Though not as commonly grown as white potatoes, sweet potatoes are easy to grow in much of Pennsylvania. Why not give growing sweet potatoes a try?
Sweet potatoes (ipomoae batatas ) are part of the morning glory family (Convolvulaceae). Like morning glories and their less popular relative bindweed, they spread and crawl on the ground. Their large heart-shaped leaves and branching stems will create a nice ground cover, choking out weeds. Sweet potatoes are native to the tropical regions of Central and South America and will thrive in the summer heat. They are not usually prone to disease or pests, and those attributes make for an easy plant in your vegetable garden. Sometimes sweet potatoes are referred to as yams, but yams are tubers related to lilies and grasses. Yams wear a dark brown skin, and their white flesh is not so sweet. Sweet potatoes are not tubers. They are true roots, called storage roots, that will continue to enlarge as long as the plant continues to grow. The Guinness World Record for the biggest sweet potato is 81 pounds 9 ounces! And unlike other crops, the quality of sweet potatoes improves with storage.
Start with slips
You do not grow sweet potatoes from seeds or seedlings but from slips. Slips are sprouts on sweet potatoes from the previous year’s crop. The slips can be bought by mail order, nurseries, or farmers’ markets. Slips from reputable nurseries will be free of disease. As a fun project or if you need a lot of slips, you can grow your own from a previous harvest or from store-bought sweet potatoes. Fresh sweet potatoes or sweet potatoes treated with sprout inhibitors will be harder to sprout.
Six to eight weeks before planting, select mature sweet potatoes (small/medium) that are about 1.5 inches in diameter and are free of blemishes. Prepare a container with 2 to 3 inches of moist potting soil or sand on the bottom, big enough to contain your sweet potatoes. Soil should be moist, not dripping wet. Place sweet potatoes on their sides and buried halfway. Put the lid on the box to conserve moisture and keep the box in a warm (75°F) and sunny place indoors. Each sweet potato will provide between three to five slips. Remove the lid when shoots appear. When the slips are about 10 inches long, you should harden the slips off to acclimate them to the outdoors. Bring the box outside a little more time every day for a week or two. First, place the box in the shade, and after, increase the sun exposure. You can separate the slips from the sweet potato when ready to plant. Cut off the bottom inch and plant in your garden. Slips do not need to have roots to be planted. If the weather has not warmed sufficiently for you to plant your slips outdoors, pot them up like regular seedlings, keep them in a warm sunny place, and water them regularly.
Note that sweet potatoes are prone to rot, and the method of growing slips by suspending the sweet potato in water carries more risk of a bad outcome.
Soil preparation
Wait to prepare the soil when it has warmed enough (65°F). In most of Pennsylvania, that should be the end of May, or the beginning of June. Sweet potato plants are very sensitive to cool soil, so plant three to four weeks after the frost-free date. Check your frost-free date with your county Extension or on the National Weather Service site (https://www.weather.gov/ctp/FrostFreeze).
Sweet potatoes need loose, well-drained soil; soil too rich in nitrogen will result in many leaves but few potatoes. Sweet potatoes in compacted and waterlogged soil will rot. Add 1 to 2 inches of compost to loosen the soil. When ready to plant, create long ridges 8 to 12 inches high and 12 inches wide with rows 3 feet apart. Wait for a cloudy day and plant in the evening to reduce plant stress. Bone meal can be added to planting holes to promote root development.
Planting
Plant the slips on top of the ridges, about one foot apart. Slips should be planted 6 inches deep, with at least two nodes underground and at least two leaves above ground (nodes are where the leaves shoot from the stem). The deeper the planting, the more nodes in the ground, and the bigger the yield. Ridges provide depth, better drainage, constant temperature, moisture, and more uniform development of the roots. Slips do not need to have visible roots when you plant them. Water newly planted slips every day for the first two weeks.
