Growing Sweet Potatoes

Sweet potatoes are one of the most under-appreciated crops you can grow. But it will reward those gardeners who give sweet potatoes what they really need: heat.

As you will hear in the podcast with Master Gardener and vegetable expert Gail Pothour, sweet potatoes have some very particular needs. A San Joaquin County (CA) listener wrote in asking how to alleviate the frustration of trying to sprout sweet potato slips from organic potatoes in a cold house in January. That’s the core problem right there — sweet potatoes are tropical plants, closer kin to morning glory than to the white potato, and they simply won’t cooperate without warmth. They’ll sit, sulk, and rot before they ever sprout.

The good news is that our region of California is genuinely excellent sweet potato territory. Twenty percent of all commercially grown sweet potatoes in the U.S. come out of Merced County here in California, so conditions here are about as favorable as it gets.

Timing is everything. Don’t rush them into the ground — mid-May is about right, once the soil has had a chance to warm up. Plant slips, not seeds — always buy certified disease-free slips from a reputable nursery or mail-order source.

More about creating sweet potato slips, from the Santa Clara Master Gardeners

* To grow your own slips:

* Start in February or March.

* Fill a shallow container that has drainage holes with moist potting soil.

* Nestle in the sweet potato so it is half covered in the soil. Place in a waterproof tray.

* Cover to retain humidity and keep it warm.

* Remove covering when sprouts appear in 2–4 weeks.

* When sprouts are 6 inches long, clip off 1 inch from potato (to prevent possible disease propagation from the mother potato) and plant directly in pots (or root in water then transplant into pots).

The Santa Clara Master Gardeners have a video, as well, about creating your own sweet potato slips.

Grow in raised beds or ridged rows, about 12 inches apart with three feet between rows. They need room to sprawl. Soil prep matters too. Work in some compost and a light dose of nitrogen before planting, and aim for something loose and sandy — sweet potatoes hate compacted ground. Water consistently for the first few weeks while the vines establish, then ease off. Once they’re growing vigorously, they don’t need much babying. For fertilizer, lean toward something a bit higher in phosphorus and potassium than nitrogen — a 3-4-4 or a 2-2-2 would be ideal.

Harvest runs anywhere from 90 to 140 days after transplanting. Watch for the vines to start yellowing — that’s your signal. Don’t wait too long, though. Cold is the enemy at this stage too: roots can suffer damage if temperatures dip below 50 degrees, even briefly. If frost is coming, get them out of the ground and into boxes in a warm, slightly humid spot right away for curing.

More about Sweet Potato Curing and Storage

From “Growing Sweet Potatoes in the Sacramento Region”:

Newly harvested sweet potatoes are not very sweet. They require 1 to 2 months of curing and/or storage before they will develop the sweet, moist taste we expect (part of the starch content turns to sugar). Freshly harvested sweet potatoes can, however, be candied or made into pies. This is a good use for roots that may have been damaged during harvest.

Sweet potatoes need to be cured if they are to be stored for long periods. The curing process allows any bruises or blemishes on the thin skin to dry so that rotting in storage is reduced.

If the sweet potatoes are not intended for long-term storage, curing is not essential; however, storing freshly-harvested sweet potatoes for several weeks will improve the flavor.

After the roots are harvested and thoroughly dry, put them in a warm, humid place (80° to 90°F at 85% relative humidity, if possible), or carefully lay the roots out in a warm, dry, and well-ventilated area for 1 to 2 weeks to cure and until all skin wounds have healed.

Commercial sweet potato growers have curing rooms with heaters, humidifiers, and evaporative coolers to maintain proper temperatures and humidity. Providing such curing conditions can be rather daunting for home gardeners, so listed below are some suggestions for curing sweet potatoes at home.

Several of these suggestions are courtesy of the Santa Clara County Master Gardeners who conducted a sweet potato trial in 2006. Sweet potatoes need to be handled gently, so be careful when placing the roots into containers (avoid throwing or dropping them) so that they will not become bruised, which will keep them from storing well and can trigger them to start decaying.

• Put sweet potato roots in a paper bag with a wet paper towel, close the bag, and cure them in a hot attic for 2 weeks. Then store them in a cool room at 60°F until ready to use.

