Sweet Corn

History

Corn is a member of the Poaceae family, also known as the Grass or Cereal family, which includes wheat, oats, barley, rye, sugarcane and rice, as well as bamboo and many ornamentals, such as the lawn grasses (Figure 1). There are many types of corn that can be grown and used for human food although there are two major types; corn that is harvested when mature as a dried grain (field corn) and corn that is harvested immature as a vegetable referred to as sweet corn.  Grain corn types include: popcorn, ornamental corn, flint corn, flour corn, waxy corn and dent corn. Dent corn, also called grain corn, is mostly grown in the United States.  Sweet corn as a vegetable was introduced to Europeans in the 1700s.

Sweet corn is a native American crop, like squash and beans. DNA evidence traces it back to a large grassy plant in the Balsa River valley of west-central Mexico that is so unlike present-day corn that one wonders how it ever was domesticated. Teosinte had ears 2 to 3 inches long, with hard, loose seeds that shattered easily on the ground and had a tough hull that had to be heated like popcorn to release the grain, but it stored exceptionally well. Selected and improved by ancient farmers, corn moved throughout North and South America (Mesoamerica, the Andes and the Caribbean) until it had become a single-stalked plant with seeds enclosed in a tight husk about 8,000 years ago. Corn was so well-traveled that cobs found on the Colorado Plateau from 4,000 years ago carried more genes from the Amazonian savannahs than the nearer Mexican races of corn. The name maize, which is used for corn throughout the world, comes from “mahiz”, the Taino word from a Caribbean tribe encountered by Columbus in Cuba in 1492. English speakers in Europe called it Indian corn (“corn” being the English word used for a grain from a specific place) and later dropped “Indian” from the name.  In the botanical name, “Zea” is Greek for cereal grain and “mays” is the Latinization of maize.

Corn was highly adaptable and was bred into biotypes that could thrive in the tropics of its origin but also in the high, cool altitudes of the Andes. After its discovery and transport by Europeans, it began to be grown throughout Europe, Asia and Africa. The genetics of corn grown in Africa come from the Caribbean lines of corn, and those grown in the Himalayas originated in the Andes.

Flint corn was a staple crop of Indigenous peoples in the Eastern United States by the 800-900’s. It was grown with squash and pole beans in a configuration called the “Three Sisters”. This was an important form of companion planting, as corn and beans together provide all of the amino acids that make up a complete protein, necessary for the human diet. All three seeds were planted together in a mound. The corn provided a pole for the beans to climb, the beans provided nitrogen for the corn, and the squash shaded the ground to exclude weeds.

Maize is divided into subspecies by its kernel type: Flint corn (“hard as flint”) from the American Southwest, Dent corn, which has a dip in the top of the kernel when dried and was developed when Europeans arrived in the U.S., and Flour corn, grown by Incas, Aztecs and American Indians for its soft, easy-to-grind kernels. Popcorn is a fourth subspecies and Waxy corn is a fifth. Flint corn and dent corn were crossed during the western migration of U.S. settlers in the 1800s to produce varieties that were highly productive and adaptable, resulting in the modern hybrid field corn of the U.S. Corn Belt.

Field corn was grown in North America before 200 B.C. and used primarily as a grain for animal feed. Sweet corn, used for consumption by humans fresh or processed, is thought to have been a mutation in a Peruvian field corn that caused kernels to accumulate twice as much sugar and significantly less starch than field corn, with a thinner seed coat. The sweet gene mutation is called the sugary (su) gene. Sweet corn has been grown by North American Indians since pre-Columbian times. It was collected by European settlers who traded with the Iroquois Indians in the 1770’s.

Varieties

There are many characteristics that distinguish between sweet corn cultivars and that affect quality and growing. The majority of sweet corn varieties are often differentiated by three categories based on kernel color: yellow, white, and bicolor (yellow and white) kernels. If white and yellow corn cultivars are grown together, they will produce bicolor corn. If bicolor corn is grown with yellow corn, kernels will be mostly yellow. There is no relationship between the color of kernels and sweetness. Sweet corn cultivars are also divided into different maturity classes based on time to harvest from direct seeding; Early maturity cultivars (mature in less than 70 days), Mid (70-84 days) and Late (more than 84 days). In general, early cultivars produce smaller plants, smaller ears, and have poorer quality than longer season varieties. Longer season cultivars also have more rows per ear and deeper kernels than short-season varieties.

