Growing High-Yielding Sweet Potatoes
In May I gave information on planting sweet potatoes. Hopefully that went well for you, and by now you have a large patch of healthy green vines. Let’s keep it that way! Here I will tell you about what the growing plants need, and the pests, disease and afflictions to avoid.
The ATTRA publication Sweetpotato: Organic Production is a good introduction.
Oklahoma State University Extension Sweet Potato Production is a clear concise publication (although it’s not organic).
Sweet potato development
This paragraph was included in Planting Sweet Potatoes, and I’m repeating it here, as a good reality check on what you can expect.
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. The second month or so is the vine growth stage. The roots begin to store starch and sugar close to the stem base. 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.
Growing sweet potatoes – Three Ws: water, warmth, weed-free
- The critical time to maintain sufficient moisture is after transplanting for at least the first 20-40 days while roots are developing. By now, most growers will be beyond this most-important watering period. But if it’s less than 40 days since you planted them out, keep the soil moist. Use your fingers to test the soil for dampness.
- Once they are established, sweet potato plants are fairly drought-tolerant. But if you want high yields, they’ll need water once a week, either from the sky, or provided by you. Note that if you are using plastic mulch, rain won’t go through it, so I hope you installed drip irrigation below the plastic. If not, lay it on top, right beside the plants. Aim to provide an inch (2.5 cm) of water per week.
- Sweet potatoes need warmth! Heat determines success; the number of days from planting does not.
- Provided the weather is warm enough, most varieties take 90–110 days from planting out to reach a good size.
- You can gain warmth in a cold climate, by planting inside a hoophouse or low tunnel covered in clear plastic. Ventilate in hot weather.
- Growing Degree Days (heat units) are a tool for measuring accumulated heat, but you don’t need to calculate GDDs to get a good crop!
- Early varieties take 1200 GDDs to grow a good crop.
- To calculate GDDs, take the day’s high temperature (max) and the day’s low temperature (min) and add them together. Divide by 2 and subtract the base temperature of 55F. (Apologies to the rest of the world – I only know this method using Fahrenheit, but I’m sure you can find out how to do the calculations in Celsius). There are phone apps that will do the calculation for you.
- Example: For a daytime max of 90F, and a night-time min of 70F, you get 25 GDDs – just about perfect for sweet potatoes. 90+70=160. 160/2=80. 80-55=25. At 25 GDDs a day, you theoretically only need about 48 days to get a crop. There are some other limits to daily plant growth – the likely minimum for a decent crop is about 76 days.
- In a plastic tunnel, you can get 20 GDDs a day or more, rather than the 5 you might get outdoors.
- Cultivate to remove weeds until the vines cover the ground, after which very little weeding will be needed.
- If you have plastic mulch, walk through pulling weeds, and drop them on the plastic to cook. If you are growing on bare soil, hoe while the weeds are small, and pull if the weeds and the vines get ahead of you.
- Weeding is generally not onerous because the sweet potato vines cover the ground within 6 weeks of planting and smother any newly emerging weeds.
- Deer eat sweet potato plants at all stages, including digging out the roots in the fall. Dogs, fences and guns are the three most effective methods of deer control. The plants can be covered with row cover or plastic net for the growing season. Motion-sensor sprayers work well if maintained.
- Rabbits eat the foliage. Plant the slips on black plastic to hold back weeds, then put wire hoops over the rows and cover with row cover for 3–4 weeks while the plants are young. Even after the plants are large rabbits can cause substantial losses.
- Groundhogs dig and eat the roots. They can be trapped with baits of fruit. What’s for dinner?
- Pocket Gophers search out sweet potatoes to eat. Their mounds may be hidden under the foliage and the plants may survive as they only eat the larger roots, leaving no crop.
- Voles move in from grassy areas to live under the mulch and feed as fast as the roots form. They eat the roots from the top down leaving the outer shell in the soil where they have feasted. Cats are the best control.
- Rats love the roots. Cats or dogs are the best methods of control.
- Field Mice build nests under black plastic and eat the roots emerging from the ground.
Human “pests” of sweet potatoes
You can eat sweet potato leaves yourself and it takes several meals to reduce yields of the tubers. Some researchers working in Vietnam, discovered that harvesting 25%, 50%, 75% or 100% of the vines every 15, 20, or 30 days (ignoring the information about the season of the year and the varieties) gave the sort of results you might expect. Harvesting tops every 20 days gave highest yields of greens. Harvesting 50% of the greens each time gave highest total yields of greens. Harvesting not more than 25% or 50% of the greens each time gave the highest eventual tuber yields, after 120 days. Researchers in Tanzania came up with the clear information that harvesting three times at one month intervals gave the highest greens production, but the tuber yield was affected tremendously. Harvesting tops twice in a growing period proved the best in leaf production as well as root yields. So, clip 25-50% of the tops of each plant up to twice in one summer, and you’ll still get a good yield of roots.
Also see the University of Arkansas Extension Nutritional and Medicinal Qualities of Sweetpotato Tops and Leaves. This publication explains how to plant slips 2” (5 cm) apart, specifically for greens (vine tips) and harvest 6 times between the end of April and the end of October.
Insect pests of sweet potatoes
See North Carolina State University, Pests of Sweetpotato for photos, drawings and details.
Although there are many insect pests that feed on sweet potato vines and leaves, most do very little damage, and hunting them down is not justified.
Pests that feed on foliage
- Sweet potato flea beetles – Tiny black/bronze oval beetles (1.6 mm long), with reddish-yellow legs, and ridged wing covers; make small shot-holes in leaves or grooves in the upper surface of the leaves. Damaged areas turn brown and die. See below about larvae.
