Growing Sweet Potatoes

growing sweet potatoes-minSweet potatoes are known for their rich, golden flesh and are enjoyed by families around the world in a variety of dishes. Surprisingly easy to grow and occasionally mistakenly referred to as “yams” sweet potatoes were first domesticated around 5,000 years ago.

The sweet potato is a root vegetable. Additionally, its leafy greens can be consumed as a source of many beneficial nutrients. Surprisingly, it is not a member of the “nightshade” family but rather belongs to the family “Convolvulaceae”, meaning it is only a distant relative of the standard potato.

Start Growing Sweet Potatoes

While all varieties of regular potatoes grow favorably in cool soil, sweet potatoes grow best in warm conditions. They are tropical plants and are very sensitive to a cold climate. When considering the location for your sweet potato crop, remember that the plant will produce many fertile vines that will provide excellent ground cover. Sweet potatoes, therefore, are an ideal crop for beds that are adjacent to areas difficult to landscape properly.

When and Where You Should Grow Sweet Potatoes

Before planting your sweet potatoes, as with most crops, you’ll want to prepare your soil. Thankfully, sweet potatoes are not terribly picky. They prefer loamy soil which is well-drained and not overly rich. We advise you mix in a one-inch layer of compost and break up any clumps. Afterward, thoroughly dampen the bed with fresh water.

This is the proper time to check your soil’s acidity. You are aiming for a pH between 5.8 and 6.2. Sweet potatoes can tolerate a more acidic soil (towards 5.0) if necessary. Amend your soil as needed until you achieve the proper balance.

Sweet potatoes prefer a more sandy soil, as opposed to a heavier clay. If you are in an area with heavy clay, consider utilizing a raised bed. The benefit of planting your crops in a raised bed is that you can build your soil to the specifications required for your crop. For sweet potatoes, you’ll want to amend your soil with compost and sand. Potatoes planted in heavy clay will grow to be thinner and misshapen. Proper sweet potato root development is dependent upon adequate airspace throughout the soil; good aeration.

If you are located in a warmer climate zone, you may wish to plant your sweet potatoes around a month after the last Spring frost. Remember, sweet potatoes prefer hotter ground and air temperatures. You’ll want to wait until you can depend upon consistently warm temperatures in both.

In Northern zones, it may be a good suggestion to cover the soil with black plastic or black fabric mulch about three weeks before planting. This will help trap the heat from the sun and speed up the warming of the soil.

Understanding Seed Germination of Sweet Potatoes

Many farmers claim that sweet potatoes are so easy to grow that a plant dropped on the ground is actually capable of producing on its own if the land is warm and moist!

While we cannot vouch for the validity of this particular claim, we do know that sweet potato is one of the simpler crops to plant and maintain. In fact, sweet potato plants are not started from seed, but rather root sprouts, called “slips,” which may be purchased from nurseries or mail order suppliers such as New Sprout Organic Farms. Additionally, slips can be grown in your own kitchen from your own homegrown sweet potatoes if desired! More on this method will be explained later.

Roughly six weeks before you are ready to begin planting outdoors, you’ll want to prepare your slips. Place the roots inside a box of moistened sand, sawdust or chopped leaves in a warm spot. Be sure to keep it at approximately 75 to 80 degrees. Keep an eye on the slips, watching for shoots to sprout. Once the shoots reach six to nine inches long, cut them off from the root. Remove the bottom inch from each slip, since that portion may occasionally harbor disease.

Starting Sweet Potatoes Outdoors

Now you are ready to begin planting your sweet potato shoots outdoors! In order to create the most habitable environment for the sweet potatoes to grow, build up the soil into long and wide ridges. You will want it to be about ten inches high and spaced between three and four apart so that the crop’s vines have plenty of room to run. You can expect each row to produce around eight to ten pounds of potatoes.

Around about three to four weeks after the last frost, plant the shoots in full sun. You will want to be sure that the soil has warmed fully. Make holes six inches deep and three inches wide for each shoot. Bury the shoot up to its top leaves, pressing the soil down gently but firmly, and water well immediately after planting, and for the next few days with a solution high in phosphorus.

Take care not to bruise your plant. Sweet potato shoots are somewhat sensitive to bumps and bruises.

