Yacon, or Smallanthus sonchifolius, is a heat-loving, distant relative of the sunflower. The plants are robust and undemanding, making them easy to grow and maintain in USDA Zones 7 to 11. Yacon dies back to the ground when hit by frost, but returns the following spring via underground storage tubers--as long as those haven't been frozen. Young plants started indoors can be moved outside in late spring after all danger of frost has passed for your area. This is typically about the same time that you would put your tomato plants out.
Start the yacon rhizome indoors in early spring, about 2 to 3 months before the last predicted frost date for your area. Plant about 1 inch deep in a 4-inch pot of organic potting soil. Water well enough to evenly moisten the soil, but not so much that it's soggy or wet. Set it in a warm spot, such as on top of the refrigerator or over a hot water heater to sprout.
Water the soil often enough to keep the surface uniformly moist. Keep it warm, and don't let it dry out. When the first leaves appear, move it to a bright windowsill.
Step the baby yacon up to the next larger pot size in a month or so if it seems to be outgrowing its container. Keep it on its warm window ledge until it's time to plant outdoors.
Choose a sunny spot with rich, well-draining, loamy soil once it has warmed in the spring. Your yacon will grow to be roughly 6 feet tall and wide, so multiples will need to be planted 3 to 4 feet apart. Cultivate the planting site well with a fork. Amend with all the organic compost that you can.
Plant the yacon after all danger of frost has passed. Water thoroughly, but just enough to uniformly moisten the soil. Mulch the area to a depth of 1 or 2 inches to discourage weed growth.
Water the yacon only enough to keep the soil surface evenly moist throughout the growing season. No fertilizer is required. Yacons need about 200 frost-free days to produce tubers for harvest, typically around the end of October