- Sweet potato weevil adults and larvae do feed on the foliage, but mostly go for the roots (see below).
- Caterpillars of three kinds:
- Southern armyworms – Gray-black larvae up to 36 mm long with green or pink tints; pale longitudinal stripes and pairs of triangular spots along the back; pale yellow heads with bright red-brown marks. They feed on leaves and tips of vines, and congregate around the bases of plants during the middle of the day.
- Sweet potato hornworms – First instar: white with a black horn; later instars (up to 90 mm long): green or brown with black diagonal lines down each side and a black horn, with a green or brown head with black stripes. They defoliate plants and often hide under leaves near the bases of plants.
- Yellow-striped armyworms – Pale gray-black caterpillars up to 45 mm long, with orange-yellow stripes along the sides and pairs of triangular spots on the back of most segments; brown heads with black markings and a white inverted V. They feed similarly to southern armyworms.
- Potato leafhoppers – Wedge-shaped insects up to 3 mm long; green bodies with yellow to dark green spots. They usually jump rather than fly. They suck sap from the underside of leaves causing yellowing of leaf tips and margins.
- Fruit or vinegar flies – Small yellowish red-eyed flies about 3 mm long. They hover around overripe or decaying produce. They may be found with their small creamy maggots in cracks in sweet potatoes.
- Tortoise beetle adults and larvae – Long-oval shaped gold beetles, up to 8 mm long, with various black or red markings on their flattened, shell-like bodies. The larvae have dull yellow, brown, or green bodies up to 12 mm long and black heads, legs, spots, and spines. Long spines on the abdomen hold excrement. Adults and larvae chew the leaves riddling them with holes.
- Spider mites – Tiny reddish or pale spider-like arthropods that feed on the underside of leaves. Heavily infested plants develop a yellowish, bronzed or burned appearance.
Pests that feed underground on tubers and side roots
- Sweet potato flea beetle larvae – Thin white, cylindrical larvae, up to 5 mm long, with 3 pairs of legs near their heads. They make shallow, winding tunnels on the surface of sweet potato roots and sweet potatoes. The tunnels darken, split, and leave scars.
- Sweet potato weevil adults and larvae –Snouted beetles 6 mm long with dark-blue wing cases, orange-red legs and thorax, and fat, legless, 9 mm grubby white larvae with pale brown heads. The beetles make small holes over the surface of sweet potatoes mostly at the stem end. The larvae tunnel inside the tubers, leaving frass, which causes the sweet potatoes to taste bitter.
- White grubs (spring rose beetles) – Dirty white grubs up to 25 mm long with brown heads and 3 pairs of legs near their heads. They leave large, shallow feeding scars on the sweet potatoes.
- Wireworms – Thin, tough, wire-like larvae with 3 pairs of short legs near their heads and prolegs at the end of the body. They initially create large shallow cavities in sweet potatoes which they later excavate into deep ragged holes. Three species, with colors from yellowish-brown to cream or yellow-grey. Heads are darker, brownish.
- White-fringed beetle larvae – Yellow-white legless, 12-segmented grubs, up to 13 mm in length, with small, pale heads. They chew into the roots.
Afflictions of sweet potatoes (these are not caused by disease organisms)
- Round chunky roots, low yield, purple color: Planted too early, too cold.
- Low yield: Flooded or crusted soil 6-7 weeks after planting? Planted too early?
- Rough irregular shaped roots: Heavy clay soils or organic matter above 2%.
- Rattails – thin, tough, tubers: Hot dry weather, insufficient water.
- Long, slender malformed roots, reduced yield: Potassium deficiency.
- Souring – Tissue breakdown caused by poor soil aeration, such as flooding.
- Water blisters – Small whitish bumps around the lenticels (breathing holes): wet soil.
- Blister – Small raised bumps appearing several months into storage: boron deficiency.
- Fine hairline cracks: Another sign of boron deficiency.
- Cracking: Uneven water supply or too much late-season water.
Diseases of sweet potatoes (mostly fungal)
More info and photos: North Carolina State University Diseases of Sweetpotatoes.
- Brownish skin patches, worse in wet years: Scurf fungus, Monilochaetes infuscans. More likely if too much compost was used. Stored roots shrivel.
- Metallic black surface lesions, maybe covering most of the root: black rot fungus, Certocystis fimbriata. Internal decay is not deep, but the fungus may impart a bitter flavor.
- Sunken brown lesions that may completely encircle the root: ring rot, Pythium
- Sunken lesions that dry and may fall out: Circular Spot, Sclerotium rolfsii. May taste bitter.
- Hard, dry, black, sunken spots developing in harvest wounds: Fusarium. Spots may become larger than 2″ (5 cm) diameter, but damage is not deep.
- Pitting: Soil rot or soil pox fungus in the presence of water stress. Roots will be small and malformed.
- Streptomyces root rot bacterium causes a similar rot.
- Fine or coarse irregular cracks, browning of the surface; dry, corky, dark-colored clumps of tissue scattered throughout the flesh, becoming worse if roots are stored warmer than 60°F (16°C): russet-crack/internal cork, feathery mottle virus (yellow feathery patterns of leaves). Do not use as seed stock.
When to harvest sweet potatoes
Unlike white potatoes, which have the annual plant sequence of vegetative growth, flowering and dying back, sweet potato plants would go on growing forever if the weather remained warm enough. Choose when to dig them up, ahead of cold weather. The longer you wait, the bigger the potatoes, but you are gambling with the weather. Usually sweet potatoes are harvested in the week that the first frost typically occurs in your region. I have written plenty already in previous years about harvesting, so I won’t go into it here. See one of the links to those posts, or my slideshow, if you want to know what comes next, or your climate is considerably colder than mine in central Virginia.