Okinawan Purple Sweet Potatoes are a lovely and delicious cultivar, featuring a creamy, beige skin encasing gorgeous purple-magenta flesh. Full of the antioxidant anthocyanin, they’re also high polyphenols, vitamin C, and soluble fiber. These sweet potatoes are easy for the backyard gardener to grow in the U.S. climate.
Hello, Blue Zones fans from the new Netflix series, welcome — I’m watching, too! I’m an avid follower of all things Blue Zones, and I hope you’ll find the information here about Okinawan Sweet Potatoes helpful.
In the U.S., they’re harvested Sept/Oct, but will probably be difficult to find anywhere but specialty stores and farmers’ markets. If you want to grow them in your backyard garden next year, you should be able to find slips — the starter plant – next March or April at the big online retailers (see info below).
Also, if you’re interested in the soup mentioned during the Sardinia segment, I have a recipe on my food blog for Sardinian Longevity Minestrone, which was inspired by one of the Blue Zones books.
What is an Okinawan Sweet Potato? What does it taste like?
Despite the geographic name famous for cultivating them, they’re also commonly grown in Hawaii and are often known as the Hawaiian Sweet Potato.
Okinawan Sweet Potatoes – a.k.a., Okinawan Purple Sweet Potatoes or Hawaiian Purple Sweet Potatoes – are a beautiful cultivar, featuring off-white or beige skin with a vibrant purple-pink interior. They’re starchy – more so than their orange-fleshed counterparts – but have no saturated fats or cholesterol.
They were, in fact, a dietary staple of the people in the famous Okinawa Blue Zone (an area designated as having a high number of centenarians).
They are, unfortunately, difficult to find in the typical American grocery store, which is a shame because they’re so delicious and are my very favorite variety of sweet potato.
Taste is subjective, of course, but I find them far less cloyingly sweet than the standard orange grocery store variety (often, Beauregard). Nutty and slightly earthy, they not only make excellent crispy fries, but are also quite lovely in salads (I’m an add-all-the-vegetables kind of green salad fan).
They cook up fairly quickly in a pressure cooker, and my favorite work-from-home vegetarian lunch is one topped with black beans, salsa, and guacamole.
Okinawan Sweet Potatoes retain their color when cooked and, in fact, deepen into a rich, dark purple, while the skins tend to take on the purple coloring.
What are Sweet Potato Slips and Where to Buy Them?
So, where do sweet potatoes come from anyway? Unlike most vegetables, sweet potatoes are not started from seed, but rather from mature shoots produced by the tubar, called slips. The shoots are removed from the potato once they leaf out, and then placed in water to develop roots.
You can definitely grow your own slips from a sweet potato, but if you don’t have a sweet potato in your possession right now, you’ll need to go ahead and purchase the slips from a reputable nursery for this growing season.
The slips will arrive carefully packaged, usually with the roots encased in a soil plug. You should remove the slips from the outer packaging immediately upon receipt, leaving the root plugs intact.
It’s important to note that sweet potato slips are not cold-weather hardy. This presents a challenge to the gardener: you either lock in your order as soon as you find them (whether you’re ready to plant or not), or you wait to order until the weather is perfect and risk everyone being out-of-stock.
For me, the decision is easy: I want my Okinawan sweet potatoes, lol, so I don’t take a chance. I order early. This year (2021), mine arrived a full month before they’ll be planted, as we’re still having floods and the occasional overnight dip into the 30°Fs here in March and April.
I purchase my slips from Baker Creek Seeds. I hit their Live Plants page early in March, and sign up for the back-in-stock notification. As soon as I get word that they’re ready to purchase, I pounce, no matter the date or weather forecast.
I keep them in their little plastic shipping bags, rolled down to expose the upper leaves for aeration, sitting firmly in a ramekin in a morning-sunny window until I’m ready to plant. Keep the soil plugs lightly watered, and the slips will thrive. This year’s slips:
When to Plant the Slips
Your slips will be ready to plant when temperatures in your area no longer dip into the lower 50°Fs. For my Zone 6a, that’s mid- to late-May. In 2020, I planted my sweet potatoes on May 17th; May 15th in 2021 (although we went on to have a freakish dip into the 30s late in May, and I had to cover the plants to protect them from frost).
But don’t go just by the calendar; let nature be your guide. If it’s still snowing after your last frost date, don’t take a chance (look up your last frost date in the U.S.).
If you received them early and have been caring for them inside, first harden them off a bit by letting them spend a few hours outdoors each day for a week, under gentle sunlight (say, morning hours), before planting. If you don’t have a safe space for this (e.g., if you live in an apartment and are planting in a community garden), place the slips by an open window.
Prepare the soil
Sweet potatoes are not terribly fussy and are, in fact, robust survivors. But, the goal of this effort is to produce beautiful, well-formed sweet potatoes, and proper soil prep will take you a long way. The potatoes form underground, so the composition of the soil is everything.
At least one week before planting, you’ll want to deeply turn the soil where you’ll be planting the slips. This central planting is often called the “crown,” and that’s how I’ll refer to it in this article.
Then, at least lightly turn the soil in a 10-foot radius in every direction: vines will drop roots at their leaf notes to secure the plant to the ground as it expands. And sometimes, if you’re lucky, the plant will produce another cluster of potatoes from the nodes, so turning the entire bed deeply would not necessarily be a wasted effort.
Amend the soil throughout the bed with compost. Smooth out the top with a rake, water lightly, and let rest.
Clay soil is not optimal for potato formation because it’s not pliable enough to allow the potatoes to form naturally. It’s not impossible, but you’ll need to add a lot of compost to the crown area to lighten the soil.
