Despite their exotic looks, growing purple potatoes (Solanum tuberosum, sometimes Solanum tuberosum andigenum) is similar to growing most other types of potato. In their native Andes Mountains of South America, more than 2,500 potato species and cultivars exist, with colors ranging from white, pink, red and yellow to purple, bluish and black. Purple potatoes contain up to 12 percent protein, a higher level than found in usually grown commercial cultivars. The flesh is dry with a slightly nutty taste. Cooked purple potatoes keep their color best when roasted or microwaved.
Rather than starting from seed, potatoes grow from whole small tubers or pieces of tubers called seed potatoes. A number of purple potato varieties exist. The deepest purple-fleshed cultivar is early season "Purple Majesty," maturing in about 70 days in U.S. Department of Agriculture plant hardiness zones 3 through 9. The dark purple fingerling "Purple Peruvian," which grows in USDA zones 3 through 11, features tubers 2 1/2 to 3 1/2 inches long and a maturation time of 100 to 120 days. It might be hard to find seed potatoes for some purple potatoes since they're not widely grown. If the seed potatoes are about the size of a hen's egg, plant them whole. If they're larger, cut them into pieces with a clean, sharp knife dipped in rubbing alcohol before you start and between each potato you cut. Each piece should have one to three eyes. To prevent disease, put cut pieces in a cool, dry place until the cut surface is hard and dry.
Planting and Spacing
Potato plants are vigorous growers and need room both above and below ground. Space seed potatoes 9 to 18 inches apart, skin side up, in a trench 8 to 12 inches deep and cover them with 2 to 4 inches of soil. For multiple rows, space trenches 2 to 4 feet apart. When potato sprouts poke through the soil, let them reach about 6 inches tall and add about 3 inches of soil to the trench. Repeat until the trench is filled. Time the planting after the last danger of frost has passed since vines grow fastest in cool spring temperatures and increasing day length. Late frosts can kill new sprouts, but regrowth usually occurs.
Purple potatoes tolerate a wide variety of soil types but do best in a loose, well-drained soil rich in organic material. If you have heavy soil, amend it before planting with a 2- to 3-inch thick layer of compost dug into the top 6 inches of soil or consider installing a raised bed. Potato plants need about 12 inches of soil depth to develop good roots and tubers. Purple potatoes require full sun. Vines grow quickly once roots become established and need ample water to support growth, usually 1 to 2 inches of water weekly. Drip irrigation saves water and reduces disease from too-wet soil conditions. Use an organic fertilizer such as 5-1-1 fish emulsion, applied monthly at the rate of 1 tablespoon of fertilizer for each 1 gallon of water used. Thoroughly soak the soil around the plants. Avoid using manure as a fertilizer because it can introduce scab disease.
Different kinds of purple potatoes have varying maturation rates, so check whether the cultivar you're growing is early, medium or late to anticipate the harvest date. After potato plants bloom, vines begin to yellow. Stop watering and wait until the vines die back. Remove the vines and carefully dig potatoes with a garden spade or fork, taking care not to damage the tubers.