By Jody Padgham
Envision a 30 to 50 percent increase in the yield of ripe fruit harvested from tomato plants healthy in the hoophouse from March through October. No, we’re not talking about the mythical Shangri-La, but instead the amazing potential of grafted tomato plants.
The concept of grafting a desired top stock (“scion” or “head”) onto a sturdy, disease-resistant rootstock has long been accepted in the tree fruit industry. As cultivars are developed for certain fruit characteristics, it can be challenging to also keep disease and yield qualities strong. By attaching the desired fruit-producing top to a sturdy, resistant root of another variety, a strong, disease-resistant, high-yielding plant can be created. In the past several years this concept of grafting has made it into the world of tomatoes and other herbaceous plants.
In grafting, a “custom” plant is created, with the optimal root for nutrient uptake, structural strength and disease resistance, and the ideal head to dictate the types and quantity of fruit. Various root stocks and head types are available and recommended for grafting. The final grafted plant must be “balanced,” with equal energy going into both development of a strong structure and productive fruiting. Grafted tomatoes will do well either in the controlled environment of a greenhouse or outside in fields or gardens.
Tomato plants grafted to Maxifort on the left, ungrafted plants of the same variety on the right. Photo courtesy of Johnny’s Selected Seeds at the Johnny’s trial greenhouse.
“Those of us growing in hoophouses or greenhouses need to maximize the use of space,” says Mike Leck, Production Manager at Gardens of Eagan Farm (GOE), in Northfield, Minn. “Grafting increases the vigor of plants, which means more production and fewer disease problems.” Leck points out that at GOE they have struggled with speck and spot bacterial problems. “Now we see healthy grafted tomato plants right next to diseased un-grafted plants. The stronger rootstock allows the plant to develop to its full potential.”
“Grafting is a great technology for organic growers specifically,” adds Andrew Mefferd, Trial Technician specializing in tomatoes at Johnny’s Selected Seeds, of Winslow, Maine. “Grafted plants are resistant to many soil-borne diseases, which organic systems struggle with due to limited alternative controls.” For instance, root stock seed available this year from Johnny’s is listed as resistant to Fusarium Wilt, Fusarium Crown and Root Rot, Nematodes, Corky Root Rot, Tobacco Mosaic Virus and Verticillium Wilt. “Even growers that have never seen these diseases will benefit from the resistance in these root stocks,” Mefferd claims. “They provide insurance, and help the plant to overcome adversity.”
Grafting Increases Yield
Grafting onto any rootstock will enhance the yield of a tomato, but certain top stocks respond better than others, and some combinations will do subtly better in different growing situations. Research done by Johnny’s in 2010 clearly shows that grafted tomatoes “greatly increased yield possibilities” with average yield per plant of the same species increasing as much as 40 to 60% over non-grafted plants. (Table 1)
Mefferd did research in the 2012 growing season comparing two types of grafted (double leader) plants to un-grafted plants, both single and double leader, and found grafting again almost doubled yield. “Even though these yields aren’t that spectacular for the variety, they clearly show the advantage of grafting,” he says. (Table 2)
The Grafting Process
Those interested in using grafted tomatoes have the choice of creating their own grafted plants or purchasing grafted plants. “Grafting can be tricky, and takes some practice,” Leck says. “Anyone trying it will want to allow time for trial and error.” Keys to success include choosing appropriate root stock and matching top stock; planning planting dates of both root stock and top seeds so that the stems will be similarly sized; making good cuts; connecting the two plants so that the tops don’t come off; and proper care of the grafts in a healing chamber. To help producers succeed, Johnny’s offers a seven-page grafting guide (see details on page 15.)
Johnny’s also sells seed specifically bred to provide disease-resistant root stock.
Leck points out that the timing of planting both rootstock and top stock must be managed carefully. You must pay attention to the specific growth habits of each seed in order to have closely sized stems of both species two weeks after seeding. Johnny’s recommends overplanting by at least 25% to be sure that you have enough stock of both kinds to choose from in making your grafts. At GOE, Leck plants seeds over a 3- or 4-day period, so that he has a variety of seedling sizes to choose from. He explains that the root seed is expensive, and at times hard to get, so he’s learned to plan well in ordering.
To make the actual graft, two-week old seedlings of both root and top are carefully cut, pruned, and joined using a grafting clip or other mechanism to set each scion on top of a root so they can grow together to make a single plant. The stems of both plants must be similarly sized and shaped for success. Cleanliness to ensure no bacterial contamination is also essential. Joined plants must be carefully maintained for up to a week in a “healing chamber.” This chamber must be specifically controlled at 80-82°F, 80-95% humidity, with no drafts, and moderate, indirect light. In this chamber the plants must heal, while not actually growing or transpiring. If moisture is too low or light too stimulating, the plants will continue to transpire and the tops “pop off.” Mike Leck says that getting the healing chamber “just right” at GOE was one of the biggest learning curves.
“If something happens along the way, it can be very discouraging, like just a crazy science experiment—lots of work and nothing to show for it,” Leck laughs. He says that if something goes bust with either seedling group, or in the healing chamber, you will be set back in your planting by as much as three weeks. He also points out that the process involves some expense–for double seeds and the cost of setting up the healing chamber. But, he also wants to make clear that once the plants are past the healing stage, they are very vigorous and hardy. Although it takes about two weeks longer to create a grafted tomato from seed to ground, because of their increased hardiness, they can generally go into the hoophouse or field two weeks earlier than non-grafted plants.
The benefits of using grafted tomatoes are clear, but the challenges of producing the plants may lead you to decide to buy grafted plants rather than make your own. Luckily they are now becoming available from many nurseries growing starts and transplants.
Buying Grafted Starts
Gardens of Eagan is in its second year of selling organic grafted tomato transplants. This year they will have two varieties available, “BHN 589” and “New Girl.” With at least eight weeks notice, GOE is also willing to custom graft plants for you, either using your seed or purchasing seed for you. Orders, either from the catalog or custom, must be placed “as soon as possible,” Mike Leck says. Growth from seed to ready-to-transplant seeding takes about five weeks.
Other nurseries also offer grafted tomatoes, but be sure that they are certified organic if you are an organic producer. Increased costs of production will make the costs of grafted tomato starts 50-100% higher than non-grafted, but with increased vigor and yield, Mike and others are positive that the additional investment is well worth it.
“Grafting is part art and part science, and won’t be for everyone,” Mefferd of Johnny’s Seeds concludes. “Ideally, we will start to see regional growers that can raise grafted tomatoes for those in the area. Then everyone that wants them can gain the benefits of grafted tomatoes.”
* Table 1 Detail: In 2010, Johnny’s research farm in Albion, Maine used the hoophouse tomato trial to quantify the yield boost resulting from using a vigorous tomato rootstock grafted to a desirable fruiting variety. Sets of three plants of each variety were grown, with three grafted to Maxifort (#2700) and three grown on their own roots. As you can see, yields averaged over 40% higher for the grafted plants, depending on the fruiting variety. Geronimo, for example, responds very well to grafting, showing a 66% yield boost when grafted to Maxifort. Individual results may vary, but this is a good illustration of the greatly increased yield possibilities of using grafted tomatoes. This trial was planted in an unheated hoophouse on May 21, 2010, and harvested until the beginning of October. Compost and organically approved soil amendments were used and plants were spaced two feet apart and trained to a double leader. No supplemental fertilizer or fertigation was used other than the original soil prep, which was amended to meet soil test recommendations for greenhouse crops.
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