There are other cultivar characteristics that vary due to variety. Suckers, or weak plants at the base of the plants, seldom produce edible ears. Most modern cultivars do not produce suckers, but some heirloom cultivars do. Suckers, or tillers, are also thought to be a response to damage. Sweet corn husk color varies by variety ranging from light to dark green, green and purplish. The husk cover of ears and a tight husk at the ear tip are important characteristics with improved resistance to worm damage, bird predation, and diseases such as corn smut. Ear size is another important cultivar attribute and in general is correlated to maturity.  Early maturing cultivars have fewer rows (12 to 14) per ear and larger kernels while later maturing cultivars have 18-row ears with smaller kernels that are considered of higher quality. Kernel characteristics are an important factor in eating quality. To reduce biting into the cob, deeper kernels are desired compared to shallow kernels. Medium wide or narrow kernels are preferred to excessively wide kernels as the later must be punctured while biting.

One of the most important quality characteristics is kernel sugar content; sugars are converted to starch as corn matures and after harvest. Sweet corn cultivars are divided into three distinct types in reference to sugar according to genetic background: normal sugary (su), sugary enhanced (se), and super sweet (sh₂). There are also varieties that have a combination of these types of genes, with different qualities that are designated as synergistic (sy) or augmented super sweet (shA).

Several hundred sweet corn varieties are grown in the United States and it is important to note their type. Standard, or traditional sweet corn varieties including heirlooms, contain a sugary (su) gene that provides the sweetness and creamy kernel texture of “old fashioned” corn flavor. This type must be cooked within an hour or two of harvest for best flavor, as sugars begin to convert to starches shortly after picking. Storage life is normally 1 to 3 days under proper conditions for the standard (su) varieties. Sugary enhanced (se) hybrids have the natural sugary enhanced gene that has higher sugar levels (up to twice) than the standard varieties while retaining a similar kernel tenderness and creamy texture. Corn with this gene is sweeter at harvest and maintains quality longer in storage (3 to 5 days) than the standard types. The super sweet (sh₂) sweet corn varieties contain a natural mutation which delays the conversion of sugar to starch which allows them to stay sweeter longer (5 to 10 days) than standard (su) varieties. The sh₂ gene provides a lower starch content and much higher sugar content, allowing better taste and longer storage. As ears of sh₂ corn dry, these lower-starch kernels shrink considerably, hence the name of the gene. Both the se and the sh₂ genetic mutations produce a more tender kernel. Many of the newer sweet corn varieties may have multiple combinations of su, se, and sh₂ genes on each ear and are called synergistic varieties (sy); they also may be referred to as mixed gene, multi-gene, sweet breeds, or extra tender. These all require isolation distances similar to the sh₂ varieties.

Several sweet corn varieties are available as transgenic seeds. They have been genetically modified to contain the Bacillus thuringiensis (Bt) toxin and/or are Round-up Ready (immune to the herbicide glyphosate) or tolerant of the naturally-derived herbicide, glufosinate. These varieties are of little interest to home gardeners, as they are much more expensive and legal agreements are required with the developer before planting.

To produce ears of corn that are good-tasting and true to variety, do not plant sweet corn with field corn, popcorn or ornamental corn. To maintain the characteristics of the super sweet (sh2) varieties (extra sweet and tender with good storage), they must be isolated from all other varieties except other super sweets.  Isolation distances are far (500 feet apart) because corn, a grass, is wind-pollinated (see below on pollination). To maintain the flavor and characteristics of the super sweet seed planted, a variety other than a sh2 corn variety must not be allowed to grow nearby and add its pollen to the mix. Pollen from other types may supplant the relatively weak pollen of sh₂ types and revert back to the starchiness and toughness of field corn. Sugary enhanced (se) sweet corn may be planted next to other sugary enhanced or traditional sweet corn varieties. If you wish to plant two varieties in the same garden, design the plantings with care or pay attention to days to maturity so that varieties do not shed pollen at the same time.

Few heirloom varieties of sweet corn exist; those whose seeds have been carefully saved by growers for more than 50 years. Sweet corn varieties are usually hybrids. Generally, it is not recommended to save seed for future planting with hybrid varieties, as they are usually not expressed properly in the next generation. Most open-pollinated heirloom corn varieties are not sweet corn but are instead varieties used for flour or are dent or flint varieties used for cornmeal or grits. If planning to save your own seeds, plant different varieties 1600 feet apart to ensure seed purity.

Table 1. Recommended Sweet Corn Varieties for Louisiana

When and How to Plant

Sweet corn is a warm season annual crop that grows well at temperatures between 55 and 85 degrees F, however, it should be planted early in the season (see Table 2). Early planting helps deal with two problems: corn earworm pest pressure and harvesting in hot weather when heat causes sugar in the ears to convert more rapidly to starch and decreases the sweet flavor. Direct seed outside when soil temperatures are at least 50 degrees F, but ideally between 60-85 degrees F, usually about 2 weeks before the last frost. A soil temperature map can be helpful for guiding planting decisions. Super sweet varieties make a better stand if soil temperature is above 65 degrees F. Planting before soil temperatures have warmed enough may lead to slow germination and possible rotting of seed. Planting fungicide-treated seeds (usually colored a bright pink color) can be helpful. Sh2 seed, with thinner pericarps (the outer layer of the seed) are especially vulnerable to attack by seed-rotting fungi and treated seed should be planted, or a poor stand may result. While direct seeding is best, for the sake of earliness, corn may be started inside in six-packs 10-14 days before transplanting outside or direct seeded rows may be covered with clear plastic mulch to help warm the soil. Remove plastic after most of the seed has germinated.

