Sweet Potato

Six to eight weeks before you intend to transplant outside, bed whole storage roots and cover them with about 2 inches of moist sand, holding the soil at 75 to 80 F. When the sprouts are 4 to 6 inches long, remove each slip with a twisting tug and plant it. Where you can, buy slips from a reputable source grown from virus-tested stock - saved and swapped roots are how disease arrives in a garden. Sweet potato is grown from rooted shoots, not seed or cut tubers, and the mother root will not push sprouts at all until it is held near 80 F - which is why the timer starts indoors, weeks before the garden is warm enough.

Wait until soil temperature at a 4-inch depth is at least 65 F - measure it, do not go by the calendar, and expect that to fall some weeks after your frost-free date rather than on it. Mound ridges 8 to 12 inches high and about 12 inches wide, then set slips about 12 inches apart in rows 36 to 40 inches apart, 4 to 6 inches deep, with several nodes buried and several leaves above ground. Water regularly after planting to help them establish. The plant is injured below about 50 F and simply sits in cold ground without rooting, so slips set early lose more time than they gain. The ridge raises and warms the root zone and gives the storage roots loose, drained soil to expand into.

In a short-season garden, lay black plastic mulch over the finished ridges a week or two before planting and set the slips through slits in it. Leave it down all season. Where the season is long and hot, skip it and mulch with straw or leaves once the vines are running. Black plastic warms the root zone and buys back a few weeks of growing season, which is the whole margin in a climate that only just clears the 90 to 120 frost-free days this crop needs. In New Hampshire trials the soil under black plastic reached planting temperature by about June 1.

Water uniformly: at least 1 inch of rain or irrigation a week through the season, aiming for steady moisture rather than swings between bone dry and soaked. Then stop watering for the last three to four weeks before you plan to dig. Uneven water is what cracks a sweet potato: a long dry spell followed by a soaking makes the root swell faster than its skin can grow and it splits, which is why the fix is steady moisture all season and then a dry finish - not letting the bed swing between extremes. Constantly waterlogged soil rots the roots instead.

Clemson's program is a preplant application of 5-10-10 at 30 pounds per 1,000 square feet tilled into the soil, then a single side-dress of 4 pounds of 5-10-10 per 100 feet of row before the vines cover the row. A soil test beats guessing on both. Do not reach for a high-nitrogen fertilizer. Too much nitrogen buys you vines instead of roots: the plant puts the surplus into leaf and stem growth and the storage roots stay small. The side-dress lands before the canopy closes because that is the last moment you can get fertilizer to the soil without walking on the vines.

Rotate: do not plant sweet potatoes in ground that grew them year after year, which is what builds weevil populations. Plant only slips from a weevil-free area; if you cannot, examine each seed root carefully and destroy any that is infested. Pull volunteers, and turn under or destroy vines and culls after harvest rather than leaving a pile. Expect wireworm damage in ground recently broken out of sod. If flea beetles have scarred your roots before, Jewel or Centennial are the resistant varieties. Sweetpotato weevil is the crop's worst pest and there is no rescue for it: the larvae burrow deep into the stems and fleshy roots where nothing reaches them, and infestation turns the roots bitter and unfit to eat. Clean stock, rotation and residue destruction are the control. NC State does refer commercial growers to the state agricultural chemicals manual, but there is no home-garden rescue spray for a pest that lives inside the root.

Dig around the time of your first frost - a light frost on the tops is survivable, but complete the harvest before a killing frost, and never leave dug roots lying in the garden to be frosted. Harvest gently, taking care not to bruise or scrape the skin, and cure the same day. Once a hard frost kills the vines, the dead stems conduct rot straight down into the roots still sitting in the soil, and the crop is at risk from that point on. The skin is thin enough that a fingernail scrape becomes a rot entry point, which is exactly the wound curing has to heal.

Dig before frost once roots are sized, handle gently, then cure warm and humid before storage. Curing heals harvest wounds and converts starches to sugars for better storage and flavor.

Cure the roots immediately after digging: hold them near 85 F at about 90 percent humidity for four to six days, in a single layer with air moving around them. Then store at 55 to 60 F at high humidity (about 95 percent), where a cured crop keeps six to ten months. Do not wash them until you are ready to use them, and do not refrigerate them. Curing does two jobs at once. The warm, humid air makes the plant wall off its own harvest scrapes with new corky tissue, which is what shuts rot organisms out, and it converts starch to sugar - an uncured sweet potato is bland and rots within weeks. Below about 50 F the roots take chilling injury, which is why a refrigerator ruins them.