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Sweet Corn - Gramineae Zeamays

The earliest written record of corn (maize) is found in the Popul Vuh, the sacred book of the Quiche Indians of western Guatemala. The records date back to the eighth century.

Modern-day maize agronomists sometimes divide corn into popcorn, sweet corn, dent corn, pod corn, flour corn, and flint corn, but this is an arbitrary, not a natural, classification. Sweet corn is distinguished from other corns by its high sugar content when in the milk, by its early dough stages and by its wrinkled, translucent kernels when dry.

Sweet corn is a warm-season vegetable that can be easily grown in any garden with sufficient space. It is especially popular with home gardeners because it is a vegetable that tastes appreciably better when it is harvested fresh from the garden. Successive planting will yield continuous harvests.

Sweet corn may be divided into three distinct types according to genetic background: standard, super sweet, and sugar enhanced.

Standard sweet corn varieties contain a "sugary gene" that is responsible for the sweetness and creamy texture of the kernels.

Super-sweet varieties contain a unique gene that makes the kernels sweeter than those of the standard varieties. The sugar of the super sweet varieties is also converted to starch more slowly, preserving the sweetness for a longer time. The kernels of the super-sweet varieties have a crispy texture and contain low amounts of the water-soluble polysaccharides that impart the creamy texture to other sweet corn varieties. Although this lack of creamy texture is not especially noticeable in fresh corn on the cob, it affects the quality of frozen and canned corn.

Sugar enhanced varieties (75% normal "sugary" kernels and approximately 25% super-sweet kernels) contain multiple gene combinations that impart a creamy texture and increase sweetness. The sugar content is not as high, however, as that of the super-sweet varieties.


Varieties

Nearly all sweet corn sold today is a hybrid. The maturity dates are relative; the actual number of days to harvest varies from year to year and location to location. All of the varieties recommended for Arkansas are in the standard sweet corn varieties.


Planting

Sweet corn requires warm soil for germination (above 55 degrees F. for standard sweet corn varieties). Early plantings should be made in mid-March if weather permits. Sweet corn may be planted from early spring to early August.

For a continuous supply of sweet corn throughout the summer, plant an early variety, a second early variety, and a main crop variety in the first planting. For example, you may want to select a 70-day maturity variety, an 80-day maturity, and a 90-day maturity. Make a second planting and successive plantings of your favorite main crop or late varieties when three to four leaves have appeared on the seedlings from the previous planting.

Plant the kernels (seeds) 1/2 inch deep in cool, moist soil and 1 to 1 1/2 inches deep in warm, dry soil. Space the kernels 9 to 12 inches apart in the row. Plant two or more rows of each variety side by side to insure good pollination and ear development. Allow 30 to 36 inches between rows.

Plan your garden arrangement and planting schedule to prevent cross-pollination between your sweet corn, field corn, and popcorn. If sweet corn is crossed with field corn or popcorn, it will not develop a high sugar content and will be starchy. Cross-pollination between yellow and white sweet corn varieties affects only the appearance of the white corn, not the eating quality.


Care

Cultivate shallowly to control weeds. Although corn is a warm-season crop, lack of water at critical periods can seriously reduce quality and yield. If rainfall is deficient, irrigate thoroughly during emergence of the tassels, silking, and maturation of the ears.

Hot, dry conditions during pollination result in missing kernels, small ears, and poor development of the tips of the ears. Sidedress nitrogen fertilizer when the plants are 12 to 18 inches high.

Some sweet corn varieties produce more side shoots or "suckers" than others. Removing these side shoots does not improve yields.

The practice of removing the tassels from the stalk is a research and breeding practice and is not a gardening practice.


Problems

Corn earworms are a problem in sweet corn every year. Early plantings are not badly infested, but later harvests will usually have severe earworn damage unless timely control measures are followed. Corn earworm moths deposit eggs on the developing silks or on the leaves near the ear. The tiny caterpillars follow the silk down into the ear, where they feed on the tip. Once the worm is inside the protective husk covering, there is no effective control.

A suggested insecticide must be applied before the worms enter the silk channel. For good control in heavy infestations, make several applications two to three days apart from the time silks appear until they turn brown. To restrict worm infestation, tighten the tip of the husk with a rubber band or clothespin after the silk appears or insert mineral oil (1/2 medicine dropper full) in the silk tube which helps to decrease the damage.

