Growing Sweet Potatoes
This member of the morning-glory family were taken from Central and South America to Spain by Columbus. After the conquest of Mexico, the Spaniards took the sweet potatoes to the Philippines, and the Chinese adapted them there. Records show that they were cultivated in is what now Virginia in 1648.
No vegetable commonly grown in the United States will withstand more summer heat than the sweet potato, indeed, very few require as much heat. This tropical plant does not thrive in cool weather. A light frost will kill the leaves and soil temperatures below 50 degrees with damage the tubers.
Commercial crops are feasible where daily temperatures are above 70 degrees, day and night, for at least 3 months. Louisiana, North Carolina, and Georgia all produce a lot of commercial crops. In the West, only the hottest summer areas of Arizona and California support commercial culture.
Although the names sweet potato and yam are often used interchangeably, they are different plants, and differ in both growth habit and culture. Yams belong to the genus Dioscorea and are rarely grown outside of the tropics. They are not as nutritious as sweet potatoes, being little more than starch. Shredded and cooked, they become mucilaginous, or gluey.
Fertilizing sweet potatoes is tricky, given too much nitrogen, they develop more vines than tubers. However, they are not a poor soil crop,and a low-nitrogen fertilizer such as 5-10-10 worked into the soil at the rate of 4 pounds per 100 feet of row will improve the yield. Prepare the soil 2 weeks before planting. In some areas potash will increase the yields and make the tubers shorter and chunkier; low potash likely means long, stringy potatoes.
Sweet potatoes are started from slips. Supermarket tubers may sprout and produce slips, but like Irish potatoes they are usually treated to prevent sprouting. Disease-free slips from a garden center or seed company are best, and you'll be sure of the variety. Plant slips after the soil is warm, since the length of the growing season is often the limiting factor. Where the season is long enough, plant 2 weeks after the last frost to ensure thoroughly warm soil.
If drainage is poor or plants are over watered, the tubers may be elongated and less blocky. To ensure good drainage, sweet potatoes normally are planted in 6 to 12 inch ridges, located 3 to 4-1/2 feet apart. Where soil is fast draining and sandy, ridge planting is of little advantage. Space the plants 9 to 12 inches apart. Rich soil may allow closer spacing. If spacing is increased much beyond 24 inches, more oversized tubers will result but at the expense of both total yield and quality.
Being deep-rooted plants often grown in moisture poor sandy soils, sweet potatoes are usually able to survive, but those planted for harvest should never struggle for water. As a general guide, they will use about 18 inches of water per season. After your first sweet potato crop, save some of the healthiest tubers to produce your own slips. Cut or split the crown and underground stems of each plant and examine for dark strands or darking of the internal tissue, a symptom of stem rot infection. Do not use tubers from stems whose internal tissue shows any discoloration, even though the sweet potato appears fine. Also examine for surface cracks and black eyes, which indicate nematode infestation. Three potatoes should produce 24 slips, enough for a 25 foot row and about 60 pounds of tubers. Plant the chosen tubers close together in sand, vermiculite, or perlite hotbeds 5 to 10 weeks before your outdoor planting date. When sprouts reach 9 to 12 inches, cut them off 2 inches above the soil, and set the slips into containers with high-quality sterile rooting soil. Keep the soil warm (80 to 90 degrees) for fast rooting. Good roots will usually form in 10 to 14 days.
Harvest sweet potatoes when slightly immature as soon as the tubers are large enough. Otherwise wait until the vines begin to yellow. Try to avoid bruising them when digging, since this invites decay. If the leaves are killed by frost, harvest immediately.
Sweet potatoes improve during storage, because part of their starch content turns to sugar. For storage, they need to be cured. Let the roots lie exposed for 2 to 3 hours to dry, then move them to a humid (85 percent relative humidity) and warm (85 degree) storage area. After 2 weeks lower the temperature to 55 degrees, where they will keep between 8 and 24 weeks.
If you lack garden space for sweet potatoes but would still like to grow some, try them in a box at least 12 inches deep and 15 inches wide. Use a light, porous soil mix and place a 4 foot stake in the center to support the vine. I've also heard of them being grown in old rubber tires and large trash cans.