The Seed Garden (Seed Savers Exchange, 2015) by Micaela Colley & Jared Zystro and edited by Lee Buttala & Shanyn Siegel brings together decades of research and hands-on experience to teach both novice gardeners and seasoned horticulturists how to save the seeds of their favorite vegetable varieties.
You can purchase this book from the MOTHER EARTH NEWS store: The Seed Garden.
Sweet potato plants can flower and produce seeds, but gardeners and seed savers typically propagate sweet potatoes vegetatively by planting slips. As with any form of asexual reproduction, planting slips results in plants—and in this case, a harvest of edible roots—that are genetically the same as the plant from which they were produced. Saving and storing roots for slip production is a common practice that most gardeners can easily manage.
Varieties of sweet potato are classified by the color of their edible roots: skin color can be white, yellow, brown, red, or purple; and flesh color can be white, cream, yellow, orange, or less commonly, reddish purple. They are also categorized by their growth habit, which may be either compact or vining, and the texture of their cooked roots, which ranges from soft and moist to firm and dry. While this crop grows well in the hot climate of the American South, some varieties, such as ‘Centennial’, are well suited to growing in the North.
Sweet potatoes are often marketed in North America as yams, but in most other regions of the world, the word yam refers to two other species: Dioscorea rotundata and Dioscorea cayennensis.
The center of origin for sweet potato is most likely in Central America. From there the crop traveled south to South America by 2500 BCE and was brought east to Oceania by 1000 CE. How the sweet potatoes traveled from the Americas to Oceania is still a matter of debate, but some scholars postulate that birds carried sweet potato seeds over the Pacific. Recent extensive genetic and linguistic studies, however, lend weight to the hypothesis that Polynesians or other ocean-going voyagers may have transported the crop.
Growing Sweet Potatoes
The vast majority of gardeners grow sweet potatoes from slips—stem cuttings produced from roots. Slips are transplanted into well-drained, warm soil—ideally when the soil temperature is above 65°F (18°C). Sweet potato slips should be planted about 10 to 12 inches (25 to 30 cm) apart in rows 40 to 48 inches (102 to 122 cm) apart.
Harvesting and Storing Sweet Potato Roots
Sweet potatoes are ready for harvest when the foliage turns yellow and dies or is killed by frost. The roots should not be allowed to freeze and should be dug carefully with a pitchfork so as not to damage their skins. Only intact, relatively unblemished roots should be saved for producing the next season’s crop. The roots should be carefully brushed clean of soil and cured in a well-ventilated space for five to seven days at 80 to 90°F (27 to 32°C) and 90 percent relative humidity, or some approximation of these conditions. After curing, roots store best at 60°F (16°C) and about 75 percent relative humidity in a dark, ventilated location. Roots should be checked periodically, and any that show signs of rot should be removed and discarded.
Following storage, there are many ways to promote the production of slips from seed roots. A root can be set in a shallow dish of water or in a warm, humid, dark place for a few weeks until shoots develop. A more controlled approach is to presprout roots by placing them in a warm, humid place—ideally 80 to 85°F (27 to 29°C) and 90 percent relative humidity—for two to three weeks. Presprouted roots can then be planted in 2 to 3 inches (5 to 8 cm) of moist sand and kept warm—80 to 85°F (27 to 29°C)—and continually moist for four to five weeks, until shoots have emerged from the surface and produced a few leaves. Slips should be cut about 1 inch (2.5 cm) above the sand when shoots are 8 to 12 inches (20 to 30 cm) tall, and then potted up or planted directly into the garden.
Vegetatively propagated sweet potato varieties only need to be separated from one another to prevent intermixing of varieties during harvest. They are clonal, thus there is little chance of genetic variability. Most differences detected in the garden are likely the result of environmental influence. Nevertheless, for clonally propagated sweet potatoes, it is best to harvest roots from multiple plants to ensure a sufficient quantity of healthy stock. The healthiest roots should be reserved for replanting.
True Seed Production
Collecting viable seeds from this crop is challenging. Many varieties require short days (approximately 12 hours of daylight) in order to flower and set seeds, while others may not produce seeds at all. The species has perfect flowers, is generally self-incompatible, and requires insects for pollination. Planting multiple varieties from diverse sources increases the likelihood of seed production, but because sweet potatoes have a complex genome, some varieties will not set seeds at all. With most sweet potato varieties, the offspring grown from true sweet potato seeds will not be true to type but will instead be highly variable and will potentially display undesirable traits. For this reason, collecting true seeds from sweet potatoes is essentially a breeding project, not an act of seed saving; growing plants from slips is the best method to maintain a sweet potato variety.
For more on seed saving, see our Seed Saving Guide.
Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.