Sweet potato

The sweet potato is not hardy. Dig up the tubers before the first frost and let them cure briefly in a warm, airy place so the skin hardens. Then wrap them individually in newspaper and store them frost-free, dark and cool at around 10 to 15 °C; check them now and then for rot. Alternatively, in autumn cut cuttings, let them root in a glass of water and overwinter them bright and cool. That way you have new young plants in spring.

The simplest and most widespread method is propagation from cuttings, known as slips. In January or February, lay a healthy, untreated tuber (organic shop ones are fine) flat or slightly slanted in moist seed compost and stand it bright and warm (20–25 °C). Alternatively, push toothpicks in around the middle and suspend the tuber with its lower half in a glass of water. After a few weeks, sprouts appear; once these are about 10–15 cm long and carry a few leaves, gently twist them off at the base or cut them off just above the tuber. Remove the lowest leaves and stand the slips in a glass of water or insert them directly into moist compost; in warmth they form roots within one to two weeks. Wait to plant out until the nights are reliably frost-free and the soil is at least 18 °C, in Germany usually the end of May (around week 21–22). Alternatively, in summer you can cut stem cuttings from the long vines (about 20–30 cm), remove the lowest leaves and root them in water or moist soil; this is how you can overwinter plants for the next summer. The second method is division: when harvesting in autumn, or in spring from storage, you can divide tubers into pieces as long as each piece carries at least one eye (a sprouting point). Plant the pieces about a hand's breadth deep and keep them warm and frost-free until they sprout. Growing from seed is not practical for the home garden, because most cultivars hardly form viable seed, the germination rate is very low, and the offspring would not come true to type.