Plant of the Week

Thanksgiving is upon us and I was recently asked why yams are more expensive than sweet potatoes? This question raises my horticultural hackles. I suspect I won't call for enforcement of the "truth in advertising" laws, but the advertisement is erroneous. To help solve the mystery, I want to make clear that any who think they are going to enjoy whipped, candied, or other yams on Thanksgiving are sadly misled. Myths in recipes, signs at the grocery or teachings from your sainted mother aside, you'll eat sweet potatoes, not yams for Thanksgiving. Confusion starts because sweet potatoes have yellow, orange, dark red or brown roots and white, orange, or yellow flesh. One type has flesh that is dry and mealy when cooked. The other type has moist flesh. Those with dark, moist flesh are erroneously called yams, but they are just different types of sweet potatoes. True yams are perennial, tropical plants of the genus Dioseorea. Members of this genus are monocots, meaning that they are botanically related to grasses and lilies. They form edible tubers, not roots like sweet potatoes. Numerous varieties of yams grow throughout the world. Water yams, from Southeast Asia, grow up to 8 feet long and weigh up to a hundred pounds. I don't recall seeing these huge tubers in any grocery stores. Chinese yams can be grown as ornamental vines and form thick tubers three feet long. Other yams form aerial tubers weighing several pounds. There is only one yam specie native to the United States, but it produces no enlarged roots. Cultivated yams are only rarely grown in tropical areas of the U.S. Botanically, Sweet potatoes are Ipomea patatas and are related to flowering morning glory. This brings up another interesting piece of misinformation. Field bindweed is erroneously called morning glory although it is a totally different genus than the cultivated flowering morning glory. Notwithstanding this misnomer, it is interesting to know that sweet potatoes will grow in Utah with special care. They need night temperatures of at least 60 degrees with 70 to 80 degrees preferred for 120 to 150 days. They grow best in light, sandy soil without excess fertilizer. Deep, rich soil stimulates too much foliage and stringy roots. Water carefully to avoid rotting the roots. Sweet potatoes grow from small plants or slips. Start the sweet potatoes yourself or buy plants from nurseries. Produce the slips by laying sweet potatoes on their sides on sand about a month before you want to plant the slips outdoors. Put 2 inches of sand around the sweet potatoes and keep the flat between 75 and 85 degrees F. Shoots will grow with a small amount of the root. The slips can be removed and planted outdoors. They will not grow well without extra heat. Wall of Waters or other devices work well to give them this boost. Purchased sweet potatoes will not produce slips if they have been treated with a sprout inhibitor. If in doubt, buy untreated potatoes from a health food store. There are many improved varieties although it is not possible to pick the varieties when you buy them in the store. Some excellent dry fleshed varieties are Orange Jersey, Nemagold and Nugget. Recommended moist fleshed varieties include Gold Rush, Centennial and Puerto Rico. Centennial produced best in my garden. Whether you grow sweet potatoes or purchase them, handle them very carefully. The tubers are very tender and bruise easily resulting in decay in storage. Cure them at 80 degrees F. for 10 to 14 days, then store them at 50 to 55 degrees F. in a dry area. Sweet potatoes last about 10 weeks in storage. They are part of the wonderful diversity of plants we enjoy from around the world. This diversity is one reason to give thanks at this time of year. Thanks for the great blessing of an abundant food supply goes to gardeners providing abundance for their Thanksgiving table. Thanks goes to the American farmer for making our food supply the safest, most abundant and least expensive in history, and finally thanks goes to the Creator for all the wonderful plants that we enjoy. Larry A. Sagers Regional Horticulturist Utah State University Extension Service Thanksgiving Point Office