Sweet corn is a popular vegetable and is relatively easy to grow. Among market gardeners throughout New England, about half of the vegetable acreage is devoted to sweet corn. The average yield for a home garden is about one-two ears per plant.
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Sweet corn ears may have all yellow, all white, or bicolor (a mixture of yellow and white) kernels. Bicolor varieties are most popular in New England, but the quality of all of these are excellent and depend on the specific variety, the growing and handling conditions, and on personal preference.
Sweet corn kernels are sweeter than field corn varieties because of a mutation at the sugary locus (su). After harvest, the sugar converts into starches, and the kernels become tougher and less sweet. The traditional (su) sweet corn varieties become starchy relatively quickly after harvest (hence the traditional recommendation to get the pot of water boiling before you harvest the corn!). Sugary-enhanced varieties (se or se+) varieties accumulate more sugar than the (su) varieties, and super-sweet (sh2) varieties are the sweetest of all, with the sugars converting to starches much more slowly than with other types. If super-sweet varieties cross-pollinate other types of corn, the quality of both is reduced. Cross-pollination can be prevented by distance and/or windbreaks, or by planting varieties of very different maturity dates so that they do not bloom at the same time. It is not necessary to isolate sugary (su) or sugary-enhanced (se) varieties from one another.
For maximum growth and yield, sweet corn should receive full sun. Soil temperature must be at least 50°F for germination and growth, but 60-85°F is ideal. Sweet corn that has germinated can withstand light frosts because the growing point is protected by the outer leaves, but it is important for the soil to be warm enough to allow germination. It is possible to transplant corn successfully. For transplant, 1-2 seeds should be sown in cells that are large enough so that it does not become rootbound, and plants should be transplanted outdoors within 10-14 days of seeding.
These sweet corn transplants are about 7 days old, and will be ready to transplant in another 3-5 days.The ideal soil for corn is well drained and fertile, with a pH of 6.0-6.8. As a general rule, plant early corn in light soil (sand or loam) and later corn in heavier soil (silt or clay), when there is an option. Light soils warm up faster than heavy soils, so seed germinates more readily. In hot midsummer conditions, heavier soils have the advantage of holding more moisture than lighter soils.
Sweet corn needs ample water from germination to harvest, but the most critical period for water is about 2 weeks before silks form. Aim for 1 inch of water per week, and supplement natural rainfall with irrigation as needed.
To get a continuous supply of corn throughout the summer, plant a small amount of the same variety every 7-10 days or simultaneously plant varieties with different dates to maturity. In this photo, each group of four rows was planted 7-10 days apart. The older corn on the right is almost ready to pick, and the corn on the left will be ready in about 2½ months. This corn planting will provide a steady harvest of sweet corn from late July through mid-October.Test soil before planting to determine the amount of lime and fertilizers needed. Soil testing can be done through a number of private and public labs. UNH Cooperative Extension offers this service. Forms and instructions are available on our website, or you can call our Info Line at 1-877-EXT-GROW (1-877-398-).
Aged manure or compost incorporated the fall prior to planting will provide nutrients and increase water-holding capacity. Generally, corn needs the equivalent of about 25 lbs of 10-10-10 fertilizer per 1,000 square feet. Equivalent rates of other synthetic or organic fertilizers can be used instead of 10-10-10. Fertilizers should be thoroughly incorporated into the soil before planting. When sweet corn is 15 to 18' high, it may benefit from sidedressing with nitrogen fertilizer at a rate of approximately 2.5 lbs of a 10% nitrogen fertilizer (or the equivalent) per 100 feet of row. Spread the fertilizer between the rows or on either side of a single row and lightly incorporate it into the soil.
Corn is pollinated when wind currents carry the pollen from the tassel to the silk. Pollination is essential, since each kernel develops only when a pollen grain lands on the silk attached to that kernel. While it is possible for corn to be pollinated effectively when planted in a single row, planting several short rows in a block formation increases the likelihood of successful pollination.
This silking corn is at the perfect stage to be pollinated.Rows should be spaced 2½ - 3 feet apart. Plants should be spaced 8-10 inches apart between plants. For early plantings, seeds should be no deeper than 1 inch. For later plantings, plant seeds 1-2 inches deep to ensure adequate moisture contact.
Each corn planting will be mature for only a short time: 7-10 days. For a continuous supply of corn throughout the summer, plant a small amount of the same variety every 7-10 days or simultaneously plant varieties with different dates to maturity.
Weeds
Broad-leaf weeds such as lambsquarter, pigweed, purslane and galinsoga, and grasses such as quackgrass and crabgrass are strong competitors of sweet corn. Weeds can be controlled by using mulches of black plastic or straw. For black plastic, lay the mulch before planting, punch small holes through the plastic and push the corn seed down to the appropriate depth. Plants will grow through the holes. For straw mulch, apply after the plants are 4-5 inches high. Shallow cultivation or hand-hoeing are also effective ways to control weeds. The best time to kills weeds is when they are very small, so frequent shallow cultivation is the most effective method.
Insects
The European Corn Borer and the Corn Earworm are two insects that attack sweet corn. Corn borers usually occur twice, in mid-June to early July and again in mid-July to September. Corn earworms occur during this later time period. In small garden plantings, these pests may not occur in a given year, and they may not require management. For more information about integrated management of these pests, please see the publication Integrated Management of Sweet Corn Insects in New Hampshire. Biological and chemical control strategies should be based on the presence of the insects, which can be determined by trapping. For more information, see the publication Setting up Traps to Monitor Sweet Corn Insects in New Hampshire.
Diseases
Sweet corn is generally free of serious diseases. Occasional a grayblack mass appears on the ears. It is caused by a fungal disease known as smut. The best control is to cut off the ear and dispose of it.
