Growing Sweet Potatoes

Sweet potatoes are planted as slips - green shoots with roots that sprout from the sweet potato root. You have two options:

Grow your own slips (6–8 weeks before planting date): Place an organic sweet potato (non-organic may be treated with sprout inhibitors) in a jar of water, suspending it with toothpicks so the bottom third is submerged. Place in a warm (75–85°F), bright location. Sprouts will emerge from the eyes in 2–4 weeks. When sprouts are 6–8 inches long with visible roots, twist them off the mother potato and place in a glass of water until roots are 1+ inch long. Each potato produces 20–50 slips. Alternatively, bury potatoes horizontally 2 inches deep in moist potting mix and harvest the sprouts when rooted.

Buy slips from a nursery or mail-order supplier. They arrive as bare-root sprouts. Unpack immediately, place stems in water to rehydrate, and plant within 3–5 days. Buying slips gives access to certified disease-free named varieties.

Sweet potatoes demand warm, well-drained, loose soil. They perform best in sandy loam - heavy clay produces misshapen, cracked roots. If your soil is heavy, build raised beds or mounds (8–12 inches high) to improve drainage and warmth.

Avoid high-nitrogen fertilizers and fresh manure - excess nitrogen produces lush vine growth at the expense of root development. Use a low-nitrogen, high-potassium fertilizer (like 5-10-20). Potassium is the key nutrient for sweet potato root size and quality. Wood ash is an excellent natural potassium source.

The ideal pH is 5.8–6.2 - slightly more acidic than most vegetables. Do not lime sweet potato beds. A moderate level of organic matter is fine, but extremely rich soil actually reduces yields (again, too much nitrogen).

For northern growers (zones 3–6), lay black plastic mulch over beds 2–3 weeks before planting to warm the soil. Sweet potatoes need soil at 65°F+ to grow, and black plastic can raise soil temperature 10–15°F above ambient.

Plant slips 2–3 weeks after the last frost date, when soil temperature is at least 65°F (70°F+ is ideal). This is typically late May to early June in most of the US - sweet potatoes are one of the last crops planted.

Set slips 4–6 inches deep at a 45-degree angle, burying all but the top 2–3 leaves. The buried stem nodes will develop roots and become sweet potatoes. Space slips 12–18 inches apart in rows 36 inches apart. Closer spacing (12 inches) produces more but smaller roots; wider spacing (18 inches) produces fewer but larger roots.

Water thoroughly at planting. Slips may wilt dramatically for the first few days - this is normal. They will recover as roots establish. Water daily for the first week, then every 2–3 days for the next 2 weeks, then reduce to weekly deep watering.

Once established, sweet potatoes are remarkably low-maintenance. The vigorous vines quickly cover the ground, suppressing weeds and shading the soil. In fact, sweet potatoes are one of the best ground-cover crops for weed suppression.

Water 1 inch per week. Sweet potatoes are moderately drought-tolerant once established, but consistent moisture during the root-bulking period (weeks 8–14) maximizes yields. Reduce watering 2–3 weeks before harvest to improve storage quality.

Side-dress with potassium-rich fertilizer (wood ash or potassium sulfate) at 6 weeks. Do not over-fertilize - remember, sweet potatoes produce best with moderate fertility. If vines are extremely vigorous but you suspect poor root development, your soil is likely too rich in nitrogen.

Some growers "lift" or redirect vines that root at stem nodes along the ground, arguing this redirects energy to the main root cluster. Research is mixed on whether this actually helps, but it does keep vines from overrunning adjacent beds.

Sweet potatoes are ready to harvest when the foliage begins to yellow, typically 90–120 days after planting. In the tropics, they can be left in the ground longer, but in temperate zones, you must harvest before the first frost. Frost kills the vines and can damage the roots, reducing storage life dramatically.

To check readiness, carefully excavate the soil around one plant to inspect root size. Roots should be 4–6 inches long and 1.5–3 inches in diameter for most varieties.

Cut the vines back to 6 inches above ground, then use a digging fork to carefully lift the root cluster from 12–18 inches out from the plant center. Sweet potatoes bruise very easily and any damage dramatically reduces storage life - handle as gently as eggs. Do not wash the roots - brush off excess soil gently.

Dig on a dry day and let roots air-dry on the soil surface for 1–2 hours (no longer - sun can scald). Move to a shaded area for the curing process.

Curing is the critical step that most growers skip - and it makes an enormous difference. Freshly dug sweet potatoes are starchy and bland. The curing process converts starches to sugars, heals skin wounds, and dramatically extends storage life.

Cure sweet potatoes at 80–85°F and 85–90% humidity for 7–10 days. Create these conditions in a small enclosed space: a bathroom with a space heater and a tray of water, a plastic bin with a damp towel and a heating mat, or a warm closet. Commercial growers use dedicated curing rooms.

After curing, the skin will have toughened and the flesh will be noticeably sweeter. Move cured sweet potatoes to a cool, dark, well-ventilated location at 55–60°F for long-term storage. At this temperature, they keep 6–12 months. Do not refrigerate - temperatures below 50°F cause chilling injury (hard, white core, off flavors). Do not store in plastic bags (traps moisture and causes rot).

Uncured sweet potatoes store only 1–3 months and have inferior flavor. The curing step is not optional if you want quality results.