Sweet potatoes require loose, well-drained soil and about 100 very warm days. Sweet potatoes are usually grown from slips started from roots (to start your own add another 40 days). Set sweet potato starts into the garden about the same time you set out tomato transplants in spring.
For sweet potato growing tips see Sweet Potato Growing Success Tips at the bottom of this post.
Common sweet potato growing problems with cures and controls:
Transplanted slips or sprouts die or do not produce vines. Sweet potato transplants (slips) must be kept evenly moist until they root. Transplant out slips that are good sized, not slips that are thin or frail. Set transplants in the ground up to their top leaves; bury the remaining nodes so that they will root.
Leaves become yellow or brownish, wilt and droop, brown to black steaks inside stems; plant become stunted. Fusarium wilt is a fungal disease which infects plant vascular tissues. Fungal spores live in the soil and can be carried by cucumber beetles. Plant disease-resistant varieties. Rotate crops. Remove and destroy infected plants. Fungicides are not effective.
Water-soaked blotches on leaves–not enlarging past leaf veins. Leaf spot or bacterial spot is a waterborne bacterium which causes irregular geometric patterns on leaves. Spots may turn yellow and crisp. Avoid wetting foliage with irrigation. Prune off infected leaves and stems. Clean up garden. Plant disease-resistant varieties. Rotate crops up to 2 years.
Small shot-holes in leaves of seedlings. Flea beetles are tiny bronze or black beetles a sixteenth of an inch long. They eat small holes in the leaves of seedlings and small transplants. The larvae feed on roots of germinating plants. Spread diatomaceous earth around seedling. Cultivate often to disrupt life cycle. Keep garden clean.
Holes in leaves and roots. Larva of sweet potato weevil is a white, legless grub with a pale brown head about 3/8 inches long; the adult beetle has a reddish snout. The grub will tunnel through leaves and roots. Pick off weevils. Remove and destroy infested plants. Sprinkle plants with diatomaceous earth. Cultivate the planting bed before planting to disturb the weevil’s life cycle.
Leaves turn yellow and then brown from the bottom up; plant loses vigor; cracks in roots. Root knot nematode is a microscopic eelworm that attacks roots. Plant resistant varieties. Rotate crops. Remove old plant debris from garden. Root cracks also can be the result of too sudden uptake of water; keep watering even; avoid dry and wet spells.
Roots are elongated, slender. Too frequent watering. Sweet potatoes require consistent, even moisture, but roots should not be constantly moist. Water deeply for 2 to 3 hours at a time then allow the soil to dry to a depth of 6 to 8 inches before watering again. Heavy, clay soil which retains moisture can cause sweet potato roots to become long and stringy. Add aged compost to planting beds to increase drainage.
Root flesh is stringy. Soil is too wet. Roots should not be kept constantly moist. Avoid overwatering; make sure soil is well drained. Water deeply for 2 to 3 hours at a time then allow the soil to dry to a depth of 6 to 8 inches before watering again. Heavy, clay soil which retains moisture can cause sweet potato roots to become long and stringy. Add aged compost to planting beds to increase drainage. Reduce watering a few weeks before harvest.
Tunnels or hole in tubers. (1) Wireworms or (2) sweet potato weevil: (1) Wireworms are the soil-dwelling larvae of click beetles; they look like wiry-jointed worms. Check soil before planting; flood the soil if wireworms are present. Remove infested plants and surrounding soil. (2) Sweet potato weevil is a blue-black weevil that resembles a black ant; the larva is a white, worm-like grub that will feed on plant stems and roots. . Weevils lay eggs in the soil and the worm-like larvae will eat stems and roots. Mound up soil around plants so that weevils can not reach roots. Dust plants with a small amount of diatomaceous earth.
Round to black spots and blotches on root surface. Black scurf is a fungal disease that favors warm soil. Remove infected plants and plant debris that harbor fungal spores. Rotate crops. Be sure slips are not diseased. Plant certified disease-free slips. Rotate crops regularly. Solarize the soil in late spring or summer. Black scurf is resting spores; peel away the surface spores before using the root.
Black sunken spots on roots and underground stems; leaves are yellow. Black rot is a fungal disease. Roots will have circular black decayed areas that become dry and corky. Use only certified healthy slip and resistant varieties. Keep garden free of weeds; some diseases are spread by insects such as the sweet potato weevil. Avoid bruising tubers at harvest.
Internal cork, hard corky spots in root. Internal cork is a viral disease transmitted in the propagation, using diseased stock and also by aphids. Leaves will become mottled and yellowish along veins. Plant certified virus-free slips. Control aphids. Do not save roots from infected crops for planting next year.
Sweet potatoes have poor flavor. Harvest roots before soil temperatures drop below 55°F. Harvest should be completed before the first frost.
Roots rot after harvest. Bacterial rots can enter roots through harvest wounds; sweet potatoes are very thin skinned. Cure just harvested roots for about a week at 75°F. Store cured sweet potatoes wrapped in newspapers at about 55°F.
Parts of roots are hard after cooking. Sweet potatoes should be stored at about 55°F, not cooler. The tissue of roots stored too cold will become dry and hard.
Sweet Potato Growing Success Tips:
Planting. Grow sweet potatoes in full sun. Work 2 to 4 inches of aged compost into the planting beds before planting. Plant rooted slips for best results. You can start your own slips from sweet potatoes grown the year before. Allow 40 days for sweet potatoes to root. Use a large, firm sweet potato; place the potato in a pot of moist sand leaving two-thirds of the root exposed. Put the root in a sunny place at about 75°F. When sprouts are 4 to 6 inches long, twist them from the potato and put them in water or wet sand to root. Transplant the slips into the garden when the roots are about 2 inches long.
Planting time. Plant sweet potato slips after the soil has warmed in spring to at least 65°F or warmer; soil temperature of 80°F is ideal. In cool regions, grow dwarf or early-maturing varieties and plant them in raised mounds or raised beds where the soil warm quickest and stays warm.
Care. Sweet potatoes prefer regular, even watering until they are established. Keep planting beds weed free.
Harvest. Begin lifting sweet potatoes when they have reached the number of days required for maturity–check the seed packet or a growing guide. Keeping track of average days to harvest is the best way to know when to harvest sweet potatoes. An unharvested sweet potato will continue to grow and grow, but it is best harvested young. Sweet potato tubers are easily injured so lift them gently after loosening the soil with a spading fork. Cure sweet potatoes for 10 days in a dry, shady place before using.