• Place sweet potatoes in a large plastic container with damp towels, keeping the damp towels from touching the sweet potatoes. Place the container in a sunny location for 10 to 14 days, rewetting the towels if they become dry.

• Cure the roots in a warm, humid place (about 80°F) for 10 to 14 days. Then wrap each root in newspaper and store at 55° to 60°F, taking care not to injure the roots.

• Place roots in a single layer in a container on the floor of a greenhouse where the temperature ranges from 80° to 90°F during the day and no lower than 50°F at night. Let them cure for 10 to 14 days.

• Lay roots in a container placed near a furnace vent for warmth. If the temperature near the furnace vent is between 65° and 75°F, the curing period should last 2 to 3 weeks. To maintain high humidity, cover the container with paper or heavy cloth. The roots can also be packed in perforated plastic bags which will keep the humidity high, yet allow excess moisture to escape.

• Place a heater in a pantry or small room and adjust the temperature to about 85°F; place a bucket of water in the pantry/room as well. Gently place the sweet potato roots in well-ventilated boxes and place the boxes on a small table or platform above the heater (several boxes can be stacked). A week of curing is sufficient with this method. Caution: be sure the heater is not near flammable items and that safety precautions are taken to avoid overheating of the heater, which can cause a fire. The heater should be placed on a non-flammable surface.

STORING

When the roots are cured, they can then be stored in a dry, dark, well-ventilated place at 55° to 60°F for several months. Sweet potato roots are very sensitive to chilling injury at temperatures below 50°F, so do not store them at lower temperatures or quality will deteriorate. Symptoms of chilling injury include fungal decay, internal pulp browning, and root shriveling.

Storing them in an unheated garage or storage shed may be too cold during the winter months. Try wrapping cured sweet potatoes in newspaper and storing them indoors in a box beneath a bed, in a closet, or in an unheated room where room temperatures are slightly cool. Storing the roots in temperatures warmer than 65°F can cause the roots to sprout.

Check stored roots periodically and remove any that begin to decay or show other signs of deterioration.

SWEET POTATO VARIETIES

Visual comparison of diverse sweet potatoes included in a trial conducted by UC Davis Plant Sciences entitled, Opportunities to Breed Diverse Sweetpotato Varieties for California Organic Production. These materials varied in root yield, size category distribution, color, and other characteristics. Many existing heirloom varieties and landraces, including purple-fleshed varieties, produce low yields and a low proportion of roots in the No. 1 size category, despite being favored for culinary traits, such as flavor and/or visual appearance. Contents of each crate are the harvested roots of a single plot (1.83 m × 0.762 m). Varieties shown are (a) Okinawa Purple; (b) Nancy Hall; (c) Murasaki; (d) L-15-39; (e) Shore Gold; (f) All Purple; (g) Nam Hai; (h) Porto Rico USDA; (i) Viola; (j) Kekori; (k) Seon-Mi; (l) Dingess Purple; (m) Camote Morado; (n) L19-53-P; (o) Beauregard [10]; (p) L-19-18; (q) Molokai; (r) Morado; (s) Carolina Ruby; (t) Diane; (u) Vermillion [13]; (v) Waimanalo; (w) L-19-42; (x) L19-56-P; (y) Red Japanese; (z) L-17-182; (aa) CA O’Henry; (bb) L-19-15; (cc) L-17-189; and (dd) Covington [12].

From the Study: Opportunities to Breed Diverse Sweet potato Varieties for California Organic Production.

SWEET POTATO VARIETIES TRIALED AT THE FAIR OAKS HORTICULTURE CENTER, 2013

Results: A total of slightly over 51 pounds of sweet potatoes were harvested from the five plants. ‘Japanese’ was the winner with 15¼ pounds, followed by 10½ pounds each from ‘Bonita’ and ‘Covington’. ‘O’Henry’ had a yield of 9 pounds, and ‘Diane’ had a yield of 6 pounds. There were a variety of sizes and shapes of sweet potatoes that were harvested, with a 5-pounder from ‘Bonita’ and a 6-pound ‘Japanese’ sweet potato.

