Planting Sweet Potatoes

Are you thinking about crops to grow that will feed a crowd next winter? Sweet potatoes are an easy crop to grow, provided your climate gives enough warm days. They store very well at room temperature, for a long time. In mid-late May, we are still eating sweet potatoes we grew last year, and they are delicious! Well grown and cured, sweet potatoes reach their peak in flavor during January and February. 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.

Order some slips online and get the ground ready. You can order from Southern Exposure Seed Exchange 11 varieties, all organic, in bundles of 6–100. For more varieties but smaller orders, go to Sand Hill Preservation Center, Iowa. Heirlooms, 225 Varieties, all Organic. Some limits on how many you can buy. Another supplier I recommend is the Steele Plant Co, 10 varieties (not Organic) of slips in small (12) and large (500) quantities, good prices, great service. Slips are young shoots with stem, leaves and, usually, some roots. Sweet potatoes are not grown from seed or from chunks of potato.

I have known people grow sweet potatoes in hoophouses if their climate isn’t warm enough outdoors. This can fit with winter use of the hoophouse for greens and roots. My book Sustainable Market Farming contains a whole chapter on growing this crop, including growing your own slips, but it’s too late to start that this year.

Sweet potatoes fit easily into the crop schedule. Planting out comes later than most spring crops. It’s late enough to precede them with a mature cover crop mix (including legumes to the flowering stage), providing all the nitrogen needed. Or add an organic fertilizer this year and plan ahead for next year. Sweet potatoes do not need lots of organic matter, or high fertility levels. Fitting sweet potatoes into a rotation is easy because it is unlikely that you are growing anything else in that family. As with other crops, a three-year gap (or more) before planting sweet potatoes in the same beds helps control disease.

Modern varieties of sweet potatoes grow to a good size in as little as 90 days, so they are not just for the South! The further north you are, the longer the daylight at midsummer and the more photosynthesizing the plants can do.

Sweet potatoes thrive in hot weather and are fairly drought-tolerant. After vining they need little care during the summer (apart from irrigation) until harvest. Their extensive vines smother most weeds, and they have few pest or disease issues. Most of the labor is the harvest in early October, in-between most other intensive harvests of summer and fall crops. I’ll never complain about a crop which has most of the work be the harvest!

Understanding sweet potatoes

 Sweet potatoes are tuberous roots (Ipomoea batatas), related to morning glory, in the genus Ipomoea. Sweet potatoes or sweetpotatoes are native to the tropical regions in central and south America. The root is long and tapered, with a smooth skin, and color ranging from orange, yellow, beige, white, red, pink, violet and purple. Sweet potato varieties with white or pale yellow flesh are less sweet and moist than those with orange, pink or red flesh. The young leaves and shoots are sometimes eaten as greens. Hot weather greens are hard to find!

Sweetpotatoes are not yams, even though they’re often called yams! True yams are a tropical species of tuber (genus Dioscorea). They come from Africa and the Caribbean. Some are huge! They have rough and scaly skin. The flavor is starchy, and usually not very sweet (more like regular potatoes). Sweet potatoes will not even cross with yams.

Sweet potatoes are sometimes mistakenly thought to be a type of potato but they do not belong to the nightshade family, Solanaceae. Sweet potatoes are only distant cousins of the nightshade “Irish” (more accurately Peruvian) potatoes (Solanum tuberosum). Unlike Peruvian potatoes, which have the annual sequence of vegetative growth, flowering and dying back, sweet potato plants continue growing as long as the weather is warm enough. They are frost-tender herbaceous perennials.

Read more

 I have a Sweet Potato slideshow.

And I have written a lot of other posts on sweet potatoes (mostly about propagating our own slips, or about harvesting). Very little on actually growing them all summer. My posts include:

Starting Sweet Potato Slips

Sweet Potato Propagation and Yields

Transplanting Sweet Potatoes (biodegradable plastic; deterring deer)

Harvesting Sweet Potatoes (2016); Sweet Potato Harvest (2015) and again (2015) and earlier (2014) and in 2013, 2012

What Makes Sweet Potatoes Sprout (during storage)

I also have a blog post for Mother Earth News Organic Gardening blog, about growing sweet potatoes. And I have several articles in Growing for Market, which you can access online if you have a Full Subscription.

When to plant sweet potatoes

Figure out your ideal planting date. Planting is usually done about 2 weeks after the last frost. You need settled warm weather. Heat is vital. The soil temperature should reach at least 65˚F (18˚C) at 4” (10 cm) deep on 4 consecutive days—don’t rush into planting early. Plants set out too early will struggle with skin fungi, and produce uneven yields. Their growth will be stunted. Forget about climate zones—those are about winter-hardiness of perennials. Sweet potato growing is all about warmth and light. Your sweet potato plants can remain ignorant about your winter cold!