Pollination is critical for successful corn production. In a garden, planting only a few rows of corn and relying on wind-pollination may result in poor yields or improper ear fill and gaps. Corn plants have separate male and female flowers on the same plant (termed monoecious) requiring pollen to be transferred from male to female flowers for proper ear development to occur.  The male, pollen-producing parts or “tassels” are branched floral structures at the top of the stalk. The female producing part is the ear with kernels that you eat and the “silks” are the female flowers. For successful pollination, pollen grains from the tassel must land on the exposed silks; pollen from the tassels at the top of plants fall on the silks and the immature ears below and remain there. The next steps in the process, fertilization, are complicated and requires the proper environmental conditions, particularly moisture. Because it is a wind-pollinated crop, corn must be planted in blocks of at least 4 rows, side-by-side, to allow pollen to drift from the tassels to the silks on neighboring plants. Plant seed ¾ – 1 inch deep, either planting 2 seeds per hole or planting a single seed every few inches and thinning to a final spacing of 8-12” apart. If grown in containers, several containers may be planted and grouped closely together. Poor pollination and drought may result in poorly filled ears (missing kernels) or poor ear tip kernel filling.

Plant only one variety of corn, unless planning to plant blocks of different varieties far apart (500 ft). In corn, the pollen of the male parent plant will influence the characteristics of the seeds in the ear. Super sweet varieties may produce a tough, starchy kernel with reduced sugar content if pollinated by standard sweet corn varieties. Besides distancing, there are other ways to avoid cross pollination: plant 2 weeks apart so different varieties will not be pollinating silks at the same time or plant varieties that have different days to maturity, i.e., a block of a variety with a 70-day maturity next to a block of an 84-day maturity variety. Seeds may be sown again every 10 days for a continuous harvest of corn through the summer.

Table 2. SWEET CORN PLANTING GUIDE

Where to Plant

Sweet corn is a warm season crop and should be sown in deep, well-drained, fertile soil with a soil pH of 5.8-6.8, that receives full sun (at least 6 to 8 hours/day). Corn plants grow 6 to 8 feet tall, so place them in a part of the garden where they will not shade neighboring plants. It is recommended to plant corn in traditional raised garden rows that are 8-10 inches tall to ensure good drainage. As with other crops, it is a good idea to add a layer of compost, peat moss, rotted hay, or other organic matter and mix it into the soil to optimize plant health.

Because it is a heavy feeder of nitrogen, corn should be rotated to a different growing location every year. Crop rotation, the avoidance of planting crops from the same family in the same row in the following seasons, also reduces the buildup of pests and diseases.

Plant Care

It is recommended to follow sustainable gardening principles.

Watering

Sweet corn needs regular watering, especially during pollination (silking) and ear-filling. One inch of water per week should be supplied, watering deeply so that water percolates 6 inches deep. Water early so that foliage can dry before nightfall.

Fertilization

Corn is a heavy feeder of nitrogen.  It is important to provide both a preplant source and several side dressings as plants develop.

Organic fertilizers such as compost, fish emulsion, composted poultry litter or manure, worm castings, and blood or bone meal originate from living organisms. They are far more environmentally sustainable and safe than traditional synthetic fertilizers. They naturally release nutrients more slowly and over a longer period of time. When applying organic fertilizer, it is important to use in unison with compost, cover crops and crop rotation, which all work together to build soil health. Learn how to convert inorganic fertilizer recommendations to organic fertilizers here.

Alternatively, a synthetic fertilizer may be used at a rate of 1.5 pounds (3 cups) of a complete fertilizer like 8-8-8 per 20 feet of row. Broadcast, or sprinkle evenly, over the soil before planting and then mix in about 3-6 inches deep using a rake. Supplemental side dressing, or reapplication of synthetic fertilizer, is recommended when plants are 12 inches tall and again when they are 36 inches tall. Side dressing is the addition of a small amount of fertilizer to the soil around already established plants when the plant begins to fruit or vine, primarily to provide nitrogen. When using synthetic fertilizer, sprinkle lightly along the side of the row, keeping it about 6 inches away from plant stems; water into the soil. If soil test results indicate good supplies of phosphorous and potassium and a complete fertilizer is not needed, ½ pound of calcium nitrate per 20 feet of row is a good choice for side dressing. Because of their slow, steady release of nitrogen, crops fertilized with organic fertilizer often do not need to be side-dressed, but corn is an exception due to its heavy nitrogen use. Fish emulsion and blood meal are good nitrogen sources for side dressing organic corn.