SWEET POTATOES PODCAST - TRANSCRIPT

Farmer Fred:

As we are fond of saying on this program, the healthiest food you can eat is the food you grow yourself. And one of the healthiest foods that you can grow in your backyard are sweet potatoes. Sweet potatoes offer great immune system support.One sweet potato can provide over 100% of your daily vitamin A requirement, and that’s essential for immune function, skin health, and eye health. It’s a high fiber food too. Sweet potatoes, a single cup, contain about 6.6 grams of fiber that promotes gut health and provides a feeling of fullness and that helps you manage your weight. It’s an antioxidant powerhouse as well. Sweet potatoes have anti-inflammatory and cancer-preventative properties. It’s a heart healthy crop, too. Sweet potatoes are a solid source of potassium, critical for regulating blood pressure and maintaining optimum heart function.

There are some tricks to growing sweet potatoes, though. And that’s what today’s Beyond the Garden Basics podcast is all about. We will be talking with Sacramento County Master Gardener and vegetable expert Gail Pothour, who has tips for planting, growing, harvest, curing and storing sweet potatoes. And it all started as a question from another California-based Master Gardener who wanted to know exactly how to plant a sweet potato.

Farmer Fred:

The listener writes: “Any experience you might have with sweet potato varieties or tips would be appreciated. It’s been very frustrating with a low success rate. As a San Joaquin Master Gardener in Lodi, I have access to resources to help with these crops. Any experience you might have with varieties or tips would be appreciated. Buying organic sweet potatoes and starting the slip-growing process in January has been frustrating with a low success rate. They are in my house, which isn’t very warm in winter. Well, when it comes to vegetables, we like to bring in our resident vegetable expert, Sacramento County Master Gardener Gail Pothour. And Gail, sweet potatoes, they really like the heat, don’t they?

Gail Pothour:

Oh, they do, and they’re warm. So you get them in too early and you’ll have problems. So you need to wait, get them in the ground when it’s like mid-May in the Sacramento Valley.

Farmer Fred:

In one regards, I understand this person’s frustration because at a lot of nurseries, sweet potatoes are available usually in late winter, and that’s not the time to be planting them. It’s a case of having to keep those sweet potatoes someplace cool and dry until the weather warms up.

Gail Pothour:

Right. Cool and dry, but not too cool because they are sensitive to temperatures below 50 degrees. So if you store them in your garage or in a shed during the winter, that could be too cold for them. One of the best ways to get these slips, and sweet potatoes are grown from slips, not the actual potato, is to get them from a disease-free certified mail-order place or a nursery. Sometimes local nurseries will carry the slips, but not often. Mail order catalogs will have them listed. But one of the main problems is most southern-grown slips cannot be shipped to California because there’s a quarantine for the sweet potato weevil. So it can be difficult to find slips that you can order, but you can also grow them yourself. If you have an organically grown sweet potato that you’ve been able to hold over during the winter, there is a process to grow out the slips yourself.

Farmer Fred:

Is there a chance of picking up a disease that way, though? Or should you actually buy the sweet potatoes, fresh seed potatoes, fresh every year?

Gail Pothour:

Well, there is a chance to have diseases that are passed along because the slips are grown vegetatively. You grow them from a slip that comes off of the sweet potato. It is possible to transfer diseases, particularly viruses and things like that. So I think if you’re careful, I mean, we always recommend you get them from a certified mail order, you know, a grower. But if you do it yourself, if you get an organically grown sweet potato at a farmer’s market, and there hasn’t been any kind of a sprout inhibitor put on it, then you can try it yourself. It can be a little bit of a challenge, but it’s kind of like the science experiment we did in high school. You know, there is a way that you can grow them. And we do have one of our Sacramento County Master Gardener publications does have information about growing slips yourself.

Farmer Fred:

It is. It’s a very handy publication called Growing Sweet Potatoes in the Sacramento Region. And you can do an Internet search for that just by using that phrase, “Growing Sweet Potatoes in the Sacramento Region”. If you add the letters “UCANR” after it, it’ll pop right up. UCANR stands for the University of California Agriculture and Natural Resources. And there’s a lot of great information there. And in the case of this particular person, I think they just started a little too early.

