Days to germination: Not grown from seed
Days to harvest: 90 to 120 days
Light requirements: Full sun, or partial shade
Water requirements: Consistent watering
Soil: Fertile and well-drained, low nitrogen
Container: Definitely not
First of all, sweet potatoes are not the same as yams. In North America, the terms are used interchangeably but they are two very different vegetables from different plants. Most sweet potatoes have orange flesh though there is some varieties that produce yellow or purple potatoes for something unique on the table.
Cooler climates are not well-suited for growing sweet potatoes but they are otherwise easy to grow if you have 4 months of very warm weather. This is one of the reasons that sweet potatoes are a common crop in the southern United State. Some varieties will mature in only 3 months, such as Georgia Jet or Centennial which would do better farther north.
Sweet potatoes are usually cooked, but the raw flesh can be added for an interesting addition to a salad or made into crunchy “chips”. The sweet root vegetable very high in vitamin A, and are also a good source for vitamin C, manganese and copper. You will also get some iron in there too.
Starting from Seed
Gardeners don’t usually start sweet potatoes from seed, but rather from “slips” which are really just pieces of potatoes that have started to sprout. It’s basically the same principle as with using seed potatoes when you grow regular potatoes.
You can start your own slips with store-bought potatoes but they can take up to 6 weeks to develop so most home gardeners buy them ready-to-plant. If you want to give it a try, take a supermarket sweet potato and put it in a jar or glass of water so that the rounded end is under water. You can hold it in place with a few toothpicks stuck into the sides to keep the potato from falling into the glass. Picture an old grade-school science experiment.
It may seem silly, but many gardeners swear by this method. Start about 6 weeks before you are going to put them out. The potato should sprout some slips. You can either separate the sprouts or just plant the whole potato.
Slips should go out in the garden after the soil has started to warm up, about 2 weeks after the last frost. Laying out black plastic can help speed up the warming process. Even though their vines take up a lot of space (see below), you can plant your slips about 2 feet apart.
You plant your slips just like you would a seedling, leaving the leaves above ground level. They may or may not still be attached to a piece of sweet potato. Purchased slips are usually just the sprouted part (no potato attached).
Sweet potatoes will vine up to 20 feet and will produce tubers underground all along the length of the vines. As they grow, they root down when they touch the soil and produce a tuber. Then the vines just keep growing. So you can’t grow sweet potatoes on a trellis and you shouldn’t locate your plants where the vines will sprawl outside the garden area (like the lawn).
It also means that you shouldn’t try to move the vines once they’ve grown or you might uproot a potential potato.
You shouldn’t need to fertilize your plants but if you do, use a low-nitrogen mixture or you will end up with leafy vines and no potatoes. Sweet potatoes don’t like to compete so keep the patch well weeded until the vines grow out. The leaves will soon shade out most weed plants.
Too much water can make your larger potatoes split, so watch the calendar and reduce the watering when you get to about 3 weeks before harvest time.
Considering how the plants will produce underground tubers along the vines, there really is no way to adapt sweet potatoes to container gardening.
Pests and Diseases
Wireworms are one insect pest that can damage your sweet potato crop by eating into the growing potatoes. They are pale yellow to light rusty brown. Unfortunately, you can’t do that much about them. Leaving a large chunk of “bait” potato in your garden can attract them, making for easier capture and disposal. There are some natural predators that can be applied to the garden, such as some species of nematodes.
A more serious pest is the sweet potato weevil. It’s a small insect with a black body, red head and long black nose. If you see them on your plant, you likely have a problem even though it’s their larvae that are causing the most damage and not the weevils themselves. They are unlikely in your soil unless you have been growing sweet potatoes in other seasons.
Keep your garden safe by only buying slips that are certified to be weevil-free. Using your own store-bought potatoes to grow slips may introduce eggs into your garden. They can also live in morning glories (closely related plant) so never plant these flowers near your vegetable garden if you are growing sweet potatoes.
Because the tubers are growing so close to the surface, you can also have a problem with mice or other rodents. Mice can be difficult to get rid of in the garden but commercial poisons, baits or traps can help.
Harvest and Storage
Average sweet potatoes are 4 to 6 inches long, and each plant will produce anywhere between 4 to 12 of them.
The leaves of the plant will start to die back when its time to start digging up the potatoes. They grow right under the surface so don’t be too rough when you dig. You can bruise them quite easily. Any potatoes that are cut by the shovel should be used first and not stored. If a frost hits before you’ve harvested, they will be fine. But once that happens, don’t put it off much longer.
You don’t necessarily have to wait until full maturity to dig up potatoes either. With a little care, you can dig small new potatoes without killing the rest of the plant.
For immediate use, keep your fresh sweet potatoes out on the counter and not in the refrigerator. They’ll last about a week this way. Unlike traditional potatoes, sweet potatoes are not good for really long-term storage. Keep extra potatoes in a cool but dry place and they will be usable for about a month. If you’re doing that, don’t wash them first. They need to be very dry and washing will add more moisture, leading to quicker spoiling.