Sweet potatoes are not grown from seed or by planting a whole tuber. The preferred method for starting a new crop involves using specialized plant cuttings called “slips.” A slip is essentially a sprout, or young plant, that grows directly from the mature storage root of the sweet potato. This technique, known as vegetative propagation, is employed because sweet potato seeds are often sterile or produce unreliable, inconsistent results. Planting the slip ensures the new plant carries the exact genetic traits of the parent tuber.
Choosing the Right Sweet Potato Tuber
The initial step in producing slips is selecting a healthy, firm parent sweet potato, often called the “mother tuber.” Choose tubers that have not been treated with sprout inhibitors, which are sometimes used to extend shelf life. Selecting organic or certified disease-free tubers increases the likelihood of successful sprouting.
Look for a variety known to perform well in your local climate, as sweet potatoes require a long, warm period. The tuber must be free of soft spots, mold, or decay, as these issues can spread quickly during propagation. A single healthy tuber can generate dozens of slips.
Propagating Slips: The Sprouting Methods
Encouraging the mother tuber to sprout involves providing consistent warmth, moisture, and light. The traditional water method involves suspending the sweet potato in a jar using toothpicks, ensuring the bottom third remains submerged. This setup is placed in a warm location, ideally maintaining a temperature between 75°F and 85°F. However, the water method can take four to eight weeks for slips to reach a harvestable size.
A more efficient alternative is the soil or sand method, where the tuber is partially buried horizontally in a tray filled with moist potting medium. This tray should be kept warm, often with the aid of a heating mat, to encourage faster development. The soil method is preferred because it encourages slips to develop small feeder roots while attached, cutting the sprouting time down to three to four weeks. In both methods, consistent warmth is the most important factor. The medium must be kept consistently moist but not saturated to prevent rotting.
Cultivating and Harvesting the Slips
Once the initial sprouts appear, they need continued attention to grow into robust slips ready for separation. If using the water method, change the water every few days to keep it fresh and prevent mold or bacteria growth. For the soil method, maintain consistent moisture in the medium as the new sprouts lengthen.
The slips are ready to be harvested when they reach four to eight inches in length and have developed several sets of leaves. To separate them, gently twist or cut the sprout away from the mother tuber, ensuring no piece of the original potato flesh is removed. Cutting is preferred because it ensures the new plant is free of any potential diseases from the storage root.
After separation, slips not grown in soil must be placed in a jar of water to develop their own root system before planting. New roots will emerge from the nodes along the submerged stem within a few days. The original mother tuber can continue producing new batches of slips for several weeks in the warm, moist environment.
Preparing Slips for the Garden
Before transplanting the rooted slips into the garden, they must undergo “hardening off.” This gradual transition prepares the tender indoor-grown plants for the harsher outdoor environment, preventing transplant shock. This process takes seven to ten days, during which the slips are slowly introduced to direct sunlight, wind, and cooler temperatures.
Begin by placing the slips outdoors in a shaded location for a few hours on the first day, bringing them inside at night. Each subsequent day, increase the duration outside and gradually expose them to more intense sunlight. The final step is leaving the slips outside overnight, provided nighttime temperatures are reliably above 50°F. Sweet potatoes are sensitive to cold, so they should only be planted once the soil temperature is consistently 60°F or higher.