How to Grow Sweet Potato Vines
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Q. I recently pulled out a decorative sweet potato vine as I was cleaning out my planters for the season and found HUGE tubers under the soil. I understand that they are not the tastiest kinds of sweet potatoes, but I would like to save them to plant again in the spring. What's the best way to store them for the winter? And should I cut them apart in the Spring and bury the pieces in my planters?
----Candy in Port Murray, NJ, "near the Delaware Water Gap"
A. A strong 'no' to the cutting apart for planting part. That IS the advice commonly given for "regular potatoes", like the buttery-delicious "Yukon Gold" and other gourmet varieties, but:
1) I plant my seed potatoes whole; I find that cutting them into pieces greatly increases the chance of rot, especially in the cold, moist soils of Spring. And
2) Sweet potatoes are not even remotely related to regular potatoes, and their propagation is very different—and as far as I know, pretty much unique in the word of edible plants. (Weird but true: sweet potatoes are actually related to morning glories.)
My huge "Plant Book" (a poor man's version of Hortus, and I am a poor man) explains that the genus "Ipomea" is a family of around 300 different vines from mostly tropical parts of the world. Members include the familiar morning glory (so named because its flowers open early in the day), its fragrant cousin the Moon Flower (whose blooms open up in the evening), and the sweet potato —which is itself a kind of a vine, but one that's always left to sprawl on the ground; which morning glories would also do if you didn't give them anything to climb.
The ornamental sweet potato vines' that have gotten so popular in planters and hanging baskets over the years are just a specialized variety of sweet potato—one that seems predisposed to grow crazy looking tubers down in the soil. My Public Radio show has gotten a ton of emails and phone calls from people who find the strangely colored treasures when they clean out their planters at the end of the season—mostly asking if these things are safe to eat.
Which they are; they just tend to not taste good. The really weird part is that many people trying to grow 'regular' sweet potatoes either report failure or tubers the size of a marble, while these ornamental vines always seem to deliver a huge harvest. Makes me wonder if we shouldn't be growing regular sweet potatoes in hanging baskets!
(I'm actually serious here—the leaves of hanging plants can collect a lot more solar energy than the leaves on a vine that's earthbound, because they don't pile up on top of each other.)
Anyway—can she grow new vines from this year's tubers?
I suspect the answer is yes. They're weirdly shaped and colored, but they're still sweet potatoes, so their propagation should be the same. But first comes proper preparation for winter storage. Experts recommend that you 'cure' freshly-harvested sweet potatoes; a mystical enterprise where they're exposed to heat and humidity in the 90s for a week. Then they go into a cool, dry spot.
Which sounds really treacherous, so I'm going to suggest that she only 'cure' maybe half of her harvest, just store the other half, and then see which ones 'slip' better in the Spring.
"Slipping" is when you grow little baby plants out of a whole sweet potato. Now, you can just bury whole sweet potatoes very shallowly if you like, but many gardeners prefer to grow slips from the tubers and then plant the slips. It's easy, fun, and one of the best gardening activities for kids. Now: All members of this plant family are very frost sensitive, so you don't want to start 'slipping' your tubers (or planting whole ones) until about two weeks before your last frost date in the Spring.
…At which point most people famously put one end of a whole tuber into a jar of water with toothpicks shoved into its sides to keep it from falling in.
But I'm not sure that method grows the best slips. The pros say to bury the tuber about halfway to three quarters in a container of sand and keep the sand wet. The green 'slips' will sprout from the sides and top of the tuber. After they reach about six inches in height, gently pluck them off and plant them in very loose, rich, non-chilly soil—or in our listener's case, a hanging basket or container.
The nice thing about only being interested in the decorative vine is that it takes a hundred days to get decent size tubers from 'regular' sweet potatoes planted in a garden bed—and that's in a season with a warm spring and summer. But the vines 'slip into sight' right away!