Why Does the Tomato-Potato Plant Exist?

​A few months ago, news of a weird new chimaera of a plant, half tomato and half potato, began making the news rounds, even appearing in a Colbert ​Report segment titled "Craziest F#?King Thing I've Ever Heard." The plant, informally known as the TomTato in the UK and "Ketchup 'n' Fries" in the US when it sees release this spring, has potatoes growing at the bottom and tomatoes growing on top. The company that created it, SuperNaturals Grafted Vegetables, is a collaboration between three long-time experts in fields related to horticulture, who spent over a decade working on the plant.

This isn't a genetically modified plant, and you can't grow one from seed. Instead it takes advantage of a very old, almost primitive technique, which might date back thousands of years, called grafting. Grafting is one of those natural wonders that those in the business take for granted and that sound completely insane to those outside it: if you cut the top of one plant and attach it to the cut end of the bottom of another plant, provided the two plants are fairly close relatives, both will survive as a single plant.

That sounds like a weird science experiment, but the technique actually has some pretty amazing possibilities for agriculture. The bottom of the plant is called a rootstock, and the top is called a scion. The scion, typically, is the fruit-producing part, and the rootstock is, well, the roots. The impressive part of grafting is that you can hack plants together to create a better plant than would ever exist in nature, almost like taking a kangaroo's legs and placing them under the torso of an Olympic high jumper.

For example! Say you have this delicious tomato variety, but you live someplace really arid, and this particular tomato needs a lot of moisture. No problem: find a tomato rootstock that thrives in that arid climate, even if that rootstock would produce garbage tomatoes or no fruits at all (as some members of the tomato family do), and graft the delicious tomato onto it. The arid-loving tomato rootstock will work its wonders, reaching deep into your shitty dry soil and pulling out nutrients, and feeding them upwards to the delicious tomato variety, which will thrive. Grafted tomatoes have been shown to produce, when done right, t​wo to three times more fruit than non-grafted varieties. Or you can pick a rootstock that's resistant to whatever blight or pest is in your area, and bingo: blight- or pest-resistant fruits. "You use less chemicals to grow grafted vegetables and you get more yield," says Alice Doyle, one of the partners of SuperNaturals. "It's a triple bottom-line profit if you use grafted plants." Doyle claims that a single TomTato plant can produce as many as 500 tomatoes in one season, and four pounds of potatoes.

SuperNaturals is taking it one step further by doing what's called interspecific grafting: taking two plants from the same family, but not the same species, and grafting them together. Potatoes and tomatoes are both members, along with eggplants, chiles, and tobacco, of the nightshade family. Andrew Petran of the University of Minnesota has spent years conducting research on grafting, including interspecific grafting within nightshades—his Masters research involved a tomato/eggplant graft. "Interspecific grafting between solanaceous crops is actually pretty easy," says Petran. "Anything within the same plant family—and thus genetically similar—has the potential to be grafted together."

There are some of the same benefits to interspecific grafting as in the more typical tomato-tomato grafts: eggplants, for example, can be more tolerant of wet environments than tomatoes, so an eggplant rootstock in a damp place would allow you to grow tomatoes that usually need drier climes. But the TomTato is more interesting because it's not designed so much to increase the yield of one plant, but to offer a single plant that produces two different edible vegetables (well, one fruit and one tuber, technically).

"You can definitely get multiple types of tomatoes onto the same rootstock, and it's been done before," says Petran. Those aren't common, exactly, but they're not unheard of, either—you can buy some, like this tomato​ plant that'll produce both white and black tomatoes, already. The TomTato, offering two separate plant species, is a rarity, though.

But that doesn't necessarily mean it's hard to do. Doyle refused to give me any specifics on how the graft was done, or on which variety of tomato and potato they chose. That's reasonable; it's rare that a variety of plant gathers much public interest, and they want to protect their investment.

"We worked for years on the combination and technique," Doyle wrote in an email. "This one is too tough for home grafting." She says that the team tried hundreds of combinations of varieties, some failing to graft properly, some failing to produce a yield, and some failing by various other means—they wanted to stay away from any greenish potatoes, for example, because the hue is caused by a toxin called solanine that can be dangerous to pregnant women.

Still, Petran is skeptical that the grafting is really all that complex. "I think they're full of it," he wrote in an email, calling grafting "a simple process using cheap materials." But he was no less impressed with the final product. "The tomtato sounds cool," he wrote, "especially for gardeners who are very limited on space—rooftop gardeners come to mind, for me."