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Grow your own sweet potato slips

publication date: Feb 1, 2007
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By Pam Dawling,

In the March 2005 issue of Growing for Market, Paul and Alison Wiediger wrote an article encouraging everyone to try growing sweet potatoes. I hope you did. Sweet potatoes store well and are a good market or CSA crop for late fall or winter. At Twin Oaks we aim to grow enough to supply us from November to early May. One baked sweet potato of 114gm (4 oz) has 185% the RDA of vitamin A, 28% the RDA of Vitamin C, 100% of vitamin E, lots of anti-oxidants, and 160 calories, none from fat.
We used to buy bare-root sweet potato slips for transplanting, believing growing our own would be very tricky. The collapse of our supplier, plus our desire to have organic plants, plus a need to reduce our expenses one year, pushed us into growing our own. We had some problems initially, so I can warn you about how not to do it. Now we have a system we really like, and we’ve found several advantages of home-grown slips over purchased ones.
With purchased slips, we had to specify a shipping date months ahead, then hope the weather sprites would be kind. We had to jump to when the plants arrived, and get them all in the ground pronto, and keep them alive as best we could. We accepted as normal a certain amount of drooping. Here in central Virginia, zone 7, we can have late frosts, spring droughts, or El Nino wet springs, and climate change is only adding to the uncertainty. With homegrown slips we can delay planting if that seems wise; we can plant them in stages rather than all on one day. The transplants don’t wilt. We can grow them big and plant them with 3-5 nodes underground, giving more chance of survival in heat or frost. We can keep some spares on hand to replace casualties. The sturdy plants get off to a strong start – this could be an even bigger advantage further north where the season of warm-enough weather is on the short side for a 90-120 day plant.

Planning ahead
Decide how much space you want to devote to sweet potatoes, or how many pounds (tons?) you want to grow. The yield is about 288 pounds per 1000 sq ft. Planting space is 6-18” in the row (wide spacing gives more jumbo roots, mostly out of favor these days). We do 15” as we like to get some jumbos. 12” would be a good start. The space between rows could be 32-48”. The vines are rampant once they get going. Calculate how many slips you’ll need and add 5-10%. Each mother root can produce 10-30 slips, depending how much time you allow.

Selecting mother roots
Ideally, seed roots would have been selected from high-yielding plants at harvest. If you didn’t do that, choose small to medium-sized roots of typical appearance (no rat-tails!) Each root will produce about the same number of slips, regardless of size, so there is no advantage in selecting jumbos. If you are buying sweet potatoes, choose a variety with a short number of days to maturity if you are in the north. Sometimes natural food stores sell named varieties. Otherwise buy from a local grower. Failing that, buy slips one year, and save your own roots for the next year’s slips. There are two tests we do after the initial two weeks conditioning, to choose good virus-free roots, so allow another extra 10% at this stage. We save 60 roots for 600 plants.

Figure out your ideal planting date and work back to find your starting date. Planting is usually done about 2 weeks after the last frost. The soil temperature should reach at least 65F at 4” deep on 4 consecutive days. For us, that’s around May 12. It takes 8 weeks to grow the slips, and the roots produce more slips if conditioned for 2 weeks (or even 4), before you start to grow slips. So start 10-12 weeks before your planting date. We now start March 1. Here’s where I made my first big mistake – following directions written for much further south, I tried to start growing slips in mid-January. Dismal fight against nature.

Put the chosen roots in flats, boxes or trays, without soil, in a warm, moist, light place for 2-4 weeks. Ideal conditions are 75-85F, 95% humidity. This can double or triple the number of sprouts the root will produce in a timely manner. We use our germinating chamber, which is an old glass door refrigerator heated by a light bulb.

After the conditioning period, test the roots in a bucket of water – the ones which float are said to yield better and produce better flavor roots. Next, test for viral streaking, also known as color breaks or chimeras, where paler spots or radial streaks appear in the flesh. Cut a thin slice from the distal end of each root – that’s the string root end, opposite the end that was attached to the plant stem. All the sprouts will grow from the stem end, so don’t cut there! If you can’t tell the difference, you can either ignore this step and plan not to propagate your own slips for more than a couple of years, or if you are a home gardener dealing with a small crop, you could keep the slips from each root separate, and before planting, cut up the mother root and then discard the slips from streaked roots. Another option for commercial growers is to check some, which become your seed stock, and are planted in a different plot from the market stock. Discard roots with pale spots or streaks wider than a pencil lead.

