By Pam Dawling
This article originally appeared in the November/December issue of Growing for Market Magazine
Sweet potatoes came originally from the coastal mountain valleys of Peru and Ecuador. By the 16th century, sweet potatoes were cultivated in what is now the southern US states, where they became a staple of traditional cuisine. Sweet potatoes do not require a lot of organic matter in the soil or high fertility. They provide a lot of good food for modest effort. George Washington Carver of the Tuskegee Institute wrote an informative Bulletin in 1936: How the Farmer Can Save His Sweet Potatoes and Ways of Preparing Them for the Table.
Sweet potatoes are grown from “slips,” pieces of stem with a few leaves from a mother root, not from seeds or replanted roots. We used to buy bare-root slips for transplanting because we didn’t know how to grow our own and had heard it wasn’t easy. We have been growing our own for many years now. We prefer the flexibility and reliability it gives us.
We did make several mistakes initially, so I can warn you about what not to do. We have a system that we really like, and I have learned a few other methods that I will share, including one I helped out with this past year: growing them in a hoop house.
Disadvantages of buying slips
You need to specify a shipping date months ahead, then hope for good weather and no shipping delays.
You might have late frosts, spring droughts, or El Niño wet springs. Climate change is only adding to the uncertainty, while slips that arrive in the mail need immediate attention.
You have to get them all in the ground promptly and do your best to keep them alive because they arrive wilting.
Some amount of drooping (transplant shock) is normal.
Advantages of growing your own slips
You can delay planting if the weather is all wrong (frost, drenching rain, heatwave).
You can grow them big and plant three to five nodes underground, giving more chance of survival if there is a late frost or an early drought.
You can plant them in stages rather than all on one day.
You can grow extra and keep them on hand to replace casualties.
The sturdy plants get off to a strong start – the transplants don’t wilt – a big advantage where the warm season is on the short side for a 90 to 120 day plant.
Self-reliance and less money going out.
Twin Oaks slips-in-flats method
Boutard single node cutting method
Traditional outdoor bedding method (Sand Hill)
Hoop house or caterpillar tunnel bedding method (Twin Oaks)
How not to sprout slips
I made several mistakes learning to grow slips. You don’t have to repeat my mistakes. My first error was following directions written for much farther south (in pre-internet days). I was to try growing slips in mid-January in central Virginia. It was a dismal fight against nature. Likewise, I was puzzled by talk of using cold frames. Ours were freezing cold at that time of year.
Next, I set up a soil warming cable in a cinder-block-enclosed bed on the concrete floor of our greenhouse. This is how I discovered most soil warming cables have thermostats that switch off the heat at 70°F (21°C). I just couldn’t get the soil warm enough.
Selecting your own seed potatoes
Choose plants with a high yield and no string (rat-tail) roots. From these plants, choose medium-sized roots (1½ inches (4 cm) diameter) with good shape and color. Don’t save jumbo potatoes for seed. They’re harder to deal with and will not produce more or better crops. Each root will produce 10 to 30 slips, depending how much time you allow, regardless of size. Do not save any roots with disease symptoms for slip production.
If you want to be sure to avoid saving roots with color breaks, you can cut a small slice from the distal end (the end distant from the plant) for examination. The cut surface will heal over during curing. Discard any roots with streaks or dots bigger than a pencil lead. Otherwise, save 10 percent extra roots and test later.
Selecting mother roots
If you didn’t save your own mother roots at harvest, go to your stored crop and choose healthy, typical-shaped, small-to-medium-sized potatoes as described above. If you haven’t got your own sweet potatoes, buy from a local grower so you get a variety that does well in your area. If you are in a cold area with a short summer, choose a fast-maturing variety.
How many slips to plant
One slip will produce a bunch of 4 to 10 roots, each weighing 3 to 17 oz (80 to 500 g). Your climate, spacing, and growing season can all affect the yield. The usual yield range is 2.5 to 6.8 lbs (1-3kg) per plant, 276 to 805 lbs per 1,000 ft² (14–40 kg/10 m²), or 6 to 17.5 tons per acre (13.5–39 t/ha).
Use a plant spacing of 6 to 18 inches in the row (wide spacing gives more jumbo roots, not wanted by small households). We use 15 inches (38 cm) as we like to get some jumbos. If unsure, try 12 inches (30 cm). The space between the rows could be 32 to 48 inches (0.8-1.2m). The vines become rampant.
Calculate how many slips you’ll need. For an acre you’ll need 15,000. Each mother root can produce 10 to 30 slips, depending how long you grow them. Save at least one sweet potato tuber (root) per 10 slips wanted. If you plan to do the two optional tests below, add 10 percent. We save 60 to 200 roots for 600 plants, 200 is definitely more than necessary, but the additional cost is tiny and who needs the worry?