Gail Pothour:

Oh, I agree. They said that they had started them in January and that’s way too early because at least in our area, in order for the soil to be warm enough and the air temperature to be warm enough, we’ll plant them in the garden in May. So you would start the slips yourself two months before that, so March. So January is a little bit early. And this person also said that their home wasn’t very warm in the winter. And in order to grow out sweet potato slips, they need to be kept quite warm. So the way you would do it is to have small sweet potatoes, put them in a shallow container, cover them a couple of inches with sand or peat moss or some material, sawdust even, keep it moist and warm. So you would ideally put the container on, say, a propagation mat, a heating mat, to keep the medium warm, 75 to 80 degrees. So that’s what they need. If it’s too early and too cold, you won’t have very good success.

Farmer Fred:

But again, you have the conundrum of sweet potatoes being available at the nursery way too early, I mean, for planting. So you buy them in winter, but hold on to them and don’t start the slips until, like you said, when the weather warms up.

Gail Pothour:

Right. And unlike the regular potato, the russet, the Irish potato that we normally associate with growing in the ground, you put that potato in the ground and that’s what sprouts. Sweet potatoes, you don’t plant the sweet potato. You do need to get slips growing from that potato, and then you would snap those off and plant the slip in the ground.

Farmer Fred:

Okay, then what is the process for doing that?

Gail Pothour:

Well, as I mentioned, you would put the sweet potato in a shallow container, cover it medium, keep it warm. The slips would grow. And when they’re about eight inches long, you just snap those off and then just stick that in the ground. And then roots will form from that slip.

Farmer Fred:

Sweet potatoes have a large growing area. They can be grown in many areas of California and also around the country, too. They can be grown in many zones, and that would include, if you’re familiar with the National Sunset Zones, zones 26 through 33, and that includes Central and Interior Florida, the Lower Rio Grande Valley, the Gulf Coast, North Florida, the Atlantic Coast to Charleston, the Interior Plains of South Texas, Hill Country of Central Texas, the Interior Plains of the Gulf Coast and Coastal Southeast, the interior plains of the mid-Atlantic states, Chesapeake Bay, southeastern Pennsylvania, southern New Jersey, and the north-central Texas and Oklahoma area eastward, all the way to the Appalachian foothills. So sweet potatoes do have a very wide growing area where you can try this yourself. So what sort of fertilization and watering do sweet potatoes need?

Gail Pothour:

Well, they need to have a fertile soil. It needs to be pretty sandy and well-draining. They tend to not do real well in, say, a heavy clay soil because when you harvest the sweet potatoes, it can get scarred up and things like that. But as far as fertilization, you don’t want high nitrogen because then you’ll get a lot of foliage and not so much flowering and the little sweet potatoes. So what we like to do at the Fair Oaks Horticulture Center when we grow sweet potatoes is give them a monthly application of something like fish emulsion that’s been diluted. Nothing too high in nitrogen.

Farmer Fred:

When are sweet potatoes ready to harvest?

Gail Pothour:

Yeah, many varieties are available after 90 to 140 days. So depending on the variety that you’re growing, you would start checking in the soil around that time. So if you were growing a 100-day variety, after 100 days, kind of, check around and see what size the little sweet potatoes are. But generally, it’s going to be later in the summer, even early fall, because it is a long growing season.

Farmer Fred:

Will the vines turn yellow first before they’re ready?

Gail Pothour:

Generally, they will. That’s one of the signs that they’re getting ready to harvest is the sign they’ll start to yellow. So if the vines have not started to naturally turn yellow, but it’s starting to become cold weather - remember I said they’re sensitive to temperatures at 50 degrees or below - then you might want to just harvest them before the cold weather because the cold would really damage the sweet potatoes.

Farmer Fred:

All right. So pay attention to those nighttime temperatures. And when the forecast starts calling for nighttime temperatures or early morning temperatures to be below 50 degrees, get out your shovel or spading fork and start harvesting.

Gail Pothour:

Absolutely. Yeah, because they will be damaged. even in just a few hours at below 50 degrees.

Farmer Fred:

How do you store sweet potatoes?

