By JEFFERY GOSS
Missouri Correspondent

HOBERG TOWNSHIP, Mo. — Grafting has been used for about 3,000 years to propagate fruit trees, and for the past 200 it has been used for propagating shrubs and woody landscape plants. Now, researchers and private firms are taking an interest in doing it on vegetable crops such as tomatoes and watermelons.

“We grew some grafted watermelons this summer, and they were harvested,” said Andrew L. Thomas, horticulturist at the Southwest Research Center near Hoberg, Mo., who noted the seedlings were grafted at Lincoln University in Jefferson City.

Grafting can theoretically be done to any dicot plant; however, practice does not always follow theory. Generally, though, plants belonging to the nightshade or cucurbit families can be propagated by graft.

Vegetable grafting is essentially the same operation as fruit tree grafting: The upper part, or scion, of the plant is attached to the root system of another plant belonging to the same or a closely related species. It is not quite as easy as grafting a tree, however. The scion must be held onto the rootstock by a physical gripping tool, such as a vise, to keep it from falling off.

The vise cannot press the two plant tissues together too hard, though, or the result may be bruising and the graft’s failure. During the time it takes for the graft union to form, the environment must be carefully controlled.

“You’ve got to have 100 percent humidity to keep it from wilting,” Thomas explained, adding this is usually done in a greenhouse.

The graft generally heals within a week, or even sooner if growth hormones are used. When the graft is healed, the vise is removed and the plant can stand on its own. Within another week growth of the scion is resumed, and the plant can be transplanted into the field or garden.

Grafting trees is worth the hassle because they live for many decades, even centuries. But one might wonder, why such trouble to graft an annual plant that will live less than a year? Why is vegetable grafting practical, even if it is biologically possible?

“You’re going to probably have double the investment on the (seedling),” Thomas said, “but you will double the yield, decrease pesticides significantly ... increase vigor ... I predicted five years ago that (this technique) would revolutionize everything.”

The idea is to graft a select variety of the crop onto a root system that is also genetically selected, but for different qualities. In the case of watermelon this is often the birdhouse gourd, Lagenaria siceraria.

For tomato grafts, they are often put on rootstocks of Lycopersicon hirsutum x lycopersicum: A cross between the common cherry tomato and a wild South American tomato relative with unpalatable fruit. The rootstocks used are chosen for their resistance to pests and their efficient uptake of nutrients from the soil.

Vegetable grafting has actually been around for about a century, according to Carey Rivard, a Kansas State University extension horticulturist in Olathe. Rivard, who teaches workshops on vegetable grafting and tomato production, asserts “the first report of melon grafting was in the 1920s; the first report of tomato grafting was in the 1950s.”

Up until the past decade, however, almost all vegetable grafters were located in Japan, Korea or certain European countries. Only in the past seven years has anyone been commercially grafting vegetable crops in North America, and even now vegetable grafters are few and far between on our continent. Although, well-known companies such as Johnny’s Selected and New England Seed have begun selling grafted tomato seedlings.

The labor-intensive, space-saving techniques of European and East Asian growers are often slow to catch on in North America, where deficits in yield are typically made up by simply planting more of the crop.

“Most of our research started about seven years (ago),” Rivard recalled. “Our first field season (with grafted tomatoes) was 2005.”

He believes only in the last five years have U.S. companies and universities been practicing vegetable grafting “with any great regularity.” Rivard credits Clemson University watermelon expert Richard Hassell with developing the main grafted-watermelon research unit in the United States.

“Watermelons are more difficult to graft ... tomatoes are pretty easy,” Rivard noted. “We’ve given a lot of our workshops to (private growers, and) I think before too long you’re going to see more and more of that.”

He referred to small private growers doing their own tomato grafting. Vegetable graft visionaries such as Thomas and Rivard see the process as a potential way of enhancing the organic method, as well as reducing the pesticide and fungicide bill for conventional producers.

Thomas points to the example of Greece, which “used to be a big watermelon production area” until hit by an outbreak of plant diseases which ruined the crop. After some time Greek melon growers began to learn grafting, and they used birdhouse-gourd rootstocks, which are resistant to the diseases.
“With grafting, they’re back almost 100 percent to where they used to be,” he said.

In Japan, grafting is used in melon, cucumber and eggplant production. For some U.S. farmers, however, the technique remains too demanding for easy implementation on the farm. Even if it is possible to buy pre-grafted seedlings, the scion selection will likely remain limited to certain widespread commercial selections.

Vegetable grafting does not involve genetic engineering, contrary to what some believe. Nevertheless, it does involve two difficult steps: keeping the humidity at relative 100 percent, and physically holding the plants together without crushing them.

Moreover, it requires access to seed for rootstock varieties, such as birdhouse gourd or the tomato cross. Rivard has found “the seed companies are usually pretty tight-lipped” about what rootstocks they are using.

It seems likely vegetable grafting will be mostly the work of large seed companies with large greenhouses and expensive equipment. As with some other aspects of plant technology, however, it also seems likely there will arise a small number of hobby experimenters and loca1 businesses specializing in the technique.

It is unlikely, by most accounts, that melon grafting will ever be as common here as it is in Asia, where melons are often grown from transplant and where (in Japan especially) specialty melons regularly sell for $100, often for use as business-party gifts.

Moreover, perhaps because of the industrial scale for which vegetable grafting is most suited, the North American organic agriculture community has yet to broadly support the practice of grafting vegetable crops.

As with any new technology, its advocates can be sidetracked by the novelty of the procedure and overwork it beyond practicality. One experimenter grafted the tops of tomato plants onto the root system of potato plants (both of them being members of the same family, Solanaceae). The idea was that tomatoes and potatoes could be grown on the same plant, thereby saving space.
“It doesn’t really work out too well,” Rivard explained, “because tomatoes are a warm-season plant, and potatoes are a cool-season plant (and) you just end up with a lousy crop of each.”