This sweet potato is starting to grow sprouts which will grow into slips. Photo by Southern Exposure Seed Exchange
I wrote Growing Sweet Potatoes in June 2016, Saving Sweet Potato Roots for Growing your own Slips in October 2016 and How to Prevent Sweet Potatoes Sprouting in Storage in January 2018. Here's the next step in the process of sweet potato self-reliance.
The Mystique of Sweet Potato Slips
Sweet potatoes are not grown from seed or from replanted roots, but from “slips,” which are pieces of stem with a few leaves, grown from a mother root. We used to buy bare-root slips, believing that growing our own would be very tricky. 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.
Disadvantages of Buying Slips
• You need to specify a shipping date months ahead, then hope for good weather.
• You might have late frosts, spring droughts, or El Nino wet springs, and climate change is only adding to the uncertainty.
• You have to jump-to when the plants arrive, get them all in the ground promptly, and keep them alive as best you can.
• A certain amount of drooping (transplant shock) is normal.
Advantages of Home-Grown Slips
• Delay planting if that seems wise.
• Plant them in stages rather than all on one day.
• Grow them big and plant them with 3-5 nodes underground, giving more chance of survival in heat or frost.
• Keep some spares on hand to replace casualties.
• The sturdy plants get off to a strong start – the transplants don’t wilt – a particular advantage where the season of warm-enough weather is on the short side for a 90-120 day plant.
• Planning ahead – how many sweet potatoes to plant
• Decide how much space you want to plant, or how many pounds (tons?) you want to grow.
• One plant will produce 4–10 roots, each weighing 3–17 oz (80–500 g).
• Yield range is 2.5–6.8 lbs (1–3kg) per plant, 276–805 lbs/1,000 ft² (14–40 kg/10 m²).
• Planting space is 6"–18" (15–45 cm) in the row (wide spacing gives more jumbo roots).
• Space between rows could be 32"–48" (0.8–1.2m). The vines become rampant.
• Save at least one sweet potato tuber (root) per 10 slips wanted.
• Calculate how many slips you’ll need and add 5–10%.
• If you plan to do the two optional tests below, include an extra 10%.
• We save 200 roots for 600 plants, which is always plenty.
Selecting "Mother Roots"
The ideal time to select mother roots is at harvest, when you can choose from the highest-yielding plants. If you didn’t do that, retrieve some from your stored sweet potatoes, selecting small or medium-sized roots (1½" (4 cm) diameter) of typical appearance (no rat-tails)!
• Do not use any roots with disease symptoms.
• Each root will produce 10–30 slips, depending how much time you allow, but regardless of size – no advantage in selecting jumbos.
• 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.
A good introduction to growing sweet potatoes can be found in the ATTRA publication Sweetpotato: Organic Production. Also see the commercial growing page of the North Carolina Sweet Potato Commission for lots of good information, including photos of problems.
How Not to Sprout Sweet Potatoes
I took several wrong directions when I first learned to grow slips. You don't have to repeat my mistakes! My first big mistake – following directions written for much further south, was to try growing slips in mid-January in central Virginia. Dismal fight against nature! Likewise, I was puzzled by talk of using cold frames. Ours were freezing cold at that time of year. Indoor spaces are much warmer than outdoors!
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 set to switch off the heat at 70F (21C). I just couldn’t get the soil warm enough.
Figure out your ideal planting date and work back to find your starting date. Planting out is usually about 2 weeks after the last frost. You need settled warm weather. The soil temperature should reach at least 65F (18C) at 4" (10 cm) deep on 4 consecutive days – don’t rush into planting too early, or you will get lower yields.
We plant May 10, between pepper and okra & watermelon transplanting dates. It takes 7-8 weeks to grow the slips using our method, 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 start 3/4.
Sweet potato roots growing slips in our germination cabinet. Photo by Kathryn Simmons
First test the roots in a bucket of water – the ones that float are said to yield more and produce better flavored roots.
Second, test for viral streaking, (color breaks or chimeras). Discard roots with pale spots or streaks wider than a pencil lead. Cut across the distal end of each root – that’s the stringy root end, opposite the end that was attached to the plant stem. 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. Plan to propagate your own slips for just 2 or 3 years (to keep the virus load low).
Sweet Potato Conditioning
Put the chosen roots in flats or crates, without soil, in a warm, moist, light place for 2-4 weeks. The cut surfaces will heal over during conditioning. Ideal conditions are 75°F–85°F (24°C–29°C), 95% humidity. Conditioning can double or triple the number of slips the root will produce in a timely manner.
Sprouting the Sweet Potato Mother Roots
The environment for sprouting the roots is similar to that for conditioning, so you can likely use the same location. You will need 12" (30 cm) headroom.
Plant the selected roots flat, almost touching, in free-draining potting compost in flats or crates. 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 with some kind of liquid feed.
Cut sweet potato slips in water. Photo by Kathryn Simmons
• After 5–7 days, the tubers begin to produce slips.
• When the slips are 6"–12" (15–30 cm) tall with 4–6 leaves, cut them daily 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 them in rubber bands and set them in water.
• The slips grow more side roots while they are in water for several days.
• Once a week I spot (plant) the oldest, most vigorous slips (with good roots) into 4" (10 cm) deep wood flats filled with compost.
• The spotted flats need good light in a frost-free greenhouse and sufficient water.
• The slips become very sturdy. If you are 2 weeks shy of your planting date and short of slips, you can take cuttings from the first flats of slips.
• 10 days before planting, start to harden off the flats.
A flat of sturdy ready-to-plant rooted sweet potato slips between two flats of more recently spotted slips. Photo by Kathryn Simmons
Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs. Pam also writes for Growing for Market magazine. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available at www.sustainablemarketfarming.com. Her blog is on her website and also on facebook.com/SustainableMarketFarming. Pam's second book The Year Round Hoophouse will be published by New Society winter 2017/18
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