Creating the ''perfect'' vegetable plants through grafting

Controlled-environment growers have long known the benefits of grafted plants. Field growers are quickly learning them too.

By David Kuack

Plant grafting of plants has been done for thousands of years. Preparing and using grafted vegetable plants is common in Asia, Europe and other regions and is gaining use in North American production systems. North American greenhouse and high tunnel growers were the first to use grafting most routinely, but field vegetable growers are showing increased interest in the benefits grafting has to offer.

Grafting joins the root system of one variety to the shoot of another variety to create one “hybrid” plant. The plant used for its roots is called the rootstock. The plant used for its stems and leaves to produce marketable fruit is the scion.

Matt Kleinhenz, professor and extension vegetable specialist at Ohio State University-OARDC in Wooster, Ohio, said the number of vegetable crops that are being grafted is steadily climbing.

“Currently the core crops include tomato, watermelon, cantaloupe, pepper, cucumber and eggplant,” Kleinhenz said. “These crops are grafted for various reasons, including their financial value and because their production can be limited by issues that grafting can address.”

 
Advantages of grafted plants
Kleinhenz said there are a number of potential benefits provided by grafting. These benefits apply to both the person who creates the grafted plants and the one using them.

“The broadest description of the benefits of grafting may be that it makes better use of genetics in production,” he said. “Single commercial fruiting varieties are often hybrids. When developing them, the breeder attempts to incorporate most or all of the traits that matter into each one. That process is resource demanding. It takes time and money. It’s technically challenging and it always involves compromise. Each and every variety is imperfect in some way. A variety may be better than its predecessors, but it is still imperfect in some way.”

Kleinhenz said there a number of ways in which hybrid varieties can be imperfect. They can be less resistant to soil-borne diseases or deleterious nematodes. They can use water or nutrients inefficiently. They can be susceptible to various forms of abiotic (nonliving) stresses including cold, heat or salinity.

“Instead of incorporating all of the desirable traits into one variety, grafting creates an instant combination of two varieties,” he said. “The attributes of the two varieties are specifically chosen, but there is no attempt to blend them into one particular genotype, as in traditional hybrid development. Instead, grafting provides the best of both varieties by splicing them together. Through that splicing a new “physical” hybrid is created for use in that production season only.”

Grafting provides the best of two plant varieties by splicing
them together.
Photos courtesy of Matt Kleinhenz, Ohio State University-OARDC

Kleinhenz said traditional development of a standard hybrid must overcome barriers to the crossing of the parents, the movement of traits from one plant to another and the possibility that bad traits tag along.

“In grafting, two varieties must be compatible to be grafted,” he said. “Grafting allows for the bypassing of difficult and time-consuming steps that are required to create a superior variety that is good from top to bottom. For this reason, grafting may increase both the range of traits available to growers and the speed into which they come onto the farm.”

Kleinhenz said in those systems that rely heavily on grafting, scion varieties are bred to produce high quality fruit and rootstock varieties are bred to power the scion. The scion does not need to resist or tolerate soil-borne stresses and the rootstock does not have to produce marketable fruit.

He said grafting combines two excellent varieties in a matter of seconds. However, an average of two to three weeks may be required to prepare the seedlings to be grafted and to allow newly grafted plants to heal before transplanting them.

An average of two to three weeks may be required for newly
grafted plants to heal before they are ready to transplant.

Grafting potential

“Grafted plants are primarily used to limit losses due to soil-borne diseases and deleterious nematodes,” Kleinhenz said. “Grafted plants have shown the ability to limit losses caused by organisms that attack the root system or the lowest shoots just above the soil line. Grafted plants are not widely used to combat foliar or fruit diseases such as late blight of tomato that essentially attack the shoot well above the soil line. Foliar disease management is still primarily the responsibility of the scion.”

Kleinhenz said grafted plants have also performed well under less than ideal growing conditions.

“Tests completed where soil salinity was high, where soil moisture was excessive, and when soil temperatures were low have demonstrated the high potential of grafted plants,” he said. “Grafted plants have also out-yielded ungrafted ones when conditions were good and they have been able to use water and fertilizer inputs more efficiently. Researchers and farmers are testing the performance of grafted plants worldwide under many conditions to discover where and when using them makes the most sense.”

The performance of grafted plants is being tested under many
conditions worldwide to discover where and when using them
makes the most sense.

Kleinhenz said the preparation and use of grafted plants is market-driven.

“If users see the benefits, suppliers will offer them,” he said. “Potential suppliers will be reluctant to prepare large quantities of grafted plants until they are confident people will buy them.

“I recommend that potential users try them. Local suppliers and extension personnel can assist in getting started. Growers can also prepare their own grafted plants with just a little practice. Hands-on and free web-based training guides are widely available.”

Playing catch up

The use of grafted vegetable plants in soil-based production systems is much more common outside North America.

“The current cost of grafted plants, unfamiliarity with the full benefits of using them, not being sure how to use them and their occasionally inconsistent performance may explain the situation,” Kleinhenz said. “Early adopters are already fairly convinced. Others are taking a more wait-and-see approach. Adoption curves for new practices and technologies tend to be similar. The benefits have to be clear, consistent and compelling to a core group of growers. Then, word spreads.”

Kleinhenz said even though grafting is not new, until recently there have been limited resources available in North America for widespread and intense evaluation.

“The demand for alternative disease management strategies and vigorous and resource-efficient crops is high,” he said. “New rootstock varieties are available. More and more people have at least heard of grafting, grafted plants themselves and/or grown grafted plants. And, the pool of research-based information to aid growers is expanding.”

For more: Matt Kleinhenz, Ohio State University-OARDC, Vegetable Production Systems Laboratory; kleinhenz.1@osu.edu; http://hcs.osu.edu/vpslab.

David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.

 

Learning how to graft
The “Grafting Guide,” available from Ohio State University-OARDC, offers a detailed, easy-to-follow look at the entire process of grafting. It would be of interest to both inexperienced and experienced grafters.
This comprehensive pictorial guide discusses the splice-and-cleft graft method for tomato and pepper. It provides information on selecting rootstocks and how to evaluate the suitability of grafted plants for use in field and high tunnel production.
Included in the guide are a tomato rootstock table, seeding calculator, stem diameter chart, seed treatment fact sheet, healing chamber design and other reference materials. New additions to the guide will be prepared as experience and research-based information become available.

Grafting symposium scheduled for Nov. 6

The 2nd annual Vegetable Grafting Symposium will be held Nov. 6, 2013, in San Diego, Calif. The event is being convened by a USDA Specialty Crop Research Initiative-Supported University-USDA-Industry Team hosted by the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions.

The symposium’s objectives include:

1. Summarizing the current status and expected future of grafting as a technology for enhancing U.S. vegetable production systems related to profit, resource efficiency and sustainability.
2. Increase the understanding of challenges and opportunities associated with preparing and using grafted vegetable plants.
3. Strengthen and diversify partnerships required to widen the application of vegetable grafting as cornerstone technology.
4. Describe the USDA-Industry Team's goals and approaches.

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