| Varieties | Irrigation |
| Lime & Fertilizer | Pest Control |
| Planting | Harvesting |
| Spacing & Seeding | Post Harvest Care |
Sweet corn is an excellent rotation crop on vegetable farms because of its ability to break disease cycles, improve soil structure, and reduce serious weed populations by the use of the excellent herbicides available. However, some products have rotational restrictions for other vegetable crops. It may be grown continuously on the same land provided that the ground is moldboard plowed to bury the stalks, particularly so if stalks are infested with corn borer. Smut may be more severe with continuous sweet corn.
The top fresh market sweet corn dollar is obtained by planting early maturing varieties 2 or more weeks before the average frost free date in your area. Although adapted to a wide range of soil types, early plantings should be on the light, well-drained sandy loam soils. Heavier soils are best for the main season crop. First plantings are made in early April in southern Iowa and late April in central and northern Iowa.
Noted for their vigor and tolerance to cold soils, early maturing varieties do not have the quality flavor and ear attractiveness of main season varieties. Thus, early maturing varieties are followed by later plantings of the higher quality main season types. There are many excellent varieties available for a given area with a wide variance in plant size, ear height, ear length, number of kernel rows, kernel size, ear quality, and time of development. Some popular types for the fresh market are the bicolors (mixture of yellow and white kernels), white varieties, particularly Ice Queen, and the extra sweet types.
A single-hand harvest is optimum, although two harvests are sometimes necessary when the maturity is variable. Machine harvesting for fresh market is becoming more common. Although brusing of ears is sometimes a problem, this can be minimized by machine selection, careful adjustments, moderate operating speeds, and the use of varieties adapted to machine harvesting.
Yields average 4 to 6 tons for processing and about 1,000 to 1,200 dozen (200 to 240 crates) per acre for fresh market. With good irrigation management, yield can reach 7 to 8 tons and greater than 2,000 dozen (400 crates) per acre.
You should select varieties that produce good yields of high quality sweet corn in your area. Numerous varieties are available, but they are not all equally adapted throughout the state. Therefore, large-scale plantings should be restricted to varieties of proven performance in a given location. Varieties listed as trial plantings in table 1 should be confined to an acre or less.
The white varieties and bicolors have become popular in local market sales. These types will have to be introduced to your customers.
Our traditional sweet corn varieties differ from field corn because they carry a recessive gene called sugary (su). Corn kernels carrying this gene develop nearly twice the amount of sugar as starchy types. Unfortunately, these sugar levels are unstable and diminish rapidly after harvest. Because of this, the industry has switched to genes that promote high sugar levels and improve postharvest storage properties in sweet corn. Over a dozen genes influencing kernel quality have been identified, but to date, only two and their combination are being extensively used.
Sugary enhancer (se).
In general, the se's will produce kernels with sugar levels and storage characteristics intermediate between those of normal sweet corn and the best shrunken-2's. The se's retain the creamy texture and tender skin of normal sweet corn. In fact, some varieties are so tender they won't stand up to mechanical harvesting or extended handling. Best suited for U-Pick and local markets. Isolation is required for optimum sweetness, but cross pollination with other corn types, except the sh2's, will not result in starchy kernels.
Shrunken-2 (sh2).
This type can produce peak sugar levels nearly double that of normal sweet corn. In addition, the conversion of sugar to starch occurs at a much slower rate giving more flexibility in harvest timing and storage (i.e., 5 to 10 days). Kernels are often described as being very sweet and 'crunchy' (the newer hybrids have eliminated the skin toughness problem). Isolation is required since cross pollination with other corn types will cause kernels to have low sugar and high starch levels. The sh2's are have gained strong acceptance as a good 'shipper' and showing potential for processing.
Synergystic (se x se x sh2 x sh2)
These types have a combination of se and sh2. Generally, both parents have se traits and one parent also has supersweet traits. 100 % se kernels, 25% sh2 kernels. Sweeter than the typical homozygous se varieties. Some have very good flavor.
