Time from harvest to eating is critical for sweet corn.
Sweet corn is a warm-season vegetable that can be grown easily in any garden with sufficient light, fertility, growing season and space. It is especially popular with home gardeners because it tastes appreciably better when it is harvested and eaten fresh from the garden. Successive plantings can yield continual harvests from early summer until frost if the weather cooperates.
Sweet corn may be divided into three distinct types according to genetic background: normal sugary (SU), sugary enhancer (SE) and supersweet (Sh2).
- Standard sweet corn varieties contain a "sugary (SU) gene" that is responsible for the sweetness and creamy texture of the kernels. SUs are best suited to being picked, husked and eaten within a very short time. In the home garden, this is sometimes possible but not always practical. The old adage was "start the water boiling, run to the patch, pick and husk the corn, run back to the pot, cook the corn, and eat or process immediately."
- Sugary enhancer hybrids contain the sugary enhancer (SE) gene that significantly raises the sugar content above standard SUs while retaining the tenderness and creamy texture of standard varieties.
- The taste, tenderness and texture are outstanding.
- SEs are the gourmet corns of choice for home gardeners because they contain the best qualities of both SU and Sh2 types.
- Fresh from the garden, virtually all current SE releases have eating quality that is superior to all other types.
- No isolation from standard SUs is necessary.
- Supersweet hybrids contain the shrunken -2 gene and have a higher sugar content than the standard SU varieties.
- The kernels of the extra-sweet varieties have a crispy, tough-skinned texture and contain low amounts of the water-soluble polysaccharides that impart the creamy texture and "corny" flavor to other sweet corn varieties.
- Although the lack of creamy texture is not especially noticeable in fresh corn on the cob, it affects the quality of frozen and canned corn, as does the toughness of the seed coat.
- Unless corn must be stored, shipped or mechanically harvested, SEs are superior in eating quality to Sh2s.
- Supersweets (Sh2) should be isolated from any other type of corn tasseling at the same time to ensure sweetness and tenderness.
- Their pollen is weak and easily supplanted by other types, which causes the kernel to revert to a form with the toughness and starchiness of field corn.
- Because corn is wind-pollinated, this isolation distance can be 500 feet or more, especially downwind.
Corn is a European word meaning kernel. The corn plant is native American in origin and Illinois is corn country. Driving through the state, you will see endless fields of sweet corn, feed corn, and popcorn. Small plot gardeners know that corn takes up a lot of space, it is greedy for soil nutrients, prone to weeds and disease, destroyed by small animals, wind and frost. So why do we go through all the trouble of growing corn? Because no corn is as fresh and sweet as the corn you grow yourself.
Growing Sweet Corn
When to Plant
Sweet corn requires warm soil for germination (above 55°F for standard sweet corn varieties and about 65°F for supersweet varieties). Early plantings of standard sweet corn should be made at, or just before, the mean frost-free date unless you use special soil-warming protection such as clear polyethylene mulch film.
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 wish to select Sundance (69 days) for the first early variety, Tuxedo (75 days) for the second early variety and Incredible (83 days) for the main-crop variety. Make a second planting and successive plantings of your favorite main-crop or late variety when three to four leaves have appeared on the seedlings in the previous planting. Plantings can be made as late as the first week of July.
Spacing & Depth
- Plant the kernels (seeds) 1/2 inch deep in cool, moist soils and 1 to 1 1/2 inches deep in warm, dry soils.
- Space the kernels 9 to 12 inches apart in the row.
- Plant two or more rows of each variety side by side to ensure good pollination and ear development.
- Allow 30 to 36 inches between rows.
All sweet corns should be protected from possible cross-pollination by other types of corn (field, pop or flint). If you plant supersweet or synergistic sweet corn varieties, plan your garden arrangement and planting schedule so as to prevent cross-pollination between these varieties and with any other corn, including nonSh2 sweet corns. Supersweet varieties pollinated by standard sweet corn, popcorn or field corn do not develop a high sugar content and are starchy. Cross-pollination between yellow and white sweet corn varieties of the same type affects only the appearance of the white corn, not the eating quality.
- Cultivate shallowly to control weeds. Chemical herbicides are not recommended for home gardens.
- Although corn is a warm-weather 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, droughty conditions during pollination result in missing kernels, small ears and poor development of the tips of the ears.
- Side-dress nitrogen fertilizer when the plants are 12 to 18 inches tall.
- Some sweet corn varieties produce more side shoots or "suckers" than others. Removing these side shoots is time consuming and does not improve yields.
Corn earworms are a problem in sweet corn every year. Earlier plantings are not badly infested in areas where the pest does not overwinter, but later harvests usually have severe earworm damage unless timely control measures are followed.
- Corn earworms deposit eggs on the developing silks or on the leaves near the ear.
- The tiny caterpillars follow the silks down into the ear, where they feed on the tip.
- Only one corn earworm will be found per ear because the caterpillars are cannibalistic, with the largest devouring any others present.
- Once the worm is inside the protective husk covering, there is no effective control.
- Anything that restricts the worm, such as tightening the tip of the husk with a rubber band or clothespin after the silk appears, or inserting mineral oil (1/2 medicine dropperful) in the silk tube-helps to decrease the damage.
Corn rootworm beetles may cause extensive silk damage that interferes with pollination. Later plantings usually suffer the greatest damage, especially where field corn is grown.