At the other end of the soil spectrum, loose, very peat-y soil is also not the best for potato formation. Potatoes need a little resistance – a guiding hand, if you will – from the soil to keep from growing too long and skinny, or to become malformed in a way that’s awkward for cooking.
The ideal soil is light and “fluffy”, but holds together well when damp and squeezed. The area should also be well-draining, as sweet potatoes like to be damp, but not drenched. Do the hole test: dig a hole 12″ deep and fill it halfway with water. The water should drain away in the soil at a fairly fast clip. Standing water indicates that the bed needs to be turned more deeply and amended with compost.
How Much Space is Needed to Grow Sweet Potatoes?
I won’t mince words here: you’ll want a big space!
Although the actual clusters of sweet potatoes do not take up too much two-dimensional area – in fact, you can absolutely grow them in a large burlap bag or container – they require nutrition from the leaves that the plant’s vines produce. And those vines grow incredibly long.
So, actual in-the-ground space needed by the potatoes: small’ish. Actual spread space needed for the vines: large.
I grow all of my sweet potatoes in a 10-foot x 10-foot plot. This plot is too small, lol, but it all works out: as the vines reach the fenced boundaries, I begin redirecting them to wrap around the plot’s edges in a circle.
Note that you can grow other plants in the same plot: anything that grows tall, whose own leaves will be well above the leaves of the sweet potato vines. Keep in mind, though, that late in the season, the area will be very densely covered in vines and leaves and will be difficult to reach. You could, however, plant something like bell peppers along the border of the plot, where you’ll easily be able to reach the peppers without squooshing the sweet potato vines by walking on them.
Last year, I grew honeynut squash in the sweet potato bed, and even though there were vines e.v.e.r.y.w.h.e.r.e, it all worked out rather well, and both harvests were quite abundant.
How to Plant Okinawan Sweet Potatoes
You’ll want to plant the slips 12 to 18″ apart, so that the potatoes have plenty of room underground to sprout, separately from each other. They don’t have to be in rows.
As you saw from my plot illustration above, I plant three slips in a little triangle, with the varieties separated from each other with additional space.
Pluck off the lower leaves of the slip, leaving an umbrella of leaves on top.
Dig a hole 4″ deep and set the slip firmly all the way in, pressing the soil firmly against the plant. Water well.
Finally, you’ll want to mark the crowns – the base of the center plant. The majority of sweet potatoes will be produced in a cluster beneath the crown. Sounds simple enough, right?
But in a good year, healthy plants will produce an abundance of leaves from numerous vines shooting out in every direction, filling the surrounding space with gorgeous greenery and obscuring the location of the crown as the season wears on.
I always drive in a 3-foot stake near each crown, so that in October, I know exactly where the concentration of sweet potatoes will be located.
Caring for Okinawan Sweet Potato Plants
One of my favorite things about growing sweet potatoes is that they’re very easy to care for and are basically hands-off for much of the growing season.
After the plants have acclimated to their new soil home (two to three weeks), I fertilize once every two weeks early in the season, but as soon as the vines are robust and leafy, I halt fertilizing altogether, unless we’ve had a lot of rain.
Regular fertilizer with a formula of 5-10-10 or 8-24-24 is just fine, keeping an eye on the nitrogen. Even though sweet potatoes need their leaves for nutrition, you still don’t want to encourage the plant to go completely overboard with the greenery by overfeeding it with a too-heavy nitrogen concentration. This would produce too many leaves and not enough tubars.
Protect the leaves!
Do deer eat sweet potato vines? Oh, yes, they do! Furry woodland creatures love sweet potato leaves — they’re edible to humans, too — so plant your sweet potato slips in a fenced-in plot, protected both from deer and bunnies.
Remember that deer can jump very impressive heights, so my fencing is 7 feet tall, with chicken wire wrapped around the lower 2 feet, sunk a bit into the ground, to keep out the hungry bunnies.
However, if your sweet potato vines do get stripped of their leaves by a clever rascal, don’t give up. Sweet potatoes are determined survivors and will usually leaf out again fairly quickly.
Because the plot I use for my sweet potatoes is fairly small in proportion to the plant size, a few of the vines inevitably escape through the chicken wire and grow from there. I consider anything outside the fencing to be fair game for the furries. 😉
There is one distinct advantage to sharing the area with sweet potato leaf-lovin’ creatures, though: at the end of the growing season, about a week or so before my planned harvest, I remove all of the fencing, and let everyone have at it.
This clears the area of leaves and reveals the sweet potato vines. It also makes digging so much easier, because I’m not struggling to get through the thick bed of leaves to find the ground in which to dig.
I hope you’ve found this article helpful! Growing Okinawan sweet potatoes is a low-hassle joy, and the resulting potatoes are absolutely delightful and worth the space they occupy in the garden.
If your crop is ready to harvest, I’ve got your back with this guide for how to harvest and cure sweet potatoes.
- Well-composted soil
- Garden shovel or spade
- Hand trowel
- Garden gloves (optional)
- Prepare the Soil: about one week before planting, when temperatures are consistently above the mid-50s, deeply turn the area where the sweet potatoes will be planted. Add well-composted soil. Rake flat, lightly water, and let rest.
- While the potato bed rests, begin hardening off the potato slips by setting them outdoors for a few hours each day, in gentle sunshine.
- To plant: dig one hole for each slip, 4" deep, and at least 12" apart in every direction.
- Carefully remove the leaf stems along the bottom of the slip, leaving a canopy of leaves at the top.
- Set the slip, roots down, all the way into the hole (one slip per hole), and fill in with soil, patting everything down to firm it up.
- Water lightly. You're good to go! Fertilize every two to three weeks with a 5-10-10 fertilizer until the plants are well-established and vining profusely.