Major problems with growing are poor germination, particularly at soil temperatures below 60 degrees F; the seed is brittle and can suffer injury during rough handling; the smaller seed size can create difficulties in planting; poor ear tip cover, lodging, poor vigor, and low yield. Breeding efforts by the major seed companies are solving many of these problems.
Bicolor sweet corn varieties for trial.
Variety |
Days |
Type |
Early Vigor |
Comments |
| Seneca Spring | 71 | se | Good | small plt., excellent early eating quality |
| Sweet Chorus | 71 | se | Excellent | sweet breed -sesush2 |
| Temptation | 73 | se su | G-Excellent | consistent high yield |
| Renaissance | 74 | se | Excellent | good flag lvs, big ear for maturity, good quality |
| Precious Gem | 79 | se se | Good | has performed well in trials |
| Absolute | 79 | se | Good | large yield, 8.5-inch ear |
| Delectable | 80 | se se | Fair | avoid cold soils |
| Providence | 85 | syn | Good | std. main season bicolor. Bt type called 'Attribute' |
| Double Up | 78 | sh2 | Good | nice ears, sweet and crunchy |
| Candy Corner | 76 | sh2 | Good | good disease resistant, fair/good eating quality |
| Obsession | 82 | sh2 | Good | high yield, excellent ear quality, tends to lodge |
| Cabaret | 83 | sh2 | Good | big, full ears, dependable |
Suggested for trial: Buccaneer, BC8005, Mirai, Ex282A, 276A, and 277A
For the latest variety research results see FG601 titled: Fruit/Vegetable Progress Report 2004 located on this web site.
10 Golden Rules
(adapted from Johnny's Seeds)
1. Isolate. There are several methods you can use to isolate the super sweets:
* A distance of 700 feet will give complete isolation but this is impractical.
A 250 feet distance will give some contamination, but not enough to materially
affect quality.
* Wind direction- isolation can be enhanced by avoiding prevailing winds,
i.e. plant on the southwest corner of a field.
* Barriers and border rows - a considerable amount of contaminating pollen
can be diluted by using 2 to 5 border rows for protection. Therefore, isolation
distances can be slightly reduced.
* Maturity - isolation can be provided by time of maturity. There should
be a 14-day minimum difference between maturities in order to receive isolation.
2. Adjust soil temperature. Use fungicide treated seed or delay planting until soil temperature is 75F. Avoid planting in soils below 60F. Try covering the planting with clear plastic mulch to raise soil temperature and give a uniform stand.
3. Adjust planter. Super sweet corn seed is smaller than normal corn; there are 1 1/3 to 1 3/4 times as many seeds per pound. Set planter for proper seed size and planting depth (3/4-1"), and allow for germination percentage to arrive at a final stand of 18,000-22,000 plants per acre (8-10" apart in rows 36" apart).
4. Assure moisture. Avoid dry soil planting. Consider trickle irrigation to assure a uniform stand.
5. Fertilize. Super sweets have the same high fertility requirements as normal corn. Fertilize as required pre-plant, and use a high phosphate starter in the planter.
6. Make a smooth seed bed. Uniform depth placement of seed is more important with Super Sweets.
7. Plant on the shallow side. With good soil moisture, ideal depth is 3/4-1" to assure an even stand.
8. Note the half-silk date. Upon tasseling, watch for and record the date that about half of the plants show silk.
9. Harvest on time. Field will be ready to pick beginning 15-21 days after half-silk, depending on temperatures during this period. Warmer weather favors earlier maturity. Delay of harvest for a few days is permissible as the Super Sweet tenderness holds longer than regular sweet corn.
10. Referigerate. Super Sweets are approximately twice as high in sugar content at harvest and sweeter to an even greater degree after picking. This is due to a slower sugar-to-starch conversion. For best quality, however, cool corn following harvest. Hold and ship under refrigeration.
Lime: To give a pH of 6.0 to 6.5. Caution: many areas of the state have high pH values, over 7.0, and do not need lime.