Choose your ideal planting date and work back to figure out your starting date. Plants set out too early will struggle with skin fungi and give uneven yields. Planting out is usually done about two weeks after the last frost. You need settled warm weather. The soil temperature should reach at least 65°F (18°C) at 4 inches (10 cm) deep on four consecutive days. We plant May 10, between pepper, okra and watermelon transplanting dates.
It takes seven to eight weeks to grow the slips using our slips-in-flats method, and the roots produce more slips if conditioned for two weeks (or even four) before you start to grow slips. We start March 4th, 10 to 12 weeks before our planting date.
First test the seed roots in a bucket of water – the ones that float are said to yield more and produce better-flavored roots. I do not actually know if this is fact or fiction. I do enjoy delicious sweet potatoes, what can I say?
Next, test for viral streaks, (color breaks or chimeras) where white or pale spots or radial streaks appear in the flesh. Discard roots with streaks wider than a pencil lead. Cut a thin slice from the distal end of each root. All the slips will grow from the stem end, so don’t cut there. If you can’t tell the difference between the ends, ignore this step, and learn the difference when you harvest.
You can propagate your own slips for a few years without testing and still keep the virus level low. If you grow a small crop, such as a specialized collection of heirloom varieties, you could keep the slips from each root separate and labeled. Before planting, cut up the mother roots, discard slips from streaked roots. An option for commercial growers is to test some for your seed stock, which you plant separately from the untested market stock.
Conditioning after testing allows the cut surfaces to heal over before they are planted in compost. Tubers are more likely to rot if you cut them after conditioning. We use our germination chamber (an old refrigerator heated by an incandescent light bulb).
The environment for the next step of sprouting the roots is similar to that for conditioning, so you can probably use the same location. Set the roots in flats or crates, without any soil, in a warm, moist, light place for two to four weeks. Ideal conditions are 75°F–85°F (24°C–29°C), 95 percent humidity. This conditioning process can double or triple the number of slips the root will produce in a given amount of time.
The Twin Oaks slips-in-flats method: I learned the slips-in-flats method from Hiu Newcomb of Potomac Vegetable Farms through the Experienced Organic Farmers’ Network run by SSAWG years back.
Many growers have success with slips with few or no roots, but we like a big root system. Set up a place with light, humidity and ventilation as described above, and 12 inches (30 cm) of headroom. Using flats is much more manageable than having the roots loose in a big cold frame. Insulated boxes could sit on a bench at a decent working height, with lights or heat lamps over them.
Plant the selected roots horizontally, almost touching, in free-draining potting compost in flats or crates. We use cedar flats that are 15x24x4 inches (38x60x10 cm). The tubers (mother roots) do not need to be fully covered with soil. Water them and keep the compost damp. If your planting medium is without nutrients, feed occasionally once sprouting starts with some kind of liquid feed.
After five to seven days in the sprouting chamber, the tubers begin to produce slips. As the slips grow to 6 to 12 inches (15–30 cm) tall with four to six leaves, cut them from the tubers. Some people twist the slips from the roots, but this can transfer diseases by including a piece of the mother root.
I bundle the cut slips loosely in rubber bands and set them in a small bucket of water. The slips grow more side roots while they are in water for several days, which gives an advantage. Once a week I spot (plant) the biggest, most vigorous slips (with good roots) 4 inches (10 cm) deep in wood flats filled with compost. We use a dibble board with pegs set at a standard spacing to fit 40 plants in each 12x24-inch (30x60 cm) flat. I press the dibble board into the compost, then plant a slip in each hole.
The spotted flats need good light in a frost-free greenhouse and sufficient water. If you are two weeks shy of your planting date and short of slips, you can take cuttings from the first flats to make more. You could skip the spotting stage and transplant the slips outside directly from the water, but I don’t think this is as good as spotting them into flats of good compost for a few weeks.
This system allows for flexibility around planting dates and a longer slip-cutting season. Ten days before planting, start to harden off the flats by reducing the temperature and increasing the airflow.
The Boutard Single Node Cutting Method: Anthony Boutard and Caroline Boutard Hunt wrote an article in Growing for Market in March 2015 about single node sweet potato propagation (which they learned from John Hart of Cornell University). They farm in Oregon and New York respectively.
Both the slips-in-flats method and this one start at the same time, planting roots in damp compost in a warm greenhouse. I corresponded with Anthony Boutard about our respective methods. He pointed out that the slips-in-flats method needs a lot of warm space to grow the slips at a time when warm space is at a premium. The process takes us from early March to mid-May.
The single node cutting method uses only 10 to 20 percent of the number of mother roots compared to the slips-in-flats method. It uses tiny plants in plug flats, saving on greenhouse space, and only needs 18 days between cutting the slips and planting in the field. The smaller plants can experience less transplanting shock than larger plants. On the other hand, I do not think they have the resilience that multi-node plants have, for example, if a late frost strikes.