- Beetles multiply in early plantings of field corn, mature and migrate to plantings of young, tender sweet corn.
- Silk and the young, tender, green leaves are preferred feeding sites.
- If infestation is sufficient to remove silk before pollination, cobs develop without a full set of kernels. Control measures must be taken as the silk emerges.
European corn borers damage stalks, tassels and ears.
- As their name indicates, corn borers bore into the plant; and the stalks break over when damage is severe.
- Corn borers also may bore into the cob and be found after cooking.
- A suggested insecticide can be applied at 5 day intervals, beginning when eggs hatch in June. Spray applications for earworms usually give adequate control of corn borers.
Flea beetles often attack early in the spring as the corn plants emerge through the soil. Flea beetles can be quite damaging when numerous and they may carry Stewart's bacterial wilt disease.
Stewart's wilt is a bacterial disease spread by the flea beetle. This disease causes yellow streaks in the leaves, stunting and death of young plants of susceptible varieties.
- The disease occurs more frequently in the southern states and is not severe after cold winters or when resistant varieties are planted.
- If possible, plant varieties with good resistance.
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 to smut than others.
- Smut occurs most frequently on white varieties and is often severe when extremely dry or hot weather occurs just before 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, but it is unpleasant to handle.
- Break off the infected part of the ear; the remainder is suitable for eating.
The immature smut fungus or "maize mushroom" is highly prized in Mexican cooking. Harvest when the fungus is expanded, but before it becomes black and dried out. The time generally is about 2 to 3 days before the sweet corn reaches peak eating quality.
Harvesting Sweet Corn
Each cornstalk should produce at least one large ear. Under good growing conditions (correct spacing; free from weeds, insects and disease; and adequate moisture and fertility), many varieties 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 fully formed but 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 kernel appears milky when punctured with a thumbnail.
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, processed, or refrigerated as soon as possible. At summer temperatures, the sugar in sweet corn quickly decreases and the starch increases.
Cut or pull out the cornstalks immediately after harvest and put them in a compost pile. Cut the stalks in one foot lengths or shred them to hasten decay.
Selection and Storage
The period of peak freshness for sweet corn is measured in minutes not hours or days. The best corn is simply the freshest corn. Proper timing for harvest is crucial to the quality of sweet corn.
- Harvest sweet corn when the ears are full and blunt at the tip.
- The husks should be tightly folded and green.
- Using your thumb nail, poke an end kernel. It should squirt forth milky white sap.
- Under-ripe corn will contain a watery liquid; overripe corn will have a tough skinned kernel with doughy interiors.
- Also look at the silk, it should be turning brown and dry on the end.
Storing sweet corn for long periods of time will destroy it. The sugar quickly turns to starch, losing flavor, quality, and most of all, sweetness. If you must store sweet corn, use perforated plastic bags and get it into the refrigerator as soon as possible. Warm temperatures hasten the conversion process. Try to use the corn within 1 to 2 days and do not husk until just prior to cooking.
Questions & Answers About Growing Sweet Corn
Q. How long does it take sweet corn to develop from the first appearance of silks to harvest?
A. About 5 days are required for complete pollination after the first silks appear. Harvest begins about 20 days after first silking.
Q. The germination of my Illini Xtra Sweet is low. How can I get a better stand?
A. The seeds of supersweet varieties are shrunken and do not germinate readily in cold, wet soil. Do not plant too early in the spring. Wait until the soil is warm, preferably 65°F. Sow the seed more thickly and thin if necessary. Fungicide seed treatments may also be helpful.
Q. Why don't my sweet corn ears fill out to the tips?
A. Several conditions can cause poor kernel development at the tip of the ear such as dry weather during silking and pollination; planting too close; poor fertility, lack of potassium; and poor natural pollination. These conditions may be overcome by watering in dry weather; planting at recommended spacing (9 to 12 inches in the row); proper fertilization; and planting short rows in blocks of two or more for more complete pollination.
Q. What is the best way to grow early corn?
A. Choose an early maturing variety, plant early and shallowly (about 1/2 inch deep) and cover the row with clear polyethylene film treated to withstand UV light. Use 1 or 2 mil film 3 feet wide and cover the edges and ends to warm the soil around the seeds. The small plants can be left under the plastic for 2 to 4 weeks. Remove the film or cut slits and carefully pull the plants through before the weather becomes too hot. It is wise to experiment with this technique on a small scale first. Unseasonable heat can quickly cook and kill young seedlings under clear plastic.
Q. How can I keep raccoons out of my sweet corn?
A. It is virtually impossible to keep raccoons out of garden, although many methods are employed. The most successful seems to be an electric fence made with two wires, one about 4 inches above ground level and the other at 12 inches. The fence must be operating well in advance of the time that the corn approaches maturity. Raccoons prefer to eat sweet corn in the early milk stage, just before it is ready to harvest. Another option is a radio tuned to talk radio at night in the sweet corn patch. Raccoons are uneasy when it comes to human voices and may avoid the area. Simply place a radio in a spot protected from rain (under a bucket or wheelbarrow) and turn it to a station that has talk radio all night. The muffled voices confuse the raccoons, and they cannot quite place where they are coming from. This will deter many raccoons except those very acclimated to humans. Also let your neighbors know as they may wonder why there are people talking in your backyard all night.
Preparing Sweet Corn for Exhibition
Former Extension Specialist Jim Schmidt demonstrates how to properly prepare corn for exhibition.