European corn borers damage stalks, tassels, and ears. As their name indicates, corn borers bore into the plant; the stalks actually break off when damage becomes severe. Corn borers may also bore into the cob and be found after cooking. A suggested insecticide can be applied at five-day intervals, beginning with eggs hatching in mid-May. Spray applications for corn earworms usually will give adequate control of corn borers.

Flea beetles often attack early in the spring as the corn plants emerge through the soil. They can be quite damaging in large quantity and may also carry Stewart's bacterial wilt disease. A suggested insecticide must be applied early to control flea beetles.

Stewart's wilt is a bacterial disease spread by the flea beetle. This disease causes yellow streaks in the leaves, stunts in growth, and death of young plants of susceptible varieties. The disease generally is not severe after cold weather or when resistant varieties are planted.

Smut is caused by a fungus that invades the kernels. It develops as a swollen black pustule (gall) in the ear and sometimes infects the tassel. Some sweet corn varieties are more tolerant of smut than others. Smut occurs most frequently on white varieties and is often severe when kernels are extremely dry and during tasseling. Remove and destroy smut galls while they are moist and firm. Do not discard these galls in or near the garden. Place in the garbage or burn them. The smut is not poisonous, and in certain areas it is cultivated and eaten. Break off the infected part of the ear. The remainder is suitable for eating.

Diseases: Stewart's wilt, a bacterial disease spread by flea beetles; smut (especially on white varieties); stunt (transmitted by leafhoppers).

Insects: Corn earworm, European corn borer, flea beetles, Japanese beetles (eat silks), corn sap beetles (damage kernels after husk is loosened).

Cultural: Poor kernel development - failure to fill out to the tip, caused by dry weather during silking states, planting too close, poor fertility (especially potassium deficiency), too few rows in a block, resulting in poor pollination. Lodging (falling over) from too much nitrogen.


Harvesting

Days to Maturity: 63-100

When to Harvest: Each cornstalk should produce at least one large ear. Under good growing conditions (correct spacing; freedom from weeds, insects, and diseases; adequate moisture and fertility), many varieties will produce a second ear. This second ear is usually smaller and develops later than the first ear.

Sweet corn ears should be picked during the "milk stage" when the kernels are not fully mature. This stage occurs about 20 days after the appearance of the first silk strands. The kernels are smooth and plump, and the juice in the kernels appears milky when punctured with a thumbnail. (if liquid is clear the corn is immature, if milky it's ready, and if no sap, you're too late). Cover unharvested ears checked by this method with paper bag to prevent insect or bird damage. Experienced gardeners can feel the outside of the husk and tell when the cob has filled out. Corn matures 17-24 days after first silk strands appear, more quickly in hot weather, slower in cool weather.

Sweet corn remains in the milk stage less than a week. As harvest time approaches, check frequently to make sure that the kernels do not become too mature and doughy. Other signs that indicate when the corn is ready for harvest are drying and browning of the silks, fullness of the tip kernels, and firmness of the unhusked ears.

To harvest, snap off the ears by hand with a quick, firm, downward push; twist and pull. The ears should be eaten as soon as possible, processed, or refrigerated. At summer temperatures, the sugar in sweet corn quickly decreases and starch increases.

Cut or pull out the cornstalks immediately after harvest and put them in a compost pile. Cut the stalks in one-foot long lengths or shred them to hasten decay.

Approximate Yields: (per 10' row) 5-10 pounds or roughly 10-20 ears

Amount to raise per person: 20-30 pounds or about 40-60 ears

Storage: Refrigerate immediately to prevent sugars from turning to starch; cold (32F), moist (95% RH) conditions; will keep 4-8 days, but standard varieties will become starchy after a few days.

Preservation: Frozen on cob or off; canned.


Questions and Answers

How long does it take sweet corn to develop from the first appearance of silk to harvest?

About five days are required for complete pollination after the first silk appears.Harvesting begins approximately 20 days after first silking.

The germination of some varieties is low. How can I get a better stand?

The sweet corn seed is shrunken especially the supersweet types and do not germinate as well as smooth seed. Do not plant too early in the spring, wait until the soil is warm, about 65 degrees F. Sow the seed more thickly and thin if necessary. A fungicide seed treatment may also be helpful.

Why didn't my sweet corn ears fill out to the tips?

