The best way to start potato plants (Solanum tuberosum) in a vegetable garden is to plant seed potatoes, which are potato tubers that have been cut into pieces. If you prepare and plant seed potatoes correctly, each piece will grow into a new potato plant. The way to prepare seed potatoes for planting depends on their physiological age. Potatoes grow in all U.S. Department of Agriculture zones as annuals.
Harvest potato plant tubers once the vine dies. If the potatoes are left in the ground they can be affected by rot.
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Choosing Seed Potatoes
Choose healthy seed potatoes. Potato plants are susceptible to at least 75 diseases, and if you use unhealthy seeds, then the resulting plants probably will be unhealthy as well. Some diseases are visible in seed potatoes. If one's surface is scabby, moldy or rotting, then it is diseased. If its interior is black, hollow or rotting, it is diseased. Infected seed potatoes may grow, but they will not produce quality tubers.
The term "physiological age" applies to when potatoes break dormancy. Determine a potato tuber's age by looking at its "eyes," which are small, round indentations on the tuber's skin; sprouts eventually grow from the eyes. Physiologically "young" tubers are just coming out of dormancy, and physiologically "old" tubers have already sprouted and even may have small tubers on their sprouts.
- Dormant: No visible sprouts on the seed potato. It should break dormancy before being planted.
- Young: Very few sprouts emerging from the "eyes" on one end of the seed potato.
- Middle-aged: Several sprouts emerging all over the seed potato.
- Old: Many long, branching sprouts that are weak and flimsy on the seed potato.
- Extremely old: Small tubers on the seed potato sprouts. That disorder is called "potato no top."
Young seed potatoes grow more slowly than older seed potatoes and produce a few large tubers. Middle-aged seed potatoes grow more quickly and have higher yields of medium-size potatoes. Old seed potatoes grow quickly, but the tubers they produce are small. Do not plant seed potatoes with "potato no top;" they have very poor yields.
Store Bought vs. Homegrown
The most reliable way to procure seed potatoes is to buy tubers that are certified disease-free for planting. Saving your own potato tubers year-to-year for planting may increase the risk of plant viruses and diseases. If you choose to save your own seed potatoes, then store them at 34 to 38 degrees Fahrenheit until you're ready to bring them out of dormancy, and keep only potato tubers that show no signs of bruising, cuts or disease.
Readying Seed Potatoes
If seed potato tubers remain dormant near planting time, raise their storage temperature to 70 F for one week. Then return the temperature to 50 to 55 F for two to four weeks while storing them in low light until they sprout. Afterward, treat them like young or middle-aged tubers, depending on how many sprouts you let develop. Both young and middle-aged potato tubers should be pre-cut two to six weeks before planting. Old seed potatoes and middle-aged potatoes that have had their sprouts removed should not be pre-cut.
Warm seed potato tubers to 50 to 55 F before cutting them. Do not wash them. Using a clean, sharp knife, cut the tubers into 1- to 2-inch cubes, with at least one eye or sprout on each piece. Let the cut pieces sit for two to give days in a high-humidity location with good air circulation. Then store them at 45 to 50 F for one to six weeks.
Cut older seed potato tubers three to five days before planting. Follow the same cutting method as for younger tubers, keeping at least one eye or sprout on each cut piece. Store these older seed potatoes in a location that doesn't dip to 32 F or go above 45 F.
Sterilize knives and other cutting tools before using them on seed potatoes. Soak all cutting tools in a solution of 1 part bleach to 3 parts water for at least five minutes, and then rinse them with clean water. If you cut into a tuber and discover it is rotting or diseased, discard the potato and disinfect the cutting tool again before moving on to the next tuber.
Planting Seed Potatoes
Plant seed potatoes when the soil temperature reaches 40 F. Ideally, the seed potatoes and the soil should be about the same temperature when you plant. Plant seed potatoes with their cut sides down, and space them 8 to 12 inches apart and 3 to 5 inches deep. Mound the soil over the seed potatoes.
The best growing sites for potatoes receive full-sun exposure and have soil that is rich, lightweight, well-drained and acidic. Ideally, plant in soil with a pH level of 4.8 to 5.5. If the soil is heavy clay or drains poorly, then add organic matter such as well-rotted compost in spring before planting. Spread compost 3 inches deep over the soil, and work it into the top 6 inches of soil.