Weeds

Corn may be cultivated (the sides of the rows disturbed to loosen and cut off weeds) to control weeds but caution should be used to avoid injuring the corn roots. One needs to hoe to a shallow depth and just under the surface. Weeds become less of a problem as plants gain height and begin to shade the soil’s surface. Hand-pull any weeds that break through.

Insect Pests and Diseases

Poor tip fill is a physiological problem caused by insect pressure, drought or extensive cloudy days while ears are pollinating. Sweet corn has several insect pests such as stinkbugs, chinch bugs, and aphids, as well as birds. By far the most damaging insect pest is the corn earworm. A moth lays its eggs on fresh corn silks and larvae travel down the silks to reach the tips of developing ears and do great damage. Heavy infestations may interfere with pollination and feeding sites can provide entry wounds for fungal pathogens. Several fungal diseases may affect corn, from damping off, caused by planting untreated seeds in cold soils, to leaf blight, downy mildew, rusts and smuts. Good preventive strategies are timely planting and harvesting, variety selection and good sanitation. See Table 3 to aid in diagnosis and management of some common corn pests and diseases.

Table 3. ORGANIC AND NATURAL MANAGEMENT OF COMMON CORN PESTS AND DISEASES

Harvest and Storage

Corn usually matures 18 to 24 days after the first silks on the corn ears appear.  Silks should be beginning to dry and turn brown, and ears should feel full and firm. Peel back the husk and puncture a kernel with a fingernail. Kernels should be plump and exude a milky white juice. Super sweet varieties will have clear, rather than milky juice. Because kernels have thinner skins in the sugar-enhanced varieties, ears should be handled carefully to prevent the bruising of kernels. Most ears of a variety will be ready for harvest at the same time.

Harvest early in the morning. It is recommended to cool ears of corn to 32 degrees F within one hour of harvest; place in an ice chest or move immediately to refrigeration to prevent sugars from converting to starch. Standard corn varieties are best eaten immediately but can be stored in the refrigerator at high humidity for 1-3 days. Enhanced varieties keep for 1-2 weeks.

Sweet corn may be canned or frozen to preserve it. Freezing old-fashioned, non-enhanced varieties right after harvest prevents starch conversion and preserves sugar content.

Corn is Nutritious and Good for You

High in Potassium

Essential for body function, especially the heart, kidney, nerves, bones, and muscles

Contains Iron

Important for red blood cells and muscles

Excellent source of dietary fiber

Important for bowel health, lowering cholesterol, controlling blood sugar, and maintaining a healthy weight

Recipes

Common methods of cooking and recipes for sweet corn

Video showing how to prepare corn

Taste-testing ideas: Grilled, cornbread, corn salad, corn casserole, corn chowder, corn salsa, maque choux, corn pudding

Other websites with many corn recipes:

USDA MyPlate Kitchen

Food Hero

Produce for Better Health Foundation

Sources

• University of Arkansas Extension, Corn-Where it Came From
• University of Arizona, Sweet Corn
• University of Wisconsin, Origin, Adaptation and Types of Corn
• University of Nebraska, A Brief History of Corn: Looking Back to Move Forward
• Penn State Extension, Sweet Corn Production
• LSU AgCenter, Sweet Corn
• University of Florida, Sweet Corn
• Texas Agrilife Extension, Growing Sweet Corn
• Southeastern Vegetable Extension Workers, 2020 Southeastern U.S. Vegetable Crop Handbook
• University of Florida Entomology Dept., Featured Creatures: Corn Earworm
• University of Georgia Extension, How to Convert an Inorganic Fertilizer Recommendation to an Organic One, Circular 853.
• Clemson Cooperative Extension, Insect Pests of Sweet Corn Fact Sheet
• Cornell Cooperative Extension, Bird Damage Management Options in Sweet Corn
• Texas Agrilife Extension, Control of Chinch Bugs in St Augustine Grass
• LSU AgCenter, Chinch Bugs as a pest of Corn and Grain Sorghum
• Purdue Extension, Northern Corn Leaf Blight
• LSU AgCenter, Southern Corn Rust: Development, Risk and Management
• New Mexico State University, Extension Plant Pathology News You Can Use
• Purdue University, Brown Stinkbug
• University of New Hampshire Extension, Growing Sweet Corn (fact sheet)
• Cornell University, Vegetable Varieties for Gardeners