Growing Sweet Potatoes in a Northern Garden
Generally when I describe my garden to someone I tell them that I am on the northern edge of a southern gardening plant zone. I usually use hosta as my clarifying example of what I am talking about. I can grow all of the hot higher light level hosta such as plantagenia and Guacamole and Calamity Jane while I have difficulty growing the cooler hosta like the sieboldianas and montanas. Unless you are a hosta nut like me this won’t mean much. Let me describe my plant growing situation this way. I have all the heat and humidity one finds in southern parts of the country. What I lack is the number of days this weather is available. I have about a month less growing time over the course of the season than say someone growing plants in North Carolina or north central California. This means I have to tweak how I grow some southern delicacies such as the beautiful sweet potato.
What are Sweet Potatoes?
Sweet potatoes as we know them in the United States are in the genus Ipomoea. This genus includes the common morning glory. In fact the flower for a sweet potato looks exactly like the annual you planted by the deck trellis this spring. It only lasts for the morning too. Sweet potatoes because of the tuber they produce are considered perennials while the seed you planted for the flower is an annual. When I grew up we usually called these yams. A true yam is not related to the lovely sweet potato. I will try to only call these starchy vegetables sweet potatoes.
Propagating Sweet Potatoes
There are two ways to plant sweet potatoes. One way is to start a piece of the sweet potato in a glass of water. I have memories from my childhood doing this at home and in grade school. It is easy to do. The easiest way is to cut a chunk from the sweet potato. Then using toothpicks stuck around the sides, one suspends the cut part in clean water and the uncut part is held above the water. If you can tell the growing end of a sweet potato from the rooting end this facilitates your efforts. This can sometimes be hard to do. Generally the round end is more likely to be the growing end than the "pointy end. You may want to consider this method. Save only the growing end and sauté the rest of the tuber in some butter for dinner.
I make my own starts in the fall for next year’s planting. This prompted me to write about this today instead of a spring hub. It is early September and I have just started my cuttings for next year. I place the end of a vine on top of a pot of soil with maybe 6 inches of the growing tip outside of the pot. I next cover the vine with a bit of soil and hold all in place with a small rock. This keeps the vine from jumping out of the pot. I do this about a month before I know a killing frost is due. This is enough time for the vine to send down roots where it is covered by dirt. Then when I know a killing frost is due I cut the vine and take the pot into the house for the winter. Place these cuttings in a very sunny window or under lights. The other way is to order rooted plants from a specialty nursery. This can be a bit expensive and sometimes a rooted cutting will fail during shipment. Most supply houses include an extra start or two just to insure against this problem.
No matter whether you make starts or try to get a cut piece of the tuber to root you need to remember that if you keep your house cold like me you may lose them. I generally keep my house about 62 in the day or whenever I am not at home. These plants don’t really care for this temperature. Most make it through the winter, though they are not happy about it.
Sometimes there are some really small tubers at the end of the season. These I frequently save for next year too. Many cooks believe these to be more tender and flavorful than a full size tuber. They are too small and slim for me to consider using them for food. To “over winter” them you should pack them in a pot with dry potting mix. You don’t really need to water these though I often water them a couple of times over the winter. It keeps them from dehydrating too badly. A plump happy tuber is less likely to fail when they are planted out in the spring. My cold house does not encourage growth. BTW this is how one should save their dahlia or cannas too. Just don’t let the soil stay too wet too long or the tubers will rot.
You will want to wait until about a week before a killing frost to dig and retrieve your sweet potatoes. Southern gardeners are able to dig their crop before and after the times a northern gardener should do this. I have waited until after a killing frost to dig and this is ok. You do want to dig your crop before the ground gets too cold. Certainly don’t let the ground freeze or you will lose your sweet potatoes. They do not like to be frozen. Be sure to wash them thoroughly. Store them in a cool well ventilated area. Use them as soon as you can. I don’t count on them lasting until spring. . . . As if they wouldn’t be eaten by then!
I have found that these plants produce growth twice during the growing season. When I first put these out in the spring there is a flurry of activity and the vines can grow fairly long. It is not uncommon to get vines that grow a good 4 feet away from where the start or rooted piece was planted. Then for a couple of months it just seems to sit there and not grow. Then about a month before killing frost the vines begin to grow again quite rapidly. That is your clue to root your cuttings and not the note you left yourself on a calendar app.
Black Plastic Traps the Heat You Need
Sweet potatoes require more than a hundred growing days to produce large beautiful tubers. This presents the greatest challenge, even for me with my mostly southern environment. Also, my soil is a rather heavy loam. It is too heavy for many vegetables. So, I solve both of these problems by planting my sweet potatoes in large black plastic containers. The container in this picture is about a 40 gallon container. I put a nice loose friable potting mix in the container. The sun heats the container to a higher degree than it would in the soil.
An alternate solution would be to spread some black plastic on top of the soil. Then cut some slits in the plastic about 2 or 3 feet apart. Since most people do this to grow their cantaloupe and watermelon this may not seem so strange. You can leave the plastic down for a couple of years, rotating melons and sweet potatoes in the same area. The plastic captures and warms the soil well and helps hold moisture too. The best is that you don’t have to weed with the plastic method either!
I tend to do nearly all my fertilizing in the spring. A plain 10-10-10 or 12-12-12 general fertilizer works just fine for me. I don’t fertilize anymore after I notice the vines stop growing the first time, although, I do and will give them some homebrewed compost tea should I have some extra. Many prefer to use only an organic fertilizer since the tubers will collect whatever is fed to the plants. I’m one of those who believe that a plant can’t tell whether the nitrogen it is absorbing came from an organic or inorganic source.
You will of course want to make sure to water once a week if it doesn’t rain. Sometimes people forget to water. I use the plant again to let me know what it needs. While most plants will “curl” in the hot sun, when a sweet potato does this it means it wants water. Remember these like heat.
I feel herbicides and pesticides are more serious and should be avoided. I rarely have any issue with pests. Sometimes in the spring I will notice some holes in the leaves now and again but nothing major. I have had problems with deer and or rabbits. I had vines chewed off in the spring. Protecting your newly planted vines in the spring may require some extra watching to make sure they are not eaten. You may need to use some chicken wire to net your crop. Steamed sweet potato leaves are fast becoming a culinary offering. So, it is understandable you may have some trouble with larger herbivores. I tried a trick my mom uses to keep deer out of her hosta quite successfully. She uses shavings of Irish Springs hand soap. I did the same for the sweet potatoes. I put some shavings in some small plastic containers and set them on top of the soil in the container with the plants. You may have seen them in the picture for this article. The damage stopped right then. I had no further problem from macro pests eating my vines.
See how easy it is to grow sweet potatoes? There is really only the black plastic to remember. Otherwise you will be growing these along with your tomatoes, green beans, corn and other favorite vegetables. Nothing tastes better than a fresh vegetable or fruit from your own garden.