Growing Your Own Sweet Potato Seed Stock
No matter where you live, you can easily grow your own sweet potato transplants or slips. In the North, where springtime weather is cool, it's better to grow them indoors. In many areas of the South, you can start them outside.
To start, you need disease-free sweet potato slips. The first spring, you'll have to buy certified plants. Set them out in the best section of your garden, keep them free of pests and diseases through the season, and after harvesting and curing them, store the best roots carefully. These top-grade potatoes from certified plants will be the ones for your own slips the next spring.
Northern gardeners don't usually store sweet potatoes over the winter, so they generally start plants in March or April using purchased certified seed stock. Most years the plants will be healthy. Occasionally, some will be diseased, but if you start enough plants in the spring, you can avoid using questionable transplants.
Start your plants indoors or in a cold frame seven to eight weeks before the average last frost date in your area. For a small garden, plant about a dozen sweet potatoes close together but not touching in flats. Use potatoes that are 1 1/2- to 2-inches in diameter. Without cutting them, lay the potatoes in the flat and cover them with 2- to 3-inches of moist commercial soil mix. Cover the flats with plastic to retain moisture until shoots appear. Then remove the plastic, and put the flats in a cold frame, near a sunny window or under fluorescent lights.
After a couple of weeks, add a little plant food to the water you give the seedlings. If the temperature in the room is about 70° F, it should take 6 or 7 weeks for the plants to reach 6- to 8-inches in height.
Before you plant the young slips in the garden, they have to be hardened off just like other seedlings. Put the flats in a protected spot outdoors for a few hours the first day of this hardening process. Then gradually increase the time they spend outside, exposing them to more and more direct sun and wind.
Growing Slips Outside
If you live where late winter and early spring weather is mild and often warm, you can grow sweet potato transplants outside. However, don't forget to be alert for a late snap of cold weather that could hurt the plants.
Southern gardeners plant their seed potatoes in cold frames or hot beds -- outdoor growing houses that protect the slips from cool weather damage. In hot beds, it's common to use fresh barnyard manure under a heavy layer of soil to heat up the bed. There's no heat source in cold frames, but they're usually glassed in to trap heat during the day and to protect the plants on cold nights.
As with the indoor method, start six weeks before you expect to put the slips in their permanent place in the garden. Whether you're bedding the sweet potatoes at the edge of the garden or in a cold frame or hot bed, lay the seed potatoes close together, cover them with 2- to 3-inches of soil and water them gently if the soil is dry.
As with other vegetables, rotate your slip-starting area each season to avoid disease. Soil from permanent hot beds and cold frames should be dug out and replaced with disease-free commercial soil mix or with garden soil from an area where sweet potatoes haven't been grown for four or five years.