At this point, I’ll mention another path not to take – the first time I tried this (reading the information from somewhere further south), I was puzzled by talk of using cold frames. Ours were freezing cold at that time of year. 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 set to switch off the heat at 70F. I just couldn’t get the soil warm enough.
Once you’ve set up a place with the right conditions, plant the selected roots almost touching each other, horizontally in free-draining light potting compost in boxes or crates. We use cedar flats 15x24x4”. Water the boxes and put them to sprout at 75-85F with light, humidity, ventilation, and about 12” of head room. Once again, we use our ex-fridge germinator. Using boxes is much more manageable than having the roots loose in a big cold frame. Indoor spaces are much easier to heat than the great outdoors! Boxes can be insulated and put on a bench at a decent working height, with lights or heat lamps over them. Keep the compost damp, and if your planting medium is without nutrients, feed occasionally once sprouting starts, with some kind of liquid feed.
For small quantities of slips, it is quite possible to sprout the potatoes half-submerged in water, either in trays of water, or suspending a sweet potato impaled on toothpicks, resting on the top of a glass of water. For larger quantities I recommend our method.

Cutting slips
After 5-7 days, the roots will begin to produce slips. Ideally, wait till the slips are 6-12” tall with 4-6 leaves, then cut them from the root and stand them in water. If necessary, cut them a bit shorter. Some people pull or twist the slips from the roots, but this could transfer diseases by bringing a small piece of the root with the sprout. I cut the slips daily and then once a week I plant the oldest, most vigorous slips into flats. The slips will grow more roots while they are in water for several days, which seems to be an advantage. It’s also possible to skip the potting stage, and plant the slips outside directly from the water, but I don’t think this is as good. The slips planted in flats become very sturdy, and this system allows for more flexibility about planting dates, and a longer slip-cutting season.
I use 4” deep wood flats filled with compost. I have a dibble board with pegs set at a standard spacing to fit 40 plants in each flat. I press the dibble board into the compost, then plant a slip in each hole. Set the flats in good light in a frost-free greenhouse and water as needed. If you are two weeks away from your planting date and are short of slips, you can take cuttings from the first flats of slips to make more. About 10 days before planting, start to harden off the flats of slips.

It works well to do two plantings a week apart, using the older slips first, and then make a third visit after another week to fill any gaps. It’s better to wait for the slips to grow to a good size in the greenhouse before planting, rather than rush them outside. This article is mainly about growing your own slips, but I want to include getting them in the ground, and a few guidelines on what to expect, for those who haven’t grown them before. For big potatoes, plant the slip vertically. For average size roots but larger crops, plant horizontally 2-3” deep. Have 3-5 leaf nodes underground and only the tips above the ground – this gives the plants a second chance if frost strikes. If, on the other hand, you are planting in hot dry weather, water the soil first, and keep the roots enclosed in damp or wet compost as you plant. Sweet potatoes are often hilled, to reduce flood damage. This can be done before planting, or a little later, once the plants are established.

The first month after transplanting is the root development stage. Roots can go 8 feet deep in 40 days. So don’t be alarmed at the lack of above-ground action. Give 1” water/week as needed, and cultivate to remove weeds. The second month or so is the vine growth stage. The roots begin to store starch and sugar close to the stem base. Cultivate until it’s impossible. One year we sowed buckwheat between the rows, intending to till it in before the vines really “ran”. You can guess what happened. We ended up wading in to hand pull the buckwheat to prevent it seeding. I’m not sure whether it would have become a problem or not. Possibly the vines would have prevented germination of the self-seeded buckwheat.
During the last month of growth for that variety (3rd or 4th month), the potatoes develop and the vining stem stops growing. Unlike white potatoes, which have a natural end to tuber growth when the tops die, sweet potatoes will go on growing until prevented either by harvest or frost. So you can dig them as soon as they are big enough for your market. Just make sure you get them up before the soil temperature gets down to 55F – usually the week of the average first fall frost is about right.
I hope to write another article about harvest, curing, storage and seed root selection in the fall. Meanwhile, I’ll mention this curious idea: another method of propagating sweet potatoes is to take vine cuttings in the fall, root them in water, then pot them up as house plants for the winter, to provide early slips in the spring.

•North Carolina Sweet Potato Commission,
Their commercial growing page has lots of information.
•North Carolina State University,
Guidelines for Sweetpotato Seed Stock and Transplant Production:
•NCSU Sustainable Practices for Vegetable Production in the South, Dr. Mary Peet:
•Southern Sustainable Agriculture Working Group Organic Farmer Network Discussion:
•Sweet Potato IPM (not organic)

Suppliers of slips
•Sandhill Preservation. Heirlooms, 82 Varieties.
•Steele Plant Co, Slips in small and large quantities, good prices, great service

Pam Dawling manages the vegetable gardens for Twin Oaks Community in central Virginia. The gardens supply the 100 residents with almost all of their fresh and processed vegetables.

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