@David. Without a doubt the history of candied sweet potato recipes can be researched in much more detail than what I have done in this blog post. I was able to identify 16th European recipes for sweet potatoes that included the use of additional sweeteners such as sugar, cinnamon, nutmeg, dates, prunes, quince, and wine. This seems to be the beginning of making sweet potatoes even sweeter by the use of sugar and why I state that the idea of candied sweet potatoes has its origins in 16th century Europe. I also agree that European immigrants to the U.S. would have brought its cooking traditions to America. I believe that the popularization, and possibly the coining of the term, candied sweet potatoes may have originated in 19th century America by the evidence of such recipes in American cookbooks. I would imagine this is because American cooks had easy access to sweeteners such as maple syrup, cane sugar, and molasses, plus sweet potatoes were cultivated in the U.S. Their might be European candied sweet potato recipes that predate the 19th century American cookbooks- I just have not come across them. Also from the cookbook evidence, it appears that candied sweet potatoes topped with marshmallows originated in the U.S. I hope this helps to answer your questions.
SWEET POTATO CULTURE. The sweet potato (Ipomoea batatas) belongs to the Convolvulaceae, or morning glory family. It is a native of tropical America and is unrelated to the true yam, which is a climbing plant of the genus Dioscorea and an Old World native. The sweet potato was first cultivated in Texas by the Spanish and was later grown by Anglo-American settlers. Several varieties of sweet potatoes have been grown in Texas, including Puerto Rico, Goldrush, Redgold, Allgold, Centennial, Jewel, and Red Velvet. By the 1950s the Puerto Rico was the most prevalent variety in the state. Sweet potatoes grow best in light sandy-loam soils with a clay subsoil. Before planting sweet potato fields should be well drained and well worked to loosen the soil and promote the growth of roots. Root sprouts and slips from vine cuttings are used in propagation and planting. By the late 1980s about half of the sweet potatoes cultivated in Texas were set in the fields with the aid of a mechanical planter. The planting season in Texas starts in April or May and extends through June; harvest takes place from August through October and sometimes into November. Texas sweet potato production is often affected by severe cold or wet weather conditions, insects such as the grasshopper, and diseases. Sweet potatoes are subject to damage from relatively few insect pests, but a variety of fungal and viral diseases attack both the growing plants and potatoes in storage. The incidence of disease can be lessened by the use of disease-free seed, treatment of the crops with fungicides, and careful harvest and curing practices that minimize injuries and allow for healing of damaged skin. Crop rotation at least every four years also helps to prevent disease. Sweet potatoes were grown in Texas in the late 1800s primarily for home consumption. The 1870 census shows 68 of 139 counties reporting production of over 10,000 bushels (5,000 hundredweight) and a total state production of 2,188,041 bushels. Only eighteen counties reported no sweet potato production in 1870.
Sweet potatoes are cultivated throughout tropical and warm temperate regions wherever there is sufficient water to support their growth. Due to a major crop failure, sweet potatoes were introduced to province of China in about 1594 from . The growing of sweet potatoes was encouraged by the Governor Chin Hsüeh-tseng (Jin Xuezeng). Sweet potatoes were introduced as a food crop in Japan, and by 1735 was planted in Shogun 's private garden. It was also introduced to in 1764.
Sweet potatoes were cultivated in Virginia in 1648, possibly earlier, and are said to have been taken into New England in 1764. They were grown by the Indians of our South in the 18th century, but we do not know how much earlier. In the South today they are generally preferred to Irish potatoes as a staple food; in the North the reverse is true.