Purple sweet potatoes are poised to become as popular as kale at trendy restaurants and farmers markets, even in northern climates where they’re more challenging to grow. Purple sweet potatoes are high in antioxidants, and their creamy texture makes delicious fried sweet potato chips. Purple varieties aren’t as sweet as other colors, but they sure are beautiful and tasty. If you want to try your hand at growing these colorful veggies, here’s how.
Consider Your Climate
Purple sweet potatoes are grown in the same manner as more traditional sweet potato varieties. They require about 100 to 120 days to mature, which is why sweet potatoes produce so well in the long, hot summers of the southern U.S. In northern gardening zones, you’ll need to start your purple sweet potato crop by warming the soil for a few weeks before planting.
To warm the soil, cover the planting rows with a layer of black or clear plastic (more later on how to create good planting rows). Tuck the edges of the plastic into the soil or pin them down with rocks, bricks, or landscape pins. The plastic should be in place for two to four weeks before planting.
Pick the Variety
There are several different varieties of purple sweet potatoes available from sources such as Baker Creek Heirloom Seeds, Sand Hill Preservation, Duck Creek Farms, and Southern Exposure Seed Exchange. Look for popular cultivars such as ‘Purple Passion’, ‘Molokai Purple’, ‘All Purple’, ‘Purple Delight’ and ‘Purple Knight’.
Create The Planting Rows
Like other types, purple sweet potatoes are best planted in hilled rows. Build 10-inch-high, berm-shaped rows by mounding the soil up, after it’s been tilled and amended with compost or well-aged manure. Pick a site that receives full sun and is well-drained. Sweet potatoes prefer a slightly acidic soil pH—somewhere between 5.5 and 6.5 is best, as it’s less inviting to the soil-borne pathogens that sometimes attack sweet potatoes.
Plant The Slips
Sweet potatoes are typically grown from rooted slips, and purple varieties are no different. Slips are simply rooted stem cuttings of sweet potato vines, usually about 4 or 5 inches in length. Before planting, soak the roots of the slips in water for a few hours.
Space sweet potato slips about 12 inches apart in a row on top of the planting berm. To plant in gardens where warming plastic is in place, cut a hole through the plastic and plant the slip through the hole. If you’d like, you can mulch newly planted slips with straw, untreated grass clippings or shredded leaves, but if you’re using the plastic, there’s no need to add extra mulch.
Do not plant purple sweet potatoes until well after the danger of frost has passed. This crop is intolerant to even light frosts.
Purple Sweet Potato Maintenance
Once they’re settled into their new home, purple sweet potato vines will grow fairly quickly. There’s not much to do in terms of maintenance other than to keep the deer out of the potato patch (sweet potato vines are one of their favorites) and make sure the vines receive about 1 inch of water per week.
Do not add nitrogen-rich fertilizer to sweet potatoes, as it encourages excessive vine growth at the expense of tuber production.
Harvesting Purple Sweet Potatoes
Although you can harvest the occasional tuber anytime after the plants come into flower, plan to harvest your main sweet potato crop just before the first frost strikes. In warm climates, the vines will begin to yellow and die back, but in the north, this may not happen in time for the arrival of fall’s cold weather. Once the temperatures drop down into the 50s at night, it’s time to start checking your tubers.
To harvest, use a pitchfork or digging fork to lift the tubers out of the soil. Because the sweet potatoes can form a good distance from the plant, work from the outer edges of the berm in toward the center.
Curing Purple Sweet Potatoes
Much like all sweet potato varieties, purple types need to be cured before long-term storage. Brush off excess soil with your fingers, but do not wash or rinse the harvested tubers. Damaged tubers should be eaten immediately, but the rest can be placed in a single layer in a warm (80 to 85 degrees F), well-ventilated, humid room out of direct sun for eight to 10 days. Curing reduces shrinkage, prolongs storage and results in a sweeter flavor due to the conversion of starches to sugars.
Once the skins have cured, store your purple sweet potatoes in wooden or plastic crates at about 55 degrees F with 85 percent relative humidity.