We plant May 10–20, between pepper, okra and watermelon transplanting dates. It takes 7–8 weeks to grow your own slips using our method, so buy slips for this year and learn how to save seed stock when you harvest, to grow your own slips next year.

Planning ahead—how many to plant

Decide how much space you want to plant, or how many pounds (tons?) you want to grow. One slip will produce a cluster of 4–10 roots, each weighing 3–17 oz (80–500 g). The yield range is 2.5–6.8 lbs (1–3kg) per plant, 276–805 lbs/1,000 ft² (14–40 kg/10 m²), or 6–17.5 tons/ac

The in-row planting space is 6–18” (wide spacing gives more jumbo roots). We do 15″ (38 cm) as we like to get some jumbos. If unsure, try 12” (30 cm). Climate, spacing, and the length of the growing season all affect yields.

The space between the rows could be 32–48” (0.8–1.2m). Calculate how many slips you’ll need and add 5–10%. For an acre you’ll need around 15,000.

Sweetpotato crop requirements

 Sweet potatoes prefer loose, well-drained soil with pH of 5.8–6.2. They will tolerate pH from 4.5–7.5. Enough potassium (K) is important for drought-resistance, but too much K makes them taste bitter. Sweet potatoes do not benefit from high nitrogen (N). They can get plenty from high-biomass cover crops, organic mulch, and soil life.

Once they are established, sweet potatoes are fairly drought-tolerant. Critical times to maintain sufficient moisture are after transplanting and for at least the first 20–40 days while the roots are developing.

Tips to increase sweet potato yields

Compacted, heavy, lumpy soils can result in misshapen, undersized tubers. If you have clay soil or drainage problems, work in lots of compost and make raised beds or ridges 8”–12” (20–30 cm) high. Ridges help heat up the soil and reduce flood damage.

Black plastic or silage tarps set out 3 weeks before planting warm the soil considerably and increase the growth rate. In colder climates, plant under low tunnels of clear plastic. Ventilate in hot weather.

If it’s too cold to plant out your slips when they arrive, keep them indoors, with water covering the roots (remove any newspaper or other packing material). If you are planting in hot dry weather, water the soil first, and keep the roots enclosed in damp or wet compost as you plant.

We like to do two plantings a week apart, using the older slips first, and then do a third session to replace any casualties.

 

Young sweet potato plants with drip tape and ripped plastic mulch.
Photo Pam Dawling

Planting out sweet potato slips

 Prepare your beds or ridges. Get ready with your irrigation system. If you are covering the beds with biodegradable plastic mulch lay it out just before you will plant. Set out stakes and ropes to mark where the rows will be planted, and gather a measuring stick, trowel and watering can.

To grow the biggest roots, plant the slip vertically, which steers the plants to develop roots only from one node. To grow average size roots but many of them, plant slips horizontally 2–3” (5–7 cm) deep, encouraging tubers to develop at several nodes. Ideally, have 3–5 leaf nodes underground and only the tips above the ground—this gives the plants a second chance if a late frost strikes. Many growers have success with slips with few or no roots, but we like a good root system.

If you are using drip irrigation, run it while you plant. If the emitter spacing matches your plant spacing, plant in the damp spots without measuring. Otherwise, use a measuring stick or a double hand-span to get the plants evenly spaced and not waste plants or land, by diverging from your planned spacing. If you have driptape under mulch, feel for where the tape is, and avoid stabbing it with your trowel.

Firm the soil around each plant, so that it is not left sitting in an air pocket, unable to reach water.

If not using drip irrigation, stop every few plants and water from a watering can. Err on the side of too much water on planting day. Keep newly transplanted slips well-watered.

If it turns cold or windy soon after you transplant, you could use rowcover to protect the plants.

Sweet potato development

I usually reckon on the first month after planting being focused on root growth, the next month on vines and the rest on roots. Regardless of how early in the season you plant them out, they will not make flowers earlier, or start making tubers sooner. Both flower and tuber initiation are triggered by day length. Each variety has its own internal clock. Most varieties take 90–110 days from planting out to reach a good size, if the weather is warm enough.

The first month or so after transplanting is the root development stage. Roots can go 8’ (2.4 m) deep in 40 days. Don’t be alarmed at the lack of above-ground action. Give 1” (2.5 cm) of water per week, 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 the vines cover the ground, after which very little weeding will be needed.

During the last month of growth for that variety (3rd or 4th month), the potatoes develop. Make sure you dig them up before the soil temperature gets down to 55˚F (13˚C) –the week of the average first fall frost is about right.

There are some pest mammals and insects, afflictions and diseases to watch out for, but usually sweetpotatoes will grow relatively untroubled until harvest time. Do watch out for deer, rabbits, and other rodents, or anything that appears to be reducing the leaf cover substantially. I’ll write more about these issues in a few weeks.

For more info, see ATTRA Sweet Potato: Organic production

North Carolina State University, Pests of Sweetpotato
North Carolina State University Diseases of Sweetpotatoes.