Sweet potatoes need at least 120 days of warm weather to develop to maturity. Nights must be warm as well to keep the tubers growing. For this reason, it is virtually impossible to grow sweet potatoes in California's cool coastal areas, mountain regions or deserts. However, they can be grown in the warm interior valleys of the state. In marginal areas, row covers, cold frames, hoop houses or other structures that artificially warm the soil and humidify the air help provide sweet potatoes the conditions they need.
Soils and Irrigation
Besides temperature, the main growing requirement of sweet potatoes is well-drained soil. They prefer a sandy loam, though the regions of California where sweet potatoes can be produced rarely have this soil type. Instead, it is necessary to plant them in raised beds or carefully cultivated mounds. Sweet potatoes need ample irrigation during the growing season -- watering twice a week is often needed in the arid regions of California.
Planting, Cultivation and Harvest
Sweet potatoes are planted as rooted cuttings, called "slips," that can be purchased from nurseries or grown indoors from pieces of the mature tubers. In California, these should be planted as soon as nighttime temperatures stay above 55 degrees. Keep the beds free of weed competition and continue to hill up soil around the stems as they grow to provide more loose soil for the tubers to form. When harvesting, take care to avoid nicking the skin of the tubers, because shelf life will be decreased. Sweet potatoes must be cured for a week in 80- to 85-degree temperatures and can then be stored at cool temperatures for several months.
"Beauregard" is one of the most commonly grown sweet potatoes in California. This type has both orange skin and flesh and is a moist variety good for baking and desserts. "O'Henry" is also grown in California and has yellow-brown skin and cream-colored flesh that is drier in texture with a nutty flavor. The other common type is the "Red Yam," which has a reddish skin and orange flesh and is very sweet and moist.