Since we will be planting our sweet potatoes in hot and sunny weather, it may be a good idea to provide them with some shelter, especially hot days. Covering your plants with overturned flower pots for three days after planting can help shield them from being scorched by the sun.

Look forward to your sweet potatoes to mature in 90 to 170 days. Remember that sweet potato plants love heat but are extremely frost sensitive. Ensure you are planting well past the last frost of the year, as there is no way to ensure the protection of your plants if a late frost comes through.

Watering and Mulching Requirements

Once all of your sweet potato shoots are planted, water them thoroughly. Essentially you want to soak your new plants, ensuring all of the surrounding dirt is wet. You will want to be sure to stop watering before the mound you’ve created begins to erode. These new plants will need to be watered thoroughly in this way every day for the first week, and every other day for the second week.

Each week thereafter your watering should begin to get slightly farther apart. Eventually, you’ll want to be watering once each week. If your ground is exceptionally wet or if your area had experienced a lot of rain, adjust accordingly. Keep in mind that sweet potatoes will withstand dry weather better than rainy spells, which can lead to rot.

In dry weather aim for providing one inch of water a week up until two weeks prior to harvest, at which time you want to allow the soil to partially dry out. Sweet potatoes will withstand drought, but the crop production will reduce as well. Ensuring that you maintain watering needs through the hottest parts of summer should manage this.

Along with your crop’s watering needs, do not forget or neglect the importance of mulching your sweet potato crops. Vines should be mulched two weeks after planting, to smother any weeds which have appeared. Mulching will also conserve moisture and help keep the soil loose. This is a necessity for sweet potato root development.

Additionally, it’s helpful and highly recommended to occasionally lift longer vines. This will prevent them from forming new roots at their joints and creating many undersized tubers at each rooted area when we want them ripening the main crop at the base of the plant. Besides this, try to handle the plant infrequently. You want to prevent wounding the relatively fragile plant, which could then be invaded by disease spores.

When needed, reshape your beds with soil and mulch. Dress the sides of the potato beds three to four weeks after initial planting with approximately three pounds of quality organic 5-10-10 fertilizer per 100 feet of planted potatoes. If you have sandy soil use five pounds of fertilizer. It is imperative that the fertilizer you choose for your garden is not a synthetic chemically-based fertilizer, but rather one derived from organic (living) products.

You will also want to practice no-till weed control. You may lightly hoe the top layer of beds to manage the weeds, but be cognizant of the living ecosystem beneath what your eye can see. We are creating life underground which is fueling our crops. Do not disturb anything beyond the very topmost layers of soil, ever. Do not prune your vines either, as you want vigorous ground cover from this part of your sweet potato plant, and a sprawling vine means healthy tubers are growing beneath the soil.

Companion Planting and Rotation Considerations

Generally, root vegetables such as sweet potatoes, beets, rutabagas, carrots, and parsnips make good garden companions. Along with root vegetables, sweet potatoes also thrive next to certain varieties of beans, such as bush beans and pole beans, if they are trained to grow along the ground, intermingled with sweet potato vines.

Contrary to popular belief, regular potatoes are not actually a close relative of sweet potatoes, but that doesn’t mean they cannot be grown together as well. They make great partners in your garden as they have similar needs once planted. Finally, many aromatic herbs, such as thyme, oregano, and dill, make excellent neighbors for your sweet potato crop.

Of course, there are some considerations to make when choosing to plant any crop next to your sweet potato plants. The largest and by far most considerable problem with sweet potatoes is their innate tendency to spread.

One way to control this is by planting within a raised bed. If you choose to forego this route, avoid planting near other crops. This is especially true if planting other crops near your sweet potato, which are also strong spreaders. One such plant is squash. Both crops will end up fighting for space, and in turn, both crops will be weakened.

Even when planting near or with appropriate neighboring plants, be aware of your sweet potato vine’s propensity to spread vastly, easily covering a large area. You’ll want to make sure the sweet potatoes beneficial neighbors are not crowded out by this vine.

For farmers located in the southern United States, one particular beneficial crop to keep in mind for companion planting purposes is summer savory. Planting this crop near your sweet potato can prevent the influx of the sweet potato weevil, a critter that will wreak havoc on your crop if not deterred. Plan to plant this crop along with your sweet potatoes and prevent the pest before it ever arrives. Always remember, an ounce of prevention is worth a pound of cure and this is exceptionally true with garden pests!