Gail Pothour:

Good question. Sweet potatoes do need to be cured before they’re stored. Otherwise, they will decay in storage. And it’s kind of tricky of finding ways to cure sweet potatoes because commercially, the growers do have curing rooms with heaters and humidifiers and evaporative coolers to maintain all the proper temperature and humidity. Well, that isn’t something that home gardeners really have access to. So actually, in the publication that I mentioned earlier about growing sweet potatoes in Sacramento area, there is a list of ways that home gardeners can try to cure the sweet potatoes without having all that fancy equipment. The thing is, you want to keep it warm and humid. So it’s kind of hard, at least in our area, to have warm weather if it’s humid because we don’t have a lot of humidity here. But there are things like storing them in a warm area, a greenhouse or something with a pail of water and covering them with plastic to keep the humidity in. So things like that. But you do need to cure them for several weeks in order for them to store properly. And another way to cure sweet potatoes, as I say, it can be rather daunting to have the right equipment, is that you could set up a heater in a small room or in a pantry and have the temperature about 85 degrees and then have a bucket of water that you would place in that room to kind of keep the humidity up and put the sweet potato roots that you’ve harvested in boxes have them well ventilated So in a single layer and place that box on a table or something above the heater so that you have the warmth and the humidity and that would help with the curing process and it may take a week or so.

Gail Pothour:

However, just be careful if you’re using a heater around anything that’s flammable that you want to take precautions so that you don’t have the heater get overheated and could cause a fire. After you cure them you’d want to store them in a room that’s about 60 degrees and so you know what’s the coolest place in your house maybe wrap them in newspaper put them in a box under your bed kind of thing or in an unused closet but that’s kind of one of the challenges of growing sweet potatoes is the curing process and then storage process now if you don’t plan on storing the potatoes for very long. You could certainly eat them and that’s fine. But just be aware that newly harvested sweet potatoes aren’t going to be very sweet. They need that curing process or at least a storage time when the carbohydrates turns to sugar. So that makes the sweet potato a little bit more palatable. So you can certainly eat them right after they’re harvested, but they won’t have that real sweet flavor. That’s what we expect.

Farmer Fred:

Yeah, and I guess storing the roots in rooms that are warmer than 65 degrees could cause those roots to sprout.

Gail Pothour:

Right. So that’s another problem is if it’s too warm, then they can sprout.

Gail Pothour:

So it’s finding that optimum about 60 degrees in a dark place in a box under your bed, maybe.

Farmer Fred:

One thing we haven’t talked about in this discussion of sweet potato tips is, is a sweet potato the same thing as a yam?

Gail Pothour:

No, it is not. That’s a marketing ploy. Actually, the true yam is from Africa. It’s a completely different family, not related at all to the sweet potato. and they can get huge. They can get three feet long and weigh 30 and 40 pounds. So you don’t find two yams grown here in the U.S. I’ve never actually seen one. It was back in the mid 20th century that farmers in Louisiana decided they wanted to differentiate their orange flesh, sweet potatoes that are a little moister than some of the white ones that are a little drier. They wanted to differentiate them from those drier sweet potatoes. So they started calling them yams. So it was just simply a marketing tour. So in California, I don’t know about other states, but if it’s labeled as a yam, they have to also tell you it’s a sweet potato. So all sweet potatoes are sweet potatoes. There’s no true yam that’s grown here in the U.S. Also, I failed to mention that the leaves on sweet potatoes are edible as well.

Farmer Fred:

Well, you just did.

Gail Pothour:

But it’s that the sweet potato leaves can be used like spinach or chard.

Farmer Fred:

I would like to eat them raw. Can you eat the sweet potato leaves raw?

Gail Pothour:

So sweet potato leaves can be eaten raw, although they kind of have a slight bitterness, apparently, when they’re raw. When they’re cooked, they become a little more mild and delicate flavored. And I have not actually eaten sweet potato leaves raw myself. But according to one of the websites that I got on Specialty Produce, they were talking about it and said that it’s very similar in taste to spinach. It can be a little bitter when raw. So if you use it in a salad, it’s perfectly edible, but it might impart a little bitter flavor. Cooking it would spell all that out.

Farmer Fred:

We’ve learned a lot today about growing sweet potatoes. Gail Pothour, Sacramento County Master Gardener, thanks for your help on this.

Gail Pothour:

My pleasure, Fred.

Thank you, paid subscribers, for your support! And thanks to the free subscribers for their growing interest.

Fred Hoffman is also a University of California Cooperative Extension Master Gardener in Sacramento County. And he likes to ride his bike(s).

This is a public episode. If you'd like to discuss this with other subscribers or get access to bonus episodes, visit gardenbasics.substack.com/subscribe