Rates:
N -- sandy soils; 80 N/acre disked in prior to planting. Sidedress
with additional 40 to 50 lb. N/acre approximately 3 to 4 weeks after emergence.
Loam, silt loam, silty clay loams; 75 to 100 lb. N/acre disked in prior to planting
for early varieties. For main season and late varieties, use 100 to 120 lb.
N/acre disked in prior to planting.
P2O5 (phosphate) -- 0 to 150 lb./acre, according to soil test report
K2O (potash) -- 0 to 150 lb./acre accordingto soil tests, according to soil test report..
Other micronutrients -- use plant analysis coupled with soil tests to detect
any suspected micronutrient deficiency.
Application: For the most efficient use of N, split the rate into several small applications during the growing season. If the crop is irrigated, N can be applied through the irrigation system. For corn following soybeans, reduce the N application by 1 lb. of N per bushel of soybean yield.
Because many fresh market consumers demand sweet corn with long green flag leaves, dark green husks, long ear length, and the best tipfill, most commercial growers maintain a high level of nitrogen in the soil. Sandy soils low in organic matter may require additional sidedress applications of 25 to 30 lb. N/acre, the final one being at the last cultivation before the rows grow in.
Granular ammonium nitrate, urea, or various liquid or gaseous forms of nitrogen may be used for sidedressing. All of them, except ammonium nitrate, must be knifed, cultivated, or watered into the soil to avoid volatilization of the nitrogen.
Phosphorus and potassium are also important (figure below). Apply some of the P2O5 and K2O in bands 2 inches below and 2 inches to the side of the row at the time of planting. Do not exceed 80 to 100 lb./acre of N plus K2O in the fertilizer band to avoid high soluble salts. A rate of 100 to 200 lb./acre of 6-24-23, 18-46-0, or 8-16-16 is a common practice.
A 400-crate per acre crop (2,000 dozen) of sweet corn removes approximately 100 lb. of nitrogen, 50 lb. of phosphate, and 50 lb. of potash per acre.
The number of days to maturity varies considerably depending on solar radiation, photoperiod, soil moisture, and the growing season temperature. Sweet corn is a warm-season crop with optimum night temperature for growth of 65 to 70 degrees F. Obviously, in cool, mid to late April, sweet corn will be slower to mature than the same variety planted near Memorial Day. Seed emergence will vary from 20 or more days in cool, wet soils to 5 to 7 days in warm soil. Unfortunately, the days to maturity listed for sweet corn in seed catalogs usually represents a value obtained with warm temperatures and rarely reflects time for development with early plantings. For instance, the maturity time for 12 early varieties planted the last week of April to early May in 1981 through 1983 in central Iowa averaged 79 days. However, the average catalog value for the same 12 cultivars was listed at 69 days.
Successive plantings are necessary for continuous supply in the market place. This can be accomplished by planting varieties differing in maturity or successive planting of a high quality variety using the Growing Degree Unit method (heat units). Actually, most growers use a combination of the two concepts.
The Growing Degree Unit (GDU) concept takes into account the effect of temperature on rate of sweet corn growth. The National Weather Service equation is:
GDU = (Tmax + Tmin) - 50 divided by 2
If Tmin < 50, then Tmin = 50 degrees F
If Tmax > 86, then Tmax = 86 degrees F
The Tmax and Tmin are the maximum and minimum temperatures in a 24-hour period. The 50 degree F value is assumed to be the base temperature below which sweet corn will not grow. The formula adjusts for maximum temperatures above 86 degrees F and minimum temperatures below 50 degrees F. The 86 degrees F level compensates for low humidity and moisture stress conditions that usually occur at high temperatures. However, recent reseasrch indicates that, with irrigation, this upper limit could be raised to 92 degrees F. The growing degree day concept is, at best, a simple relationship to explain a complex physiological growth development process.