Harvesting and Storing Sweet Potatoes

Most sweet potatoes are harvested during the week of the first frost, which varies by location. Typically, farmers located in cooler regions will harvest well before those who are located in warmer regions. Some sweet potatoes may start to poke through the ground before the first frost, and some farmers will pluck these early.

However, the longer you leave your sweet potatoes in the ground, the sweeter they will be! Another good rule is to harvest when tubers reach six to eight inches in length, as larger potatoes can become “woody.” Harvesting is best completed on a day with moderate to mild temperatures, when the air is warmer than 50 degrees Fahrenheit, so as to avoid chilling injury to the crops.

If your garden has been experiencing a time of drought, be sure to water your sweet potato rows immediately prior to harvest. This will irrigate the soil and prevent scratching of the delicate skin with hard chunks of dried out dirt. Some growers may have heard of a supposed toxin in frozen leaves which can seep into the roots of your plant if the plants are exposed to frost during this stage; this is not true. Sweet potato leaves are entirely edible. Furthermore, there is no harm in allowing exposure to a couple of light frosts provided that the days and soil remain warm. Still, when frost does come, do not wait. It is imperative to harvest your sweet potatoes within a couple of days.

While frost itself is not dangerous, cold temperatures have drastic effects on sweet potatoes. Once soil temperatures drop below 55 degrees Fahrenheit sweet potatoes can become subject to a permanent chilling injury called “hardcore.” When this occurs, the potatoes will remain hard regardless of how long they are cooked. Therefore, becoming useless and ruining the crop. Furthermore, if roots are exposed to cold without proper leaf cover, they will lose their ability to absorb water from the soil.

When ready to begin your harvest, start by removing the vines from your sweet potatoes. Only remove the vines from the plants to be harvested that day. If you have several days worth of harvest to complete, leave the vines intact on all of the plants that you will not be digging that day. They will provide protection until you can get to the rest of the crop.

Clip the vines and roll them, leaving gaps between the rows. You should leave stumps to show where to dig for your sweet potatoes. Rakes do not tend to be helpful at “grabbing” the vines, but digging forks can be useful.

Roots will often stick up out of the ground. Therefore, mowing or similarly, potato digging machines will do far too much damage to delicate sweet potatoes. Sweet potatoes need to be dug by hand, often with the assistance of digging forks. Using these forks, dig up the tuberous roots. The roots grow underground in the shape of a bunch of bananas. We advise you begin digging about a foot from the center of the plant so that you avoid damaging them.

To start, dig about six inches straight down. Then, turn at an angle towards the center of the plant, and very gently remove the sweet potatoes by hand. Be very careful not to drop or throw the sweet potatoes. In this stage, the roots are very sensitive to bruising. The sweet potato skin is also very fragile and any abrasions need to be avoided as well.

Place the potatoes on the ground beside where they have been grown so that it will be easy to identify the most productive plants. You will likely want to save a few of these sweet potatoes for making slips later.

Allow the sweet potatoes to dry in the sun for about an hour, provided the weather allows. Do not leave roots exposed to temperatures above 90 degrees Fahrenheit for more than 30 minutes, or you risk them being scalded by the sun. Conversely, if it is below 55 degrees, they will suffer from dangerous chilling.

If you decide you would like to grow your own slips for the next season, select the best seed potatoes from the most productive crops. These should be easy to determine if you laid your potatoes out according to the above recommendations. Place these potatoes aside, then grade the rest of your potatoes into ones to “use first” and ones to store.

Any potatoes with soft, wet, damaged areas or deep holes will not store and should be pulled for immediate home use or composting. Larger, open or broken surfaces will cure. Curing forms a second, tougher skin over this fragile outer layer and potatoes with these minor blemishes can be safely stored.

After laying in the sun for up to an hour, spread your sweet potatoes out and allow them to dry somewhere away from the sun that is at least 45 degrees Fahrenheit for a few hours. Remember, prior to this stage, we have not washed our sweet potatoes. You can shake off excess soil, but washing will encourage rot. You only want to wash your sweet potato immediately prior to cooking. It is still critical to handle the sweet potatoes very gently, as their delicate skin is still very susceptible to scratching and bruising.