Maintenance
A mulch of compost, dried leaves, non-treated grass clippings, or straw can be placed around the slips to cut down on weeding at the beginning. Sweet potatoes will cover the soil very quickly and will shade out most weeds. Maintain regular watering, 1/2 inch of water a week after the two first weeks, and let them grow until ready to harvest. Easy crop!
Alternate growing methods
Sweet potatoes can be grown in large pots. Porto Rico and Vardaman are bush sweet potato varieties that can be grown successfully in containers. They are easier to manage and spread less, about 2 feet per plant. Most varieties of sweet potatoes will grow roots wherever the stems will touch the ground. The vines can reach 20 feet.
The young leaves and shoots of sweet potatoes are also edible. You can grow sweet potatoes vertically to ease harvesting the leaves or for space-saving. It can be done in the garden or in large containers. Choose a sturdy trellis and weave the vines through the trellis as they grow. (Remember that sweet potatoes are naturally crawlers, but not climbers.) If you keep the main shoot growing and intact, you can prune and harvest young shoots coming from side stems. If you grow sweet potatoes in the ground only for root storage, avoid pruning, as that can cause the roots to sprout.
Varieties
Sweet potatoes come in many varieties. Some can have yellow, orange, and even purple flesh. Yellow and purple sweet potatoes are usually on the drier side. Skin can be from yellow to deep purple, and the vines can be decorative, with purple stems and lavender or white flowers. Choose for your taste and the size of your garden.
Harvest
Sweet potatoes require 90 to 120 days to mature from the time the slips are planted. They need to be harvested before the first killing frost. A hard frost can damage the roots near the surface. The cold temperature in the soil (below 50°F) will result in root rot. Harvest your sweet potatoes when leaves are beginning to turn yellow before a light frost. If a light frost burns the leaves, do not wait to remove your sweet potatoes. If harvest needs to wait, cut the vines so that the decay does not spread to the roots. To harvest, be gentle; the tubers will bruise easily and not keep as well if broken or bruised. Find the crown and dig out the tubers by hand. You can loosen the soil below with a fork; the roots will be in the top 8 to 10 inches of soil. Ideally, let the potatoes dry for a couple of hours in the garden. If it is near or below freezing, let them dry inside.
Curing
You need to cure your sweet potatoes to keep them longer, heal wounds, and improve their quality. Curing will allow the starch to be converted to sugar. Do not wash sweet potatoes before storing them; just place them in a single layer in plastic bags to keep humidity high (85-90%). Close the bags tightly, cut some slits for aeration, and place them in a sunny part of your house where you can achieve a high temperature (80 to 85°F). The curing will be done in two weeks. To make the sweet potatoes even sweeter after curing, you can wrap them individually in newspapers, stack them in a box, and leave them in a cool place like a basement for six weeks. They will be perfect for Thanksgiving! Properly handled and cured sweet potatoes, stored in a dark location with a temperature of 50 to 60°F, should store well until spring.
Common problems
Voles, mice, and deer can become a problem. The first line of defense will be a garden fence, electric or otherwise. Growing sweet potatoes in raised beds or large containers big pots will keep any rodents out. Check regularly and apply control measures as needed.
Scurf is a fungal disease of sweet potatoes, which appears as dark spots on the skin. This disease is limited to the skin, and sweet potatoes will still be edible but can continue to spread in storage. It is a soil-borne disease, and the best remedy is to maintain a four-year rotation of your crops in the garden. It also can be controlled in growing sweet potatoes in more acidic soil (pH 5.8 to 6). You can check your soil’s pH with a soil test available at your county Penn State Extension office. Weather can sometimes be an issue if there is excess rain or if your soil become compacted. Be sure to provide good drainage. Sweet potatoes thrive in hot dry weather.
Sweet potatoes as a cover crop
Sweet potatoes are not only tasty vegetables but also a recommended cover crop, according to the Food & Agriculture Organization of the United Nations (FAO). As described above, sweet potatoes’ rapid and dense growth shades the soil and competes with weed seeds. There are so many reasons to raise sweet potatoes, a real winner in the garden!
Author:
Michèle Pique
Master Gardener, Lancaster County
