Sweet Corn

Flowering. The tassel is the male flower and the silk and ear together form the female flower. Since both male and female flowers are found on the same plant, corn is monoecious in flowering habit.

Time to harvest for sweet corn ranges from 64 to 94 days from planting. Popcorn and Indian corn types require up to 110 days because ears are left on the stalk to mature. Corn is a short-day plant and most cultivars will not flower at photoperiods longer than 13 hours. When days are too long, the stalk grows very tall but never flowers. This is sometimes seen when tropical corn varieties are grown in the northern United States or when U.S. cultivars are grown in England.

Kernel color. Ears of commercial sweet corn cultivars can be yellow, white, or have mixed yellow and white kernels (bicolor). Ears of Indian corn can be solid red, purple, blue, or multicolored. Individual kernels can be solid, striped, or have starburst patterns.

Kernel sugar content. In standard sweet corn cultivars, such as 'Silver Queen', kernel sugar content peaks at 5 to 11 percent. Peak levels last in the field only 2 days at 80 degrees F or 5 days at 60 degrees F before sugar is converted to starch. Even if ears are picked at peak sugar content, quality decreases rapidly after picking because of loss of sugar. In 24 hours after picking, sugar content declines 8 percent at 32 degrees F and up to 52 percent at 86 degrees F. Thus, it is difficult to pick sweet corn before it becomes starchy and difficult to get it to the consumer before sugar levels decline. For these reasons, 'supersweet' cultivars are now preferred for commercial sweet corn production. These cultivars start out sweeter, convert sugar to starch more slowly on the plant and lose sugar more slowly after harvest. In 1988, 98 percent of the sweet corn acreage in Florida was in sh2 supersweets. Acreages of sh2 are also increasing in New York and North Carolina. While any lines with more sweetness than the standard are sometimes referred to as supersweets, technically only those lines with the sh2 or shrunken 2 gene should be called 'supersweets'. The name 'shrunken 2' reflects the fact that so little starch is present in the kernel that it appears shrunken, even when compared to standard sweet corn lines with the su-1 gene. Peak sugar levels of sh2 hybrids range from 22 percent to 40 percent,compared to 5 to 11 percent in standard sweet corn.

While it is reasonable to expect commercial sweet corn crops in the future to be dominated by sh2 hybrids, some people say the sh2 hybrids are too sweet and lack the characteristic sweet corn flavor and tender kernels. Kernels of many of the newer supersweet releases are as tender as those of standard cultivars, however. The original supersweets were also harder to grow and even the newest sh2 hybrids challenge the grower because of isolation requirements from other types of corn. See the appropriate sections of this chapter for special management practices for supersweets including isolation requirements.

Sweet corn cultivars with the se gene offer a third option for roadside marketers or those who will use or sell the corn within 1 to 2 days of harvest. These sugar-enhanced types start out with higher sugar content than standard sweet corns, but convert sugar to starch at the same rapid rate after harvest. Cultivars which are homozygous for the se gene have peak sugar contents of 12 to 20 percent while those heterozygous for the se gene have sugar levels of only 7 to 15 percent. The advantages of these cultivars are that they have the same tender kernels and creamy texture (sometimes described as 'real corn flavor'), as standard corn, but seedling emergence characteristics are better and isolation requirements less stringent than for the sh2 hybrids. Se hybrids are susceptible to some soil pathogens, however, and, like standard sweet corn, must be isolated from sh2, field and popcorn types. If planted next to standard sweet corns, individual kernels may be like those of the standard sweet corn, but this is much less objectionable than when starchy, hard kernels develop on the ear, as is the case when standard sweet corn pollinates sh2 gene corn cultivars.

Climatic Requirements

Corn is a warm-season crop that requires high temperatures for optimum germination and rapid growth. At planting, soil temperatures of 70 to 80 degrees F are best and should not be below 50 degrees F for standard sweet corn, or below 60 degrees F for supersweets. Frost will injure sweet corn at any stage of growth and even long, cool spring weather or early warm weather, can cause ear abnormalities. For example, an ear may appear in the tassel, or a tassel or sucker where the ear should be.

Moisture must be continuous through the growth period and is especially critical from silking through kernel growth. Leaf rolling is a symptom of moisture stress. Water stress at silking leads to poor pollination which shows up later as missing kernels. Kernels at the end of the ear may not develop (poor tip fill) or whole rows of kernels may not develop along the cob. With temperatures above 95 degrees F or hot, drying winds at silking, tip fill is especially poor. In rainy or very damp weather, pollen sticks to the tassel and pollination may also be poor.

Location of Production

The top states in production of sweet corn by acreage in 1992 were Wisconsin, Minnesota, Washington, Florida, and Oregon. In 1992, 11.2 percent of the total U.S. acreage was located in the Southern Region. The top five states (based on harvested acreage) were Florida, 50,907 acres; Georgia, 9,986; North Carolina, 5,669; Alabama, 2,882; and Virginia, 2,699. Most of the corn produced in the Southern Region is for the fresh market rather than for processing.