As a note, the very small or damaged potatoes may not be exceptionally sweet, as they have not yet been cured. Some people compare the flavor as more similar to Irish Potatoes, so you may wish to use discretion when using them in recipes at this stage. The rest of the potatoes can be placed into baskets or cardboard boxes for further curing and storage.

Simple Curing Method

There are several methods for curing sweet potatoes. One method is to store the potatoes in a well-ventilated spot with a consistent temperature of 85 to 90 degrees Fahrenheit for ten to fifteen days. Ensure the potatoes are not touching one another. When the curing process is complete, check for any potatoes that still show signs of bruising and discard them.

After this, the sweet potatoes should be stored in a room around 55 to 60 degrees with a humidity of 75 to 80 percent. These sweet potatoes should keep for several months. Whenever you need a sweet potato, remember not to dig through the box or bin, as this may bruise and damage the sweet potatoes.

Using a Curing Chamber

Another method of curing sweet potatoes is to create a curing chamber in your home oven. First, place an open pan of water on the floor of your oven for humidity. Place a standard household or workshop light with a 40-watt incandescent light bulb on one of the shelves of the oven. This light is your heat source. Place a thermometer on the middle shelf where you will be able to read it through the oven’s window with the door closed. Close the oven door almost entirely. Our recommendations say to place a wooden ruler or similar flat object in the opening to provide just a crack of ventilation. Turn on the light. Leave the arrangement for approximately an hour, and then check the thermometer. You are aiming for a temperature between 80 and 85 degrees Fahrenheit. If the temperature is out of this range, change to a thicker or thinner spacer, accordingly.

Another method is to use a large picnic cooler and electric heating pad to create a curing chamber. Whatever your method, once your chamber is between 80 and 85 degrees, gently place the sweet potatoes inside, and close the door (remember your spacer!). Check the temperature twice a day, adjust as needed, and plan to leave the sweet potatoes for seven days.

Storing Your Potatoes

Sweet potatoes should be stored six to eight weeks in a cool environment (between 55 and 60 degrees Fahrenheit) with high humidity before cooking for the best flavor. In the south, some farmers choose to bury them outdoors in “clamps” (mounds of straw and soil) to save indoor space. Beware of storing anywhere with temperatures below 55 degrees as this can darken the flesh and alter the texture of the sweet potato. Similarly, sweet potatoes should never be stored in the refrigerator. Stored properly, cured sweet potatoes will last six to eight months.

Growing Your Own Sweet Potato Root “Slips” for Future Crops

Sweet potatoes are not grown from seed, like most vegetables. Rather, sweet potatoes are started from “slips.” These are shoots which are grown from a mature sweet potato. Slips are available from nurseries as well as from mail order catalogs and online vendors, such as New Sprout Organic Farms.

However, we think the best way to grow the perfect sweet potato year after year is to grow your own slips, too! Remember, however, that if you are buying your sweet potatoes from the grocery store, it’s likely that it’s been treated to prevent sprouting. If you do buy a potato that hasn’t been treated, make sure to find out if you’re buying a bush type or a vining type.

In order to begin growing slips, you’ll need several healthy, clean (untreated) sweet potatoes. If you have recently grown your own harvest, we recommend you consider propagating your own crop by taking vine cuttings in the fall, placing the roots in water, and potting them as houseplants for the coming year. By the next Spring, you’ll have cuttings for early slips.

Another option is to save one sweet potato root for every ten slips wanted. Choose plants with the highest production (yield) and no rattail or stringy roots. From these plants, you’ll want to pick medium-sized sweet potatoes. When examining, you want sweet potatoes that are approximately one and one-half inch in diameter with desirable color and minimal blemishes.

Each potato will produce around fifty sprouts for slips. Carefully wash your potatoes and cut them into large sections. Place each section in a clean glass container or empty jars half filled with fresh water. Use toothpicks to hold the sweet potato piece half below the water and half above.

Moreover, sweet potato slips require warmth. A window ledge or radiator is ideal for placement. Within a couple weeks you should notice leafy tops on your sweet potato piece and roots coming from the bottom. After the sweet potato roots have sprouted, you’ll need to separate the plantable slips. Remove your sweet potatoes piece from the glass and carefully take each sprout in your hand. You must gently twist each slip off of the sweet potato to release it.