Several conditions can cause poor kernel development at the tip of the ear: dry or cool wet weather during silking and pollination; planting too close; poor fertilization, especially lack of potassium; and poor natural pollination. These conditions may be overcome by watering in dry weather, planting at recommended spacing (9-12 inches. apart in the row), proper fertilization and planting short rows in blocks of two or more for self-pollination.

What is the best way to grow early corn?

Choose an early maturing variety, plant early and shallow (about inch deep) and cover the row with clear polyethylene film. Remove the film, or cut slits and carefully pull the plants through before the weather becomes too hot. Floating row covers can also be used effectively for early corn production.

Should garden corn be planted in several short rows rather than in one or two long rows?

Yes. Corn is a wind pollinated plant. Planting corn in blocks rather than in long rows makes it easier for the plants to pollinate one another during tasseling.

Should the suckers, or side shoots, which emerge near the ground level on sweet corn be removed?

It is not necessary but modern cultivars of sweet corn have been developed to have a minimum number tillers.

How long does it take for most sweet corn varieties to produce edible ears?

Most sweet corn varieties will mature between 65 to 90 days after seeding. Maturity rate varies from season to season depending on temperatures.

Why are ears of corn underdeveloped at the tip end?

The causes for this condition include nutrient deficiency, cool temperatures during ear maturity and low moisture. Corn is pollinated by wind-borne pollen from the male flowers or tassels at the top of the plant to the female flowers or silks about midway up the stalks. Each kernel develops from an individually pollinated silk. Kernels which develop near the middle and base when conditions occur such as those mentioned above, those kernels pollinated first will take precedence over those pollinated last. This often results in failure of the kernels near the tip to develop properly, or in irregular kernel development.

Is there a best time of day to harvest sweet corn?

Harvesting during early morning is recommended to insure the sugar will be at its highest level if the corn is mature but not overripe. Cool the corn quickly to prevent loss of sugar.

How often should sweet corn be fertilized to produce high yields of good quality corn?

Sweet corn should be lightly fertilized prior to planting. It should be fertilized again when the plants are approximately 4 inches tall and when they are 8 to 10 inches tall. Approximately 1/4 lb. of complete fertilizer for every 10 ft. of garden row is sufficient.

This year my sweet corn produced yellow and white kernels on the same cob. What's wrong?

This could be caused by a bicolored varieties, such as Honey and Cream. New varieties, such as those with an super sweet character, produce white and yellow kernels on the small cob.

What is meant by advertisements in catalogs referring to "Super Sweet" varieties of sweet corn?

Newly developed "Super Sweet" hybrid varieties may contain up to 40 percent more sugar than some of the standard varieties. Super Sweet hybrid carry a gene (sh2) which results in a high sugar content. The super sweet character is lost if the corn is pollinated by ordinary sweet corn or field corn, so the super sweet hybrids should be planted away from any other type of corn. Another type is the SE or sugar enhanced sweet corn.

What is the difference between roasting ears and sweet corn?

Roasting ears a field corn harvested at an immature stage. Field corn is preferred by some because the ears are larger and the corn is not as chewy.

My sweet corn produced normally. However, as the ear formed, the tip of it became covered with a large white mass that grew until it broke open and exposed a black, powdery mass.

This is corn ear smut, a type of fungus carried in the seed. To avoid this, use only high-quality seed from a reputable source. There is no chemical control for this disease.

My sweet corn grew for a while and then had a mosaic appearance. The corn did not develop properly. The ears that formed were poorly filled.

This is maize dwarf mosaic virus. It overwinters in johnson grass around a garden. To control the problem, remove the johnson grass, and follow a good insect control program. Some varieties are more resistant to this disease than others.

I planted corn in my garden this fall and it turned out beautifully, but the corn ear worms ate more corn than I did. What can I do to prevent this?

Spray or dust the ear silks with Sevin (carbaryl) to prevent adult insects from entering and laying eggs. Begin dusting and spraying at an early stage and repeat every two days. A drop of mineral oil on the silks is recommended to prevent earworm damage.

Are there any earworm resistant varieties of sweet corn available?

No. Some varieties seem to be bothered less by corn earworms than others, but none are truly resistant. Varieties of sweet corn which have a tight shuck near the silk end seem to be bothered less by earworms than those that have loose and open ends.

The center of my corn plants are full of little green insects. What do I do about them?

Corn leaf aphids infest the sworl of young corn plants. The plants will tolerate large numbers of these aphids. If plants begin to wilt or die, spray them with malathion. Use as directed on the label.


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This page was last updated on November 16, 2002