The sweet potato is a "canoe" plant brought to the Hawaiian Islands by Polynesians. The Hawaiian names are uala and uwala.
Sweet potatoes are often referred to as tubers, plants with a fleshy underground stem. The "tuber" in this case is actually a tuberous root. The roots and leaves of the plant are both edible.
A soil test is recommended before you begin to plant sweet potatoes in your garden. Sweet potatoes grow best in acidic soils, with a pH range from 5.1 to 6.8. In addition to nitrogen and phosphorus, potassium is necessary for a healthy plant. High-nitrogen fertilizers should be avoided as they will cause excessive growth of vines, making the plants more susceptible to pests such as aphids.
It is recommended that manures not be used when planting sweet potato because the plants are more likely to develop a fungal disease called scurf that grows on the surface of sweet potato skin. Contact your local cooperative extension office for information on testing your soil.
Sweet potatoes require good drainage. Planting them in ridges can help: Ridges should be about 18 inches wide to allow for the soil to dry more quickly. If your soil is heavy and drainage is slow, it’s best that the ridges are about 12 to 15 inches high. Ridges about 8 to 10 inches high will work for light, sandy soils.
In order to allow enough space for plants to grow, space the ridges about 30 inches apart. It’s best to allow the ridges to settle for a few days before planting. Once they’ve settled, you can smooth out the tops of the ridges. At about five weeks after planting, you can pull up soil from both sides of each ridge. This will increase the height and width of each by about 1 to 3 inches and may help reduce sweet potato weevil damage.
You can purchase transplants, called slips, at your local plant nursery. The best months of the year to plant sweet potatoes in Hawaii are March, April and May. Plants should get at least an inch of water per week by rainfall or supplemental irrigation. Inconsistent watering during development may cause the tubers to crack. You should stop irrigating about a month before harvest.
The sweet potatoes can be harvested any time they reach a suitable size. Care should be taken when harvesting because they are easily bruised.
If the sweet potato tubers are riddled with holes, weevils are the likely culprit. Damage is done by the larvae, which feeds on roots. To manage this pest and other pests and diseases, do not plant in the same spot more than once every three to four years. Remove the vines to prevent the weevils from multiplying. Planting sweet potatoes in ridges can help reduce the incidence of sweet potato weevil.
If wilting is a problem, bring in the whole plant, including the roots, to your local cooperative extension office for analysis. If you have been regularly watering your plants and providing good drainage, it’s possible the wilt may be a result of a fungal infection, such as southern blight or fusarium wilt. With both of these diseases, it is difficult to use crop rotation as a management strategy. Southern blight affects different species of plants, and fusarium wilt can exist in the soil for many years.
In the case of southern blight, spacing the plants properly can help reduce disease incidence. Keeping the area clear of weeds can also help, as the weeds may be harboring the disease.
Prevention is the best way to manage fusarium wilt in sweet potatoes.
Begin with resistant or tolerant cultivars such as Jewel, Redgold, Nemagold and Centennial.
To find a Master Gardener Helpline near you, go to www.ctahr.hawaii.edu/uhmg.
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Lance Mohler is a Maui Master Gardener. Lorraine Brooks is a University of Hawaii extension agent and Master Gardener coordinator at the Maui County Extension office in Kahului.