Generally, GDUs accumulate twice as fast in mid-July (20 per day) as they do in late April and the first of May (10 per day). Thus, for a 3-day harvest spread in July, plant the same variety every 6 days in late April to mid-May. This ratio of 2 to 1 shortens to 1.4 to 1 for the end of May. Therefore, a 3-day spread in harvest in July is equivalent to a 4-day planting spread at the end of May.
The daily GDUs are reported for almost all Iowa weather stations and can be accessed through the world wide web. Or, you can calculate the GDUs by hand for your farm location. On graph paper, plot the average 40-year accumulative GDUs (on y-axis) for the season against the date (x-axis) from the weather station closest to you. Connect the points to draw a curvilinear line. Then, plot the GDUs as the season develops using the formula above. You should be able to see whether the season is behind or ahead of normal and adjust plantings accordingly. For example, if May is behind about 80 GDUs and normal temperatures occur the rest of the season, harvest will be about 4 days behind (80 divided by 20 GDU/day in July = 4 days). Thus, you may want to use an earlier maturity variety in your next planting to maintain a 3-day harvest spread.
Once 50 percent silk has occurred, time to maturity is more precise. For central and northern Iowa, we have found that silking occurring in mid-July would be approximately 18 days to maturity; silking at August 1, about 19 days; and in mid-August, 20 days to maturity. However, if extreme heat occurs in late July and August the time interval may shorten to only 15 days.
To use the GDU method effectively, you need to know the heat units or GDUs for the varieties you are planting. A few seed companies do give the GDUs for their varieties. Ask your local seed dealer.
Rows -- 30 to 38 inches apart; early variety plants 8 to 10 inches apart
in row; main season variety plants 9 to 12 inches apart in row.
Seed -- 10 to 15 lb/acre.
In a 4-year study on central Iowa soils with irrigation, the best overall plant population was 23,000 plants per acre. If the plant population is too high, the most noticeable qualitiy differences will be a shorter ear and a lack of tipfill. If irrigation is not available, you should drop the popularion back to 16,000 to 18,000 plants per acre.
Early varieties are usually vegetatively smaller and greater plant populations are suggested than for the more vigorous midseason varieties. Most growers allow for an 80 to 90 percent field germination rate under good conditions. Optimum yields are generally recorded at 20,000 to 23,000 plants per acre or with 8 inches between plants in 38-inch rows.
Precise planting of uniform seed size is essential for a full stand of uniform plants. Uniformity of growth and maturity is very important for sweet corn in that entire fields are harvested at one time. Excessive planting speed may result in poor stands. Use a dependable planter with plates designed for specific grade of seed. While air or vacuum planters permit the use of ungraded seed at planting rates as fast as 8 miles per hour, with sweet corn it is still important to use seed of uniform size to ensure uniform emergence and vigor.
A deep, firm seedbed free of clods, trash, and surface irregularities is necessary for uniform germination, good stands, and uniform maturity. Uniform maturity, because of its effect on kernel size, depth, and color, is especially important for processing sweet corn. Plow under stubble or green manure crops that precede sweet corn, and disk the soil 3 or 4 weeks before planting to allow partial decomposition of the organic matter. Disk and harrow as many times as necessary to prepare a uniform seedbed and keep weeds under control. However, an excessively worked seedbed will be subject to packing and crusting. If a crust forms before germination and emergence of seedlings, a reduced stand will result with reduction in uniformity at harvest. This problem can be partially alleviated by breaking the crust with a rotary hoe.
Planting too deeply is a common error. Seed should be planted only deep enough to be placed in the moist soil below the dry surface layer. In a loamy, silty, or silty clay loam soil, the depth should not be over 1 inch. On sandy loams, 1 1/2 inches is good, and on the very light sandy soils, such as the Muscatine region, the depth may be 2 inches. Deeper planting may interfere with germination and prevent emergence if packing and crusting follow hard rains.
