(Polly Becker for The Washington Post )

If we think biotechnology was invented in the 21st century, consider the somewhat bizarre practice of sticking the top of one plant onto the bottom of another: Grafting, this is called, and it’s been around for at least 4,000 years.

Those ancient Greek and Chinese gardeners knew that the right match could produce a plant greater than the sum of its parts — the roots of the first would make the leafy second more vigorous and fruitful.

Many of the woody plants in your garden are grafted — that weeping cherry tree now in bloom, for example — but the idea of growing grafted vegetables at home is new in the United States, with the first plants arriving five years ago in limited markets. It is still too soon to tell whether this is a passing fancy or something here to stay, though mail order catalogues and garden centers are betting on the latter. With each passing spring, they are increasing the quantities of grafted plants and the number of varieties.

The main focus of this phenomenon is the tomato, but this year consumers will also find grafted peppers, eggplant, cucumbers, cantaloupes and watermelons.

“We went from zero [grafts] in 2009 to over a million this year,” said Alice Doyle, who with partners John Bagnasco and Tim Wada produces and distributes vegetable grafts to wholesale growers and farmers under the brand SuperNaturals.

A grafted tomato plant. (W. Atlee Burpee Co. )

This year, the enterprise is offering a line of plants that will yield two varieties on one rootstock and a true Frankenstein plant that promises trusses of cherry tomatoes on top and a bed of potatoes below. It is being marketed as Ketchup ’n’ Fries.

Beyond the novelty value, why would gardeners grow grafted veggies?

Various rootstock hybrids have been developed to bestow different traits, including resistance to a slew of diseases that afflict such popular plants as tomatoes, including blight and wilt pathogens. Rare is the tomato patch in Washington that doesn’t have early blight, where leaves yellow from the bottom up and plants can become not just unsightly but feeble if the gardener doesn’t stay on top of it.

Grafts are particularly useful for gardeners who don’t have the space to move plantings from bed to bed annually. Tomato plants grown in the same soil each year will perform fine for four or five years but then will take a tumble as diseases and pests such as nematodes build up, said Steve Dubik, a professor of horticulture at Montgomery College.

Grafted plants can also achieve other miracles, such as producing more fruit, extending the harvest period and even tolerating hotter or colder climes. The selling point for heirloom varieties such as Brandywine or Mortgage Lifter is that grafted versions fix the general problems associated with antique varieties: poor yield and disease susceptibility. A researcher at the University of Maryland’s extension service found that the yield of grafted Cherokee Purple tomatoes increased by 25 percent.

Chelsey Fields, the vegetable product manager for W. Atlee Burpee & Co., said heirlooms tend to have been developed or discovered by gardeners in a specific locale and may be ill- suited to other regions, climates and soils. “They aren’t necessarily as adapted nationwide as people would like them to be. The thing with grafting is that it really makes them adaptable to all these different environmental pressures,” she said.

The promise of grafted tomatoes. A grafted plant, left, shows a markedly larger harvest than the same variety grown on its own roots. (Robin Bachtler Cushman/AP)

Burpee started selling grafted tomatoes in 2012 and now has a dozen tomato varieties for sale and more in the works as they are being trialed, Fields said. “Last year, I dropped Yellow Pear and added in Mr. Stripey, Green Zebra and Marglobe based on garden performance,” she said. This spring, Burpee added three pepper varieties and an eggplant to its catalogue.

The downside to grafted veggies, apart from their added expense, is that their garden performance may not be as dramatically different as billed, and in some cases it’s worse because the highly efficient root system may produce lots of moisture to the fruit but at a cost to flavor.

Jeff Gillman, a horticultural instructor at Central Piedmont Community College in Charlotte, conducted field trials with students and found that when the growing conditions were optimal, the non-grafted tomatoes actually fared better than the grafted ones. In difficult conditions of poor soil and inadequate irrigation, he found that the grafts were superior.

“I entered the experiment skeptical, and I left convinced that I would try grafted tomatoes if I were to grow them in a difficult situation,” he said.

Grafted vegetables “may not be a huge advantage to all gardeners,” Dubik said, “but for those gardeners who have to plant the same spot year after year and the disease pressures build up, they don’t have a lot of options.”

And then there is the cost.

You might expect to spend between $2 and $5 for a conventional transplant, depending on pot size and the retailer. A grafted plant is typically sold in a one-gallon container and retails for $12 or more. If you want to grow a fairly modest five plants, that starts to add up. Burpee sells three in 2 1/4 -inch pots for $26.85 plus shipping costs.

Like many gardeners, I start my tomatoes from seed in March. (Starting now would delay the harvest until late summer, but it is still an option.) Tomato seeds can be as much as 10 to 20 cents each (I find this shocking), but if you were bean counting, so to speak, you would also have to calculate ancillary costs such as seed-starting equipment, electricity for growing lights and your time.

As some gardeners are discovering, you can reduce the cost of grafted plants by making them yourself. This is still more expensive than starting conventional seedlings. You need twice as many seeds (one for each rootstock, one for each scion), extra equipment and an appetite for failure.

After the grafting, the plants must go into a “healing chamber” where high humidity and low light levels must prevail if the graft is to take.

Dubik teaches grafting to his students, and at first he found that he could count on only 20 percent of the grafts surviving. With experience, that is now up to 50 percent or more, and climbing, he said.

“You’ll know in about 48 hours if it’s going to work. Some of them take really easily, others don’t,” he said.

Grafted plants differ in another fundamental way from traditional tomatoes. Tomatoes are typically planted deeply to encourage more root development along the lower stem, but the union in a grafted plant must be kept above the soil line to prevent the scion from sending down roots and negating the benefits of the rootstock. Also, the plant must be supported and kept from sprawling so that stems do not touch the ground, where they would also put down roots.

Seed companies such as Johnny Seeds sell specifically developed rootstock varieties to commercial growers and home gardeners alike, though the cost of seed is high. A packet of 50 seeds sells for $23.65.

For demonstration purposes, Dubik will use seedlings of a robust, disease-resistant hybrid such as Rutgers or Jet Star and graft various heirlooms on to them. He uses a large plastic storage bin with a lid as a healing chamber and prefers a tube to a clip in securing the graft. The tubes are of different diameters to allow for variations in stem size.

He offers this tip: Don’t water the rootstock the night before grafting. If it is too hydrated, the water pressure in the stem will separate the graft.

But fixating on the cost or even the results of grafting may be missing the point: Gardeners like novelty and find delight (and frustration) in observing and growing new things.

“It’s a little bit of a fad but relatively easy,” Dubik said. “I think a lot of people are going to get into it and stick with it.”

More from Home & Garden:

How to graft tomatoes

Taking a bite out of tomato blight

Ask Adrian your questions about growing tomatoes