Soybeans are generally quite specific in their adaptation requirements dictated in large part by latitude and temperature regime. They are daylength-sensitive “short day” plants, which flower when daylength reaches a certain length or less. So there are so-called maturity groups of soybeans ranging in North America from latitudes in Southern Canada to the Gulf Coast. And in some cases there are daylength insensitive cultivars available for specific growing environments. Vegetable soybeans are a special class of soybeans, which have been selected for larger seeds and other desirable eating characteristics. There are not nearly as many commercial cultivars of vegetable soybeans as common grain soybeans, and much less specific information is available about their adaptation. The edamame soybeans are also special among the vegetable soybean group because they have been selected over the years for specific quality characteristics. The following characteristics are especially valued for Japanese edamame:

The cultivar affects many of these traits, but growing conditions are also important. Plant spacing, for example, affects pod color, seed and pod size, and the percentage of two- and three-seeded pods. Other factors, such as water and fertility management, can also affect a number of these traits. Temperature interacts with the daylength and cultivar to determine flowering date and eventually affects seed and pod size. Clearly, because of the wide range of factors affecting important edamame characteristics, it is critical to match the cultivar to the growing environment.

Several commercial cultivars were initially evaluated in unreplicated small plots on a number of farms in San Luis Obispo, Santa Barbara and Ventura Counties in 1997 and 1998. We also established a replicated field trial at the CalPoly, San Luis Obispo, experimental farm in June of 1998 to evaluate fifty-four vegetable soybean cultivars. These cultivars came from commercial seed companies as well as the National Soybean Germplasm collection at the University of Illinois.

The trial at CalPoly was conducted in collaboration with Dr. Wyatt Brown of the Crop Science Department, and students Paul DeCarli and Francisco Rivera. DeCarli and Rivera used the trial to satisfy their senior project requirements. Sakata and Takii Seed companies provided seed of their commercial edamame cultivars. Additional commercial cultivars were also obtained from Johnny’s Selected Seeds, Albion ME, and Vermont Bean Seed Company.

Cool, wet spring weather lasted very late in 1998. Soybeans require relatively warm soil conditions (60°F) for uniform germination. The trials were planted May 30, 1998, and most plots emerged in 7-10 days. The point of harvest is also critical with edamame soybeans. It is customary in Japan to pull the plants or cut them off at ground level when the plants have an optimum number of pods which are developed enough for market. Some pods will be insufficiently filled at this point and, occasionally, one or more pods will be overripe. Edamame soybeans are sensitive to high temperatures during pod development, and high temperatures and/or stress will cause inadequate pod fill and pod yellowing. Pods will turn yellow and lose marketability if they are not cooled quickly to 32°F to 40°F.

Production data for the best performing cultivars from this trial are presented in Figures 1 and 2. These data are for "marketable" production, which takes into account the color, size, and quality characteristics discussed previously for edamame. These figures are summaries of results of this trial from a June planting ( May 30). It is important to remember that because of the strong relationship between edamame cultivars and planting date, the relative performance of these cultivars would likely to be quite different on other planting dates and in other areas. Only the Kegon and Yuzusumi cultivars are available commercially (Sakata Seeds). The other edamame soybean cultivars available commercially include the following:

One or more of these cultivars may have excellent horticultural (market) characteristics when grown at a different date of planting or in a different production area. One or more growers have observed that Envy appears to tolerate cold soils somewhat better than many of the other cultivars. All of the other better performing cultivars are only currently available for experimental use. It is extremely important at this point to trial as broad a range of germplasm as possible in as many environments as possible to identify the more promising cultivars. These figures will serve to guide those growers who are interested in giving edamame a try. The other commercial cultivars should generally have good horticultural characteristics, and growers should not be afraid to trial them at other planting dates.

We also have data on post-harvest keeping quality, which I will summarize at a later date. The trials are continuing this season (1999) with an extended range of planting dates with a smaller number of cultivars. Look for future reports of data from these ongoing trials.