To speed maturity of early plantings on light, sandy soils, clear plastic mulch may be used on a trial basis. The use of clear plastic mulch will improve stands, conserve moisture, and produce earlier maturity. Corn is seeded in the usual manner except 10 to 20 days earlier in double rows that are 18 to 24 inches apart. Best results have been obtained by punching through the mulch. Some growers have been successful with the mulch concept in marketing sweet corn 7 to 10 days earlier.
Apply fertilizer and herbicide before laying the clear plastic. Punch holes in the plastic to allow rain to drain either by drilling into the roll before laying the plastic or punching holes manually in the field.
At low and intermediate plant populations, particularly with high nitrogen fertility and under high rainfall, sweet corn will produce tillers or suckers. Cutting out these tillers is not recommended. Usually they do not have ears and they actually serve as feeders contributing to the yield of the main stalk. The practice of removing the side or secondary shoots arising from the base of the sweet corn plant is known as suckering. Suckering is an old practice dating to the 1930s when hybrid sweet corn first came on the market. Benefits claimed by growers were higher yield, increased earliness, and larger individual ears. However, research by a number of states int he late 1940s and early 1950s showed that there was no consistent increase in ear size or yield of sweet corn. In some cases, yields were reduced by suckering, particularly when done late in the season. Removal of the suckers allowed greater ease of harvest, but the high labor cost was not warranted and commercial growers have discontinued the practice. New varieties adapted to machine harvest produce no suckers.
Young sweet corn plants have rather coarse, shallow root systems, but as the plant approaches maturity, the root systems become more fibrous and can penetrate to a depth of 3 feet. Start regular irrigation when plants are 3 to 6 inches tall. This first irrigation can be quite important for obtaining maximum yields. Because of the relatively shallow root system, water may be needed even though the soil appears to be fairly moist. The most important time period for irrigation is 2 weeks before tasseling through harvest. Rates of sprinkler irrigation depend upon soil type, temperature, relative humidity, and rainfall.
Usually sweet corn requires about 1 inch of water per week during the growing season. Drought stress, as evidenced by slow progress and rolling of leaves during the day, will result in delayed maturity and uneven harvest. During the critical silking and tasseling stages of growth, sweet corn may use about 1 1/2 inches of water per week per acre. Applications of 3/4 inch twice a week is preferred on sandy soils with low capacity for holding water. Higher application rates would increase nutrient leaching.
Insect pest such as corn borer, corn earworm, and aphids are the major pests of sweet corn production. For control measures obtain a copy of FG 600, titled: Midwest Vegetable Production Guide for Commercial Growers, from your local county extension office or Ag Publications and Distribution, Printing Bldg., Iowa State University, Ames, IA 50011.
Initial insecticide application should be made at initial silk and followed up with one or two additional aplications during the flowing silk stage of growth. Corn earworm moths lay eggs on the unprotected new silk.
Aphids can be a particular problem during hot, dry weather conditions
.
Black cutworm can be troublesome early in the season and scouting and monitoring of insect traps is effective. Iowa maintains several traps throughout the state and the results of the collection can be found at the Iowa Pest Newsletter site.
Corn smut can be serious some years, particularly on certain varietes. Use a smut resistance variety and be sure to rotate your land.
There are a number of good herbicides available for effective weed control. Please read all label directions carefully. This field shows the result of applying 2, 4-D at the wrong stage of growth.
Near maturity treatment.
Avitrol Corn Chops-99 is suggested, however it is a restricted-use pesticide. Usually applied in dilute form on about 1/3 of the acreage treated. Confine treatment to areas more than 50 feet from field edge. Read the label thoroughly.
Grackles, redwing blackbirds, and starlings ingesting Avitrol react with
distress calls that spook the rest of the flock. Acetylene exploders or
firecrackers are effective, but birds will soon resume feeding when exploders
are not operating. For full effectiveness locate them above the corn stalks
and move them around frequently. Constant patrolling and shooting during
morning and evening feeding periods are effective but expensive. Plant away
from marshy and wooded areas. Choose varieties with good husk cover.
