Planting Sweet Potatoes
The roots of sweet potatoes need to swell and expand easily; therefore, the slips are usually planted in raised rows ranging from 6 to 10 inches high and from 2 to 3 feet wide. Raising the soil in hills or "ridges" is especially important with clay soil. Heavy soils tend to compact, which restricts the underground growth of the roots and sometimes resulting in rough, odd-shaped potatoes. Heavy soils also tend to drain poorly.
Just as with white potatoes, you can improve heavy problem soils for better sweet potatoes by mixing in plenty of organic matter such as leaves, grass clippings, hay, compost or peanut hulls whenever and wherever you can.
You can prepare your ridges a week or so before planting, while the indoor-grown slips are hardening off or when your bedded slips reach 6 to 9 inches in height. Till or spade the soil first, working crop residues into the soil, making it as loose as possible. The soil pH should be in the 5.6 to 6.5 range, slightly on the acid side. As soil pH nears 7.0 (neutral), sweet potato plants become more susceptible to certain diseases.
Stake out your sweet potato rows, allowing about three feet between them so you can walk between the rows to weed or cultivate. Use a hoe to draw up the tilled, loose soil and form a fairly level planting bed. You can easily smooth out the ridges with a rake before planting the slips.
Sweet potato plants gobble up a lot of nutrients from the soil to feed their burgeoning roots and extensive vines, which for some varieties may reach 15 to 20 feet in a long growing season.
Fertilizing is a delicate matter, too much plant food -- especially nitrogen -- will produce skinny potatoes. With too much nitrogen, the vine portion of the plant dominates the growth, and root formation is delayed. But with too little fertilizer, the harvest won't meet your expectations.
When using commercial fertilizer, the basic guideline is about four to five pounds of 5-10-10 for each 100-foot-row. If your soil is fairly fertile, you can cut this recommendation to two or three pounds for each 100-foot-row.
The simplest way to apply the fertilizer is to broadcast it before making your ridges. Spread it uniformly over the row and include some of the soil area you will use for building your ridge.
Ready to Plant
Follow this rule of thumb on planting slips: For the best results, wait two or three weeks after the average last frost date, until the air and soil temperatures are over 60° F. The planting season in central Florida, for example, ranges from March to July.
In the North you will have to gamble. If the weather is warm enough, set plants out close to the average last frost date; if you wait much longer you won't have a long enough frost-free growing season. Place a floating row cover over the plot to protect the slips from frost and to raise the air temperatures inside the row cover.
If you're using your own slips, pull them out of the ground or flats with a twisting motion. This will easily free them from the seed potato under the soil and all other slips, yet retain many of their tiny roots. Some people recommend cutting the stem of the slips one inch above the soil to reduce the chance of spreading soilborne diseases from the bedding area to the sweet potato rows. These rootless slips must be kept well-watered after transplanting so they'll reroot quickly.
Set the slips 12 to 15 inches apart in the row, and five to six inches deep. Make a small hole for them, set them in it, firm the soil around them and give them a gentle watering. If you have some short slips, that's okay. Put them into the soil with just one leaf showing above ground. They'll do fine.
If you have a long growing season, you can take slips or "cuttings" from young sweet potato plants after they've been growing a few weeks. It's a good way to avoid the work of raising and caring for slips, and this "second crop" is a simple way to extend the season. Take the cuttings from anywhere on the vine -- they can be up to 15 inches long. Plant them right away, 12 to 15 inches apart and five to six inches deep on ridges. Keep them shaded from bright sun, and be sure to water them several days in a row, they'll root quickly.
This article is a part of our Vegetable Gardening Guide for Potatoes and Sweet Potatoes / Planting.