When these plants grow in the field, their root production is from the single node. This can lead to fewer, but very large tubers, an advantage or a disadvantage, depending on your goals. Better in climates with a short warm season than in hot places, although digging after a shorter growing period would be possible in warm climates.
This method can be used to grow more plants from purchased cut slips, which is very helpful if you are growing a rare heirloom with limited propagation material available. Plant the bought slips in the greenhouse and grow them on until you can make cuttings. Like the slips-in-flats method, this one buys time if the weather turns bad and you need to backpedal on your planting out date.
To prepare single-node cuttings, cut a slip from the mother tuber and cut off about ¼ inch (6mm) above a leaf node (the swollen point where the leaf emerges). In the leaf node is a bud which will grow a shoot, and just below the bud is a ring of cells that can grow roots. You can trim back the leaf stem, or leave the leaf on. Make the second, lower cut just above the next node down. You can make several cuttings from one regular slip.
Fifty-cell plug flats work well to grow the cuttings. Push the lower end of the shoot cutting at an angle into the cell, creating an even V with the leaf stem. If a cutting is too tall to fit your cell plugs, you can cut more off the lower end. The new shoot will then grow upwards easily from the bud in the leaf axil.
Keep the trays warm and moist, and plant out after only 18 days, into well-prepared damp soil with drip tape in place. Delaying planting for 10 days or so is not a problem.
The Sand Hill Preservation Center field bedding method: Glenn and Linda Drowns are genetic preservationists in Iowa. They sell small quantities of about 225 varieties of slips and have developed a version of field bedding of slips that works well for them. In late April or early May, around their usual last frost date, they prepare beds by digging a wide trench several inches deep.
They set the mother roots in the trench, cover with peat moss and wet it down. They cover the beds with clear plastic and wait several weeks. The soil is still very cold for sweet potatoes. If they have warm weather, the roots start sending up slips in about 20 days. Cool, cloudy weather means added time, fewer slips – and the parent roots can rot. Exercise patience.
They typically plant their slips out June 20 to June 25, and their sweet potatoes are ready to harvest by mid- to late-September. This seems a surprisingly short growing season (barely three months) but it works. Glenn says: “Slips set out when the weather is very warm will outgrow and out-produce ones set out even as much as a month earlier. A slip set out in cold soil will many times become stunted and not produce as large a yield.”
The Twin Oaks hoop house method: I learned this method this spring from my friend and colleague River Oneida, who grew slips for Southern Exposure Seed Exchange. He planted up four beds 45 feet (14 m) long with three rows per bed in a single-layer hoop house and grew an average of 4,000 slips per week for a six week sales season (24,000 slips). Caterpillar tunnels could also work.
Store your saved roots at room temperature. From March 1 onward start the sweet potatoes sprouting indoors in stages, as heated space permits, at 85°F (29°C) and high humidity. Remove any rotting potatoes. When each potato grows ¼-inch (6 mm) sprouts, take it out of the heated space and replace it with new ones. Put the ¼-inch (6 mm) sprouted ones at room temperature to slow them down for batching with later ones.
In March, cover the bed area in the hoop house with a single layer of clear plastic, and kill any weeds that pop up. Four weeks before shipping starts (on April 1 for us), prepare the beds in the hoop house. Dig out a flat-bottomed bed 1 inch (2.5 cm) deep, and set the sprouted sweet potatoes an inch (2.5 cm) apart in rows. One or two sprouts are enough. If possible, don’t bed unsprouted roots.
Spread some compost, not lots. Replace the soil you dug out on top of the sweet potatoes. Add soil from the aisles, putting 1 to 3 inches (2.5-7.5 cm) of soil on top of the potatoes. Not more. Set out drip tape and irrigate regularly.
After bedding, stab holes in the plastic every 4 inches (10 cm) for respiration. Check regularly, opening the plastic on warm days once slips are visible. In mid-April or whenever you see slips emerging, remove the plastic in the daytime, but put it back on frosty nights. Add more soil later to cover exposed tubers if needed. Regulate temperature if you want a faster or slower rate of production.
Some slips will be ready from the third or fourth week of April onwards. If too many slips are ready at any point, pull and heal them indoors. Remember to water.
Pam Dawling has grown vegetables at Twin Oaks Community in central Virginia for 27 years, feeding 100 people from 3.5 acres. She is the author of Sustainable Market Farming and The Year-Round Hoophouse. She is a contributing editor with Growing for Market magazine, a workshop presenter, a consultant and a weekly blogger at sustainablemarketfarming.com, and facebook.com/SustainableMarketFarming.