Shoot or trap raccoons and skunks in season. An electric fence with three strands placed at 3- to 4-inch intervals from the ground has given satisfactory control.
The only proven effective control for raccoons is a fence with 2 electric wires near the ground.
Sweet corn is different from field (dent) corn in that the kernel has the ability to accumulate sugars instead of starch. However, the natural sugars do not remain for long and this is why proper handling is important. The most important factor affecting the flavor of sweet corn is the time and temperature between picking and eating.
As the ear matures, the kernels pass through stages called pre-milk, milk, early dough, and dough during which sugars change to starch and the hull becomes tougher. The higher the field temperature the quicker the kernel goes through these stages. At field temperatures of about 60 degrees F, an ear may remain in prime condition for 5 days. At 85 degrees F it will pass this stage in 1 or 2 days. Thus, it is very important to check fields daily during mid-July for early sweet corn harvest.
Sweet corn kernels at the best stage for eating contain 70 percent water. If harvested too young, the kernel contents are thin, watery, and lack flavor.
Sweet corn is ready for harvest when 70 percent or more of the primary ears are marketable, i.e., the ears are filled out. It should be harvested when the kernels are just beginning the milk stage. At this stage the silks are brown and dry beyond the end of husks and the ear is large enough to fill the husk tightly to the top. The kernels have nearly attained maximum size, but they are still soft and tender and filled with a clear to milky juice when punctured with a thumbnail. The number of days from silk emergence to harest is usually 18 to 22 days. However, if days and nights are exceedingly warm the prime maturity may be reached in 15 days or less after silking.
Probably 10 or more acres of sweet corn are needed to justify a mechanical harvester. Harvest is generally accomplished by driving down two rows with a tractor and trailer picking the rows ahead of the tractor and picking four or more rows on each side of the trailer. The ears are picked by hand with a downward twisting motion and put into sacks or bushel baskets and dumped into the trailer.
Normal sweet corn loses its sweetness very soon after harvest if held at temperatures above 50 degrees F. Approximately half the sugar is lost in 24 hours at 86 degrees F. Even when stored at 32 degrees F, sweet corn loses 8 percent of its sugar in 24 hours. The extra sweet or supersweet corns maintain sweetness or sucrose content longer.
If corn is sold retail the day of harvest, no pre-cooling is needed. However, if the corn is to be shipped to market or held for an extended period of time, it should be cooled below 40 degrees F as soon as possible. Leaving the crop in a refrigerator for 24 to 48 hours to cool down is not the proper procedure. The longer the crop is at a high temperature, the greater the decline in quality. It must be cooled quickly. Usually corn is hydro-cooled to hear 32 degrees F and top-iced for transit or storage to keep husks fresh. Hydro-cooling with top-icing removes field heat very quickly, but is expensive. Vacuum cooling is also satisfactory if the corn is pre-wet and top-iced after cooling.
When using ice-cooled systems, remember the ice must melt to cool the water. The cool water is then returned to the shower or immersion tank for reuse. The water will cool faster with crushed ice than with chunck ice. The smallest hydrocooler system including water bath assembly movement along with refrigeration costs several thousand dollars. For interstate shipment, cooling and top-icing involves 15 to 25 lbs. of crushed ice distributed throughout the box during packing.
For local marketing, if cooling facilities are not available, harvest the corn as early in the morning as feasible and cool the corn with cold water and keep it in the shade during the hot part of the day. Corn harvested in the early morning will be 10 to 25 degrees cooler than that harvested mid to late afternoon.
Corn is usually sold by the count -- a dozen ears, 5 dozen ears per crate or sack. For roadside or pick-your-own, corn may be handled in bulk. Some markets may require trimming at the base of the ear (shank).
............Justice for All
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and maritial or family status. (Not all prohibitied bases apply to all programs). Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, room 326-W, Whitten Bldg., 14th and Independence Avenue, SW, Washington DC 202590-9410 or call 202-720-5964, issued in furtherance of Coop[erative Extension Work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.