It must be clear.
So now it's the third week in May. The vines should be about three feet long or more. Show plant. Now is the time to start hardening off the plants.
JUNE: Aim to plant the vines on June 1st, and not before. A few days after is okay. The plants need as long a growing season as possible, but they also can't take the cold. June 1st seems to be a good date.
So on June 1st, mark out on your plastic covered bed where you are going to plant the plants. Plan to put them in one foot or more apart. Don't go less than this. I put down stones to mark the spots.
Choose the longest slips for planting. Hold the tuber down with one hand and grasp the slip at its base with the other hand and yank it out. It may or may not have any roots attached – probably not but it doesn't matter.
Take a pair of scissors and cut a slit about 8 inches long in the plastic where you want to plant the plant. Stir in the fertilizers you use. Put in the vine. Bury the stem vertically, no more than 6 inches deep, or horizontally, your choice. Remember about heat – the soil is cooler the deeper you go. Water. Try to keep the slit closed with sand or dirt or rocks to keep the heat in. Water frequently the first two weeks. The vines may look sick at first, but they put out roots quickly and soon you will see some new green growth. The plants can take drought very well, but to keep growth going, it is best to water once in a while. Warm water is best if possible.
OCTOBER: Harvest October first. Apparently, sweet potatoes don't mature, they just stop growing when the soil temperature drops below 16 degrees C (60 degrees F). They start rotting once the soil temperature goes below 10–13 degrees C (55–50 degrees F). Bearing this in mind, October first in general seems to be a good time to harvest the potatoes.
Sweet potatoes should be cured starting on the day they are harvested. Curing develops the flavour and ensures good storage. The best conditions for curing are 30–32 degrees C (85–90 degrees F) for 5 to 7 days. If the temperature is reduced to 27–30 degrees C (80–85 degrees F) then it takes 10-–14 days. Again, you'll have to figure out a way to create these conditions in your home.
I cut off the vines, remove the plastic, and dig the tubers out carefully by hand. At this stage they are quite 'crisp' like a cucumber so it is easy to accidentally break one, that's why I do it by hand. I brush most of the dirt off and let the potatoes dry out in the sun a little. Just for fun, I weigh my harvest. Then I put them into paper bags, fold the bags up and put them in cardboard boxes, put a blanket over the box and put them in the warm area I've prepared for curing. You want them loosely wrapped to provide some air circulation. After the curing is over you can move the potatoes to a cool area of the house but not below 13 degrees C (55 degrees F). Keep them in the bags and boxes that you cured them in. Do not store in the refrigerator! I store mine in the basement but off the floor. Now you can start eating them. If properly cured, they will keep for months until the next planting season!
My information comes from a wonderful book called Sweet Potatoes for the Home Garden by Ken Allan. Ken Allan is a grower in Kingston, Ontario. He wrote this book specifically for growing sweet potatoes in the north. This is the sweet potato bible as far as I'm concerned. This book is a treasure trove of information and I am not going to lend it out! I keep going back and back to this book, and every time I learn something new. I bought it at Brian Minter's Country Garden Store in Chilliwack. But – the book came out in 1998 and so is now 18 years old and appears to be out of print. But it is available as a used copy on Amazon.com. I've done some searching and can't find anything more recent on growing sweet potatoes in the north so my opinion is that Ken Allan's book still stands as an authority on the subject. I did find that Ken Allan had written an article for the magazine Mother Earth News in 2011, so I've made some copies and if anyone is really interested in trying sweet potatoes you can take a copy. I've also got copies of what I've said tonight.