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Maximizing Production with Multiple Seeds Per Plug

The Tomato Magazine
February 2007

By Lisa Lieberman

Years ago when processing tomato growers used direct seeding instead of transplants in their fields, their planters would only allow them to plant the seeds in small groups instead of individually. When the technology became advanced enough where the planters could dispense seeds one at a time, growers soon realized that they actually got more production from planting multiple seeds per plug rather than singly. So, they went back to planting multiple seeds—
usually between two and four seeds—per plug to get maximum production.

Higher Production
Following this same logic—that using more seeds per plug equals higher yields— researchers have also begun experimenting with multiple-planting of transplants and are fi nding that, in many cases, multiple plants per plug can lead to signifi cantly higher yields.

In processing tomatoes, transplants originally became popular in the 1990s in Woodland, Calif., when the processing industry began following the lead of fresh
market tomato growers who were using transplants in their fi elds instead of direct seeding. In fresh market tomatoes, the idea of using multiple plants per plug was untenable because the goal is to produce large-sized, aesthetically perfect tomatoes.In processing tomatoes, however, the size and shape of the tomatoes doesn’t matter compared with fruit quality and yield.

Processing growers were reluctant, though, to try multiple plants per plug because, at fi rst, it didn’t seem economically feasible. The cost of growing multiple plants in greenhouses would be higher. But back in 2002, researchers began experimenting with double and triple seeding of plants in Colusa, Fresno and Yolo counties. Although the double and triple seeding didn’t seem to have any impact on yield in Yolo County, researchers did see significant yield increases in Fresno and Colusa counties.

Even factoring in the zero yield gains in Yolo County, the average yield increase per acre in Colusa, Fresno and Yolo counties was 1.4 tons per acre. In Colusa County, which produced some of the best results, there were yield increases of 10 percent in double-seeded fi elds. Triple-seeded fields had yield increases of 15 percent, Miyao said.

The upshot of all the test results was that in six out of 22 trials using different varieties of tomatoes around the state at various times of the year, multiple-plantings per plug resulted in signifi cant yield increases, Miyao said.

“Six out of 22 times is a little over 25 percent, which is statistically a significant yield increase,” Miyao said.

Curly Top Question
In Fresno County, UC researcher, Michele LeStrange did the experiment with multiple-seeding. Her research, though, wasn’t aimed at increasing yields. Her
focus was on using low populations of plants in the fi eld — about 3,500 per acre — to see if thinner stands of plants reduced the incidence of curly top virus. Curly top can be a problem in transplants.

Although LeStrange wasn’t able to test her theory about curly top virus in low stand fields, since there was none of the virus in her fi eld during her trial, she did find that multiple-seeding in thin tomato stands increased yield seven out of 10 times.

Jeff Karr, who works for TS & L Seed Company in Huron, said that curly top virus was also the reason many of his growers got started double seeding plants about fi ve years ago. Curly top infection is caused by the beet leaf hopper which stings tomato plants and spreads the virus.

“One of the reasons we started double seeding was that it seemed that when beet leafhoppers come into the fields, they would sting one plant and then move on. The virus isn’t contagious, so it’s not like if one plant gets infected the one right next to it gets infected, too,” Karr said.

In a sense, the idea of double seeding was to spread the risk around the field. If one plant got stung by a leafhopper, chances were that the second plant
wouldn’t get stung also, he said.

“For counties south of Los Banos, we noticed that with double seeding, most yielded an extra one or two tons per acre,” Karr said.

Generally, it costs about $30 per acre more to double-seed plants, Karr explained, adding that if growers make $60 per ton on a ton of tomatoes, that’s a $30 profi t plus insurance against curly top.

“One hundred percent of our growers use double-seeded plants,” Karr said.

One challenge in moving to doubleseeding, however, is that it’s harder for greenhouses to grow them. Plants tend to compete with each other and put out
more vegetative growth. There are also twice as many plants per square foot in the greenhouse, which means that greenhouses have to be more careful with their fungicides, Karr said. Miyao agrees that growing doubles in greenhouses is a challenge.

“Since these plants are more competitive in the greenhouses, they have smaller stem diameters,” he pointed out. “This is important in the field for survivability. We hope, though, that by using plant growth regulators we can control plant growth in the greenhouses and get shorter plants with wider stem diameters.”

Ultimately, more work needs to be done before researchers can make a general recommendation that all growers use double-seeding.

“ The central question is why are we seeing increases in yields in some fields in Fresno and Colusa counties but not in others such as Yolo? What’s so different
between Colusa and Fresno counties and Yolo?

If you are inclined to take a gamble by double seeding, it’s a little bit of a gamble for a possible good payoff,” Miyao said.

© 2007 Columbia Publishing

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