When this is done, place each sprout in a shallow bowl with the bottom half of the stem submerged in clean water and the leaves hanging over the top of the bowl. Shortly, new roots will emerge from the bottom of each slip. When these roots are about an inch long, the slips are ready for planting. To maintain the health of the slips, change the water regularly and discard any slips that aren’t producing roots or that appear to be wilting.

Another method for producing slips is to use a one and one-half gallon pot for every two potatoes. Do not forget to install drainage holes if your pot does not come pre-drilled. Store your potatoes in a well-lit room with a temperature between 65 and 70 degrees Fahrenheit until about three months before the last frost date. At this time, you’ll want to fill the pot with about three inches of mulch, followed by a quality soil. Plant your chosen potatoes (whole) in the pot at a 45-degree angle. You are doing this so that the sprouts will grow above the soil. When the slips reach six to twelve inches tall, you can plant them outdoors. Of course, provided the final frost has passed.

Choosing the Best Sweet Potatoes for Your Climate

Among the many types of sweet potatoes, we’ve chosen a few that you may enjoy depending on your personal needs and preferences.

  • “Jewel” – Copper-colored; Disease-resistant; Excellent storage life
  • “Stokes” – Purple-color; Known for a wide variety of health benefits
  • “Centennial” – Orange-color; Widely-grown; Good storage life; Good producer in Northern Climates
  • “Bunch Puerto Rico” – Copper-color; Very flavorful; Excellent for limited spaces
  • “Oriental” (also known as “Japanese Sweet Potato”) – Pink to purple skin, White flesh; Sweet, nutty flavor; Chef’s favorite
  • “Georgia Jet” – Reddish-purple skin, orange flesh; Great for Northern farmers; Fast growth; Extremely high yield

Additional Growing Tips for Sweet Potatoes

  • If you’ve discovered signs that frost has gotten to your sweet potato plants, act quickly to prevent the rot from spreading to the roots! For example, if you’ve noticed the tips of the leaves on your vines have turned black. When you notice this, clip the dying vines off at ground level as soon as you can. Then, you will be able to leave your sweet potatoes in the ground a few days longer, provided that the ground itself is not yet freezing.
  • Most tubers form directly below the center of the plant. However, some varieties form individual sweet potatoes much further away, or quite deep into the ground. Some sweet potatoes may even be found when a vine was resting on the soil and sent down roots. Make sure to dig through the entire area, and leave no sweet potato behind!
  • Pests tend to be more of a nuisance for southern gardeners than those located in the Northern region for a variety of reasons, but mostly because critters prefer warmer temperatures, too!
  • The sweet potato weevil is a one-quarter-inch long insect with a dark blue head and wings and reddish-orange body. It will puncture stems and roots in order to lay its eggs inside. The larvae will then feed on the flesh of the plant. Adult weevils attack vines and leaves. Weevils also spread rot, which can be identified by growing brown or black areas on stems near the soil and at the ends of the stem. Weevils multiply quickly and are difficult to get rid of. The best way to prevent this problem is to use certified disease-resistant slips and practice four-year crop rotation. If you find an infected plant, destroy it along with its roots.
  • Black rot can be identified by the presence of circular-shaped, dark-colored depressions on roots. To treat, discard infected sweet potatoes and carefully cure undamaged roots of the same crop. Be aware, black rot looks very similar to the much-less-serious “Scurf.”
  • Scurf shows up as round darkened spots on root surfaces, but these spots are small and do not affect the edible quality of the crop.
  • Stem rot, also known as wilt, is a type of fungus which enters through an injury to the plant. Usually, it is caused by an insect, improper or careless cultivation, or harsh winds. Stem rot may not kill your crop, but it will drastically reduce the harvest. To minimize your sweet potatoes’ susceptibility to this type of fungus, again plant only certified and healthy slips.
  • Dry rot occurs when stored sweet potatoes become mummified. Prevent this by maintaining the temperature at an ideal 55 to 60 degrees Fahrenheit.
  • When cooking sweet potatoes, if you’d like for them to retain their bright orange (or purple!) color, try cooking them with a slice of lemon!