University of California
UC Master Gardener Program of Sonoma County
In produce markets, sweet potatoes are often labeled as yams, but the two are not the same and have very different tastes and textures. In general, sweet potatoes have a moist, sweet interior whereas yams are starchier, more like true potatoes. Each belongs to a different genus in different botanical families. True yams are rarely sold in produce markets; vegetables labeled as yams are actually sweet potatoes.
Recognizing Sweet Potatoes
- Of the many different sweet potato varieties, most have purplish or orangish red skin, although some may be yellow-skinned.
- Flesh varies from white to pale yellow to deep orange; some are considered stringless. All have a moist, creamy interior.
- Sweet potatoes are actually tuberous roots. They may be somewhat round, plump, or narrow, normally less than 12 in. long, and weigh several ounces or a few pounds.
- The young, green leaves and stems of the sweet potato plant are edible.
- True yams usually have dark, rough skin with a starchy, pale interior, may grow to several feet in length, and weigh up to 100 pounds.
- A quarantine in California to prevent the introduction of diseases and the sweet potato weevil means that you cannot order sweet potato roots or starts from most sources outside the state.
- Sweet potatoes are cultivated from “slips,” sprouts grown from a sweet potato harvested last year. Slips can be purchased from certified California growers, or occasionally from a nursery, or started from seed potatoes.
- The surest way to acquire slips is to start them indoors from a locally purchased, organically grown sweet potato to avoid any that may have been treated to prevent sprouting.
- In Sonoma County, starts must begin as early as February or March in warm, bright, indoor conditions to be ready for planting outdoors after the soil warms to 70 degrees in May or June.
- To produce slips, place a half or an entire organic potato in water or in consistently moist germinating mix in a very warm, bright site. In a soil mix, partially cover a whole sweet potato laid on its side or a half potato set vertically; in water, halfway submerge a whole or portion of potato set with the pointed end down. Change the water weekly.
- In 6-8 weeks, leafy sprouts appear, many with fine, white roots attached. Sprouts or slips grow 8-12 in. tall. Several slips grow on each sweet potato.
- If roots have not developed when slips are 6 in. or more tall, opt to place them in a cup of water for rooting or plant directly into soil. Larger potatoes develop from rootless slips.
- Gently remove each sprout, now called a slip, from the sweet potato. Pull gently, break, or cut slips and plant immediately into warm, prepared ground or into pots for additional growth if soil in garden beds is too cool.
Hot Summer Growth
- Because they are extremely frost and cold sensitive, it is possible to grow sweet potatoes only in the warmest Sonoma County microclimates.
- Sweet potatoes need sandy soil and a long, frost-free growing season, 3-6 months. Most production is in the southeast U.S., although crops are also grown in California’s central valley.
- Plant in full sun in warm, compost enriched soil 3-4 weeks after the last frost. Make holes at least 6 in. deep to accommodate slips, 12 in. apart, in rows 3 ft. apart. Keep moist until established. Water regularly thereafter, allowing the ground to dry slightly between waterings.
- Do not allow meandering stems to root or the harvest will be diminished.
Harvest and Curing
- Before the first frost in fall, when ends of vines begin to turn yellow, stop irrigating and allow soil to dry for 1-2 weeks prior to harvesting. Cut the vines and remove them to the compost pile.
- Carefully lift potatoes with a digging fork at least 12 in. away from the point where vines emerged from the ground to avoid damaging the tuberous roots.
- Without removing any soil clinging to skins, allow sweet potatoes to lie on top of the ground in the sun for several hours to thoroughly dry before moving them to a curing site.
- Place dry sweet potatoes in a cardboard box or on a nursery flat and move to a warm, humid, 80-degree, well-ventilated location for 2-3 weeks, the time needed for starch in the tubers to be converted to sugar. Artificial heat and moisture may be needed to maintain a suitable environment.
- After curing, sweet potatoes may be wrapped in newspaper and stored for many weeks in a dry, 60-65 degree area.
- Sweet Potatoes (Master Gardener Association of San Diego County)
- Sweetpotato (Vegetable Research and Information Center)