Most Purple Sweet Potato varieties take a tediously long time to grow, a growing season of 140-160 days is needed to produce mature tubers. When they are harvested not fully mature the taste does not suffer, they still make a great swet potato - but they tend to be elongated and a tad scrawny. Plant in the same way you would standard potatoes or sweet potatoes, from slips or starts.
The Purple Sweet Potato slips require a warm environment to get going. If you live in a shorter season -cooler area it is a good idea to start them indoors. Place the slips in a glass of water, half of the potato should be above the waters surface. Using toothpicks to hold the slip in place works just fine.
Once they have sprouted they are plantable. Unlike with regular potatoes, you do not need to plant the entire sweet potato, although you can if you'd like. It is best to carefully separate the sprouts from the start - you should get several from each tuber.
Sprouts that you have been separated from the sweet potato are laid out in a bowl filled half way with water with the stem submerged. Root development from each sprout generally takes only a few days. Roots should be roughly an inch long before planting in soil. In short season regions it is best to plant them indoors to allow them to produce some healthy top growth before transplanting them into the garden.
Sweet potatoes need well-drained, non compacted soil to form the best tubers. The less resistance and obstacles the roots encounter, the better and bigger tubers it will yield. Loose soil is crucial.
After planting be sure to soak the bed thoroughly. Water them daily for the first two weeks but don't drown them. As the season progresses cut back on the water gradually till they are only being watered once a week towards season end. Excessive and persistent watering can lead to substandard mushy tubers as well as rots and fungi.
They are somewhat tolerant of drought, but naturally under extreme heat conditions you should bump up the water to curtail unnecessary stress on the plant. In warmer regions, where there is no hard frost they will over winter.