The demand for fresh vegetables by American consumers has provided some niche markets for growers in the Southeast.

High tech production techniques and genetically altered new varieties provide more varied, yet more complex, opportunities for sweet corn growers in the area.

Planting and harvest dates and isolation are critical factors in super sweet corn production. On the positive side some of these varieties show some extra ability to produce toxins that reduce insect damage.

A number of genes affect sweetness in corn. These are recessive mutants of the starchy gene found in field corn (Su) and their modifiers, and other genes. Four types of sweet corn are available: standard (su), sugary-enhanced (se), supersweet (sh2), and synergistic or triplesweet (sy).

The su type is the old-fashioned sweet corn that has long been a tradition among Southeastern vegetable growers. It must be consumed quickly after harvest, or the sugars rapidly turn to starch. Su sugary corn is the standard corn grown for processing and much of the fresh market. Sugar content at normal maturity is 5-10 percent.

The se types contain more sugar than the su type and, if cooled, will remain sweet for several days after harvest. Sugary enhanced corn results in increased sugar levels, in the range of 12-20 percent. Kernels are very tender with good “corn” flavor. Seed germinates well at temperatures of 55-60 F.

The sh type also contains more sugar than the su type but converts very little sugar to starch. If properly cooled, a sh2 variety will remain sweet for 7 to 10 days after harvest.

Super sweet or extra sweet (sh2) corn produces kernels with two to three times the complex sugars of the standard corn varieties (20-30 percent). Texture is crispy rather than creamy as with the standard and enhanced varieties.

Fresh market shelf life is extended because of the slower conversion of sugars to starch after harvest. Seed kernels are smaller, lighter in weight and shrunken in appearance (giving the gene the name “shrunken”).

Synergistic (sy) or triple sweet varieties have even more enhanced sugar, but do not store nor grow nearly as well as the sh2 and se varieties.

The different classes of sweet corn require at least some isolation, from two points of view, color and kernel quality (sugars and texture). Since colored kernels in white varieties are very obvious, a 500 foot or more isolation distance is recommended between white and colored varieties. Altered planting to provide a two week difference in silking may also be used, but is less reliable

Supersweet corn varieties and other new types of corn requiring isolation from standard sweet types should be isolated based on their Isolation Class categorization. The use of 2-4 border rows helps minimize contamination.

Isolation may be accomplished in three ways, by distance, time of pollination, and blocking. Isolation by distance is the preferred method.

If no isolation is used between standard sweet (Isolation Class II) and supersweet types (Isolation Class III), outcrossing of kernels in adjacent rows and extending for 6 to 10 rows into each type, is high enough to render the ears from these rows unmarketable.

This outcrossing can result in over 50 percent of the kernels on ears in adjacent rows being starchy. Outcrossing drops off rapidly beyond 10 rows, until at about 100 feet, only up to 1 percent of kernels (up to 4 kernels per ear) may be starchy. This level of outcrossing is probably not discernible by fresh market buyers or consumers.

Where large plantings are made for fresh market production, a distance of 250 feet is recommended between Isolation Classes I, II, and III. Where isolation of fields is convenient, maximum isolation would not need to exceed 600 feet, which is a conservative assumption based on distances used for seed production, where isolation is even more important.

Out-crossed kernels are more apparent in supersweet varieties. Hence, supersweet varieties, which are in Isolation Class III, work best when planted upwind of varieties in all other isolation classes.

If the 2-3 week pollination time difference is to be used as a means of isolation, growers should consider the following:

  • Later plantings must be based on growth stage or heat units, not on calendar day difference.

  • The early planted sweet corn must germinate uniformly to obtain an effective 2-3 week spread in pollination.

  • Early planted sweet corn must be topped, either mechanically or by hand to remove late flowering suckers before later planted varieties begin to silk.

In tests at several grower and research center locations in South Carolina several SH2 varieties proved highly adaptable for production. Richard Hassell, associate professor of horticulture at Clemson University, says these new varieties may take some extra management time, but can be very profitable to growers.

“We only looked at sh2 varieties because of the broader range of harvest options. Some people like the extra crunchiness and extra tenderness, which are positive traits of these varieties,” Hassell says.

Hassell notes that the Clemson tests were conducted both in replicated plots and in grower trials. “We feel the grower trials add a real world look at yield potential and quality,” he adds.

Hassell says several Xtra Tender varieties performed well in both grower and research tests. Among the yellow varieties Xtra Tender 1273, Xtra Tender 1178 and Xtra tender 1575 performed well.

Xtra Tender 1178 ranked first in grower trials. It produced fat ears that are excellent for canning. It is a high yielding variety (418, 48 count boxes per acre in grower trials) and matures in 77 days.

Mirai 130 performed well, but produced shorter ears and did not do well in the packing end of production. It is known for producing an extra creamy kernel and very sweet taste. Though not being able to handle dry weather has been a knock on the Mirai varieties, Hassell says Mirai 130 handled it well in grower trials.

Vision is a variety that performed better in grower trials than in research trials. In the on-farm tests it was the highest yielding variety, but ranked third in overall production ratings.

GGS 0966 is a yellow sweet corn variety that has the Bt gene included. Hassell points out that in this variety, approximately one-third of the kernels do not have the Bt gene, so though it is a good choice when fall armyworms are a problem, growers will still have to spray to avoid some worm damage.

Bi-color varieties

Mirai 301, a 78-day maturing variety was the top sweet corn in one on-farm test and performed well across all tests. It has the characteristic ultra sweet taste of Mirai varieties, but without some of the production problems.

Fantastic produced 496, 48 count boxes per acre, making it among the highest yielding varieties in the test. It was the top variety in one on-farm test. And, Fantastic produced a short flag, but the light green shuck coloration may be a concern for some buyers, Hassell notes.

Xtra Tender 270A produced a large ear and very sweet taste, but yields were lower than other top-rated bicolor varieties. An advantage for Xtra Tender 270A is its early maturity — only 71 days.

Double Up was one of the top yielding bi-color varieties, producing 419, 48 count boxes per acre. It is a 75-day maturity variety.

Mirai 308, like its sister yellow varieties, produces a very creamy, very sweet kernel. It too, showed some production weaknesses, compared to some of the other varieties in both research and on-farm tests.

Xtra Tender 282 showed high quality and high yields, but is an 82-day maturing variety. “Like most of the varieties we tested, this one is acceptable, but there are so many good varieties out there that have the same production advantages, plus 8-10 day earlier maturity dates,” Hassell says.

Accelerator is an older variety that performed well in Clemson tests for a number of years, according to Hassell. It is a 77-day maturing variety that did better yield-wise in on-farm tests than in research tests.

Xtra Tender 378 is one of the most promising white varieties, according to the Clemson researcher. It produces long-fat flags, with high yields under dry conditions. It is an 80-day maturing variety.

As the demand for higher sugar content corn varieties by ethanol plants skyrockets when projected plants come into production, the demand for high yielding, high sugar sweet corn may be an additional option for Carolina growers. The new genetically improved sh and se varieties already provide many options for edible sweet corn.