Birds and Other Verterbrates
Germinating corn is often pulled up by birds or crows as they search for the kernels. One control method is to cover the planted rows with polyester rowcovers (available at most garden centers) at the time of planting. Secure edges with rocks, soil or pins, and leave enough slack for the corn to grow. Remove after 3-4 weeks when the plants are too large for the space under the cover. Ripening corn is a favorite food of raccoons. Electric fencing with two relatively low electrified strands (6' and 12') is an effective way to exclude raccoons from sweet corn plantings. The fence should be installed (and turned on) before the corn is ripe, and it is important to keep the area underneath the fence clear of grass and weeds that can ground it out.
Harvest and Storage
When sweet corn is ready to harvest, ears are plump and the silks have turned brown and started to dry up. You can test for maturity by gently peeling back the husk to check kernel size.
The temperature at which sweet corn is harvested and stored can have a dramatic effect on eating quality. After harvest, sweetness rapidly declines as the sugar in the kernel is converted to starch. This conversion is accelerated by high temperatures. For example, the rate of sugar loss is 10 times greater when corn is held at 70°F than at 32°F. Although super-sweet types retain sweetness much longer than other types, it is important to harvest all types of corn in the early morning or late evening when temperatures are cool, and to refrigerate it to maintain quality.
Sweet corn prefers full sun and fertile, well-drained soil for maximum yield. Incorporate plenty of organic matter and a complete fertilizer into the area before planting. When soils are above 60ºF, space rows 24-30 inches apart and plant seeds 1 inch deep and 7-9 inches apart in the row. Plant corn in blocks to ensure good pollination and ear development.
Sweet corn may be transplanted or seeded under clear plastic for early production. Plant corn every 10-14 days for a continuous crop throughout the season. Side dress sweet corn with nitrogen fertilizer when plants have 8-10 leaves and again near silking. Corn requires regular watering, so maintain soils near field capacity.
Water stress will reduce yield and ear quality. Organic mulches help conserve water, supply extra nutrients, and reduce weed growth. Control insect and diseases if they occur. Harvest when ears are plump, silks are dry, and kernels are milky. Use ears immediately for best quality.
There are many different sweet corn varieties for the home vegetable garden. Major differences include maturity dates and sugar content. Sweet corn has a maturity range of 60 to 90 days from planting depending on variety. Early varieties produce smaller ears and are less sweet than later maturing types. Early varieties do well where the growing season is short and temperatures are cool. Late maturing varieties are better adapted to long seasons and warm temperatures. Sugar content in the kernels should also be considered. Sweet corn varieties are classified as standard sugary (su), sugary enhanced (se), or super sweet (sh2). Standard types germinate better than the se or sh2 types in cool soils. All types germinate well in warm soils. Eating quality (sweetness) may be affected when su, se and sh2 are planted together and tassel at the same time. If cross pollination occurs, the extra sweetness of the se and sh2 types is lost and they taste more like the standard types. Time your plantings so they tassel at different times. All sweet corn varieties will grow in Utah, but not all are available. Most garden centers and nurseries carry varieties proven to grow well and produce high quality, flavorful ears. Most varieties grow well in Utah.
Sweet corn will grow in all Utah soil types that are nutrient rich, well-drained, and fertile.
Before planting, determine fertilizer needs with a soil test and then follow the recommendations given with the test report. If fertilizer applications are warranted, work the fertilizer into the top 6 inches of soil. If you fertilize with compost, apply no more than 1 inch of well-composted organic matter per 100 square feet of garden area.
Sweet corn is a warm weather vegetable that requires soil and air temperatures above 60ºF for best germination and plant growth. Plant after the last frost-free date for your area. For earlier corn, some gardeners grow transplants for planting after the frost-free date. Seeds may be sown 3-4 weeks earlier if planted through or under clear plastic tunnels (see photo above).
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To plant 100 feet of row, you will need 3-4 ounces of seed. Planting in blocks of three or more rows rather than in a single long row. This improves pollination and ear production. Plant corn seeds 1 inch deep. Seeds should be spaced 7-9 inches apart in the row with 24-30 inches between rows. For high-density corn, plant in 15 by 15 inch squares. Plant the same variety or type every 10-14 days until early July for continuous production throughout the season. Corn requires 60-90 days to mature depending on variety. Planting a 65, 70, 75, and 80 day variety of the same type (su, se, or sh2) at the same time will ensure production over a longer time period. With a little planning, a continuous supply of fresh sweet corn can be produced.
Sweet corn requires regular watering throughout growth for best production. Soils should be maintained near field capacity. Water needs are critical during tasseling, silking, and ear formation. Drought stress during ear development will decrease yield, lower kernel quality, and affect flavor. Watering amounts depend on soil type.
In addition to the fertilizer used when preparing the site, sweet corn needs additional nitrogen fertilizer to produce optimum yields. Side dress sweet corn with ½ lb. of 21-0-0 per 100 square feet when plants are near knee high (8-10 leaves) and with an additional ¼ lb. when the first tassel or silks appear. Place the fertilizer 6 inches to the side of the plants and irrigate it into the soil.
Clear plastic mulches help conserve water, provide some frost protection, and allow earlier planting and maturity, but stimulate weed growth under the plastic. Fabric row covers also protect young plants from frosts. Organic mulches like grass clippings, straw, and shredded newspaper also help control weeds and reduces soil water loss.
Control weeds with regular cultivation especially when plants are small. Once sweet corn begins to grow vigorously, it will out-compete the weeds. When cultivating, avoid root damage which can slow plant growth.
Disease
Symptom
Control
Smut
White fungal galls that form on the tassel, stem, or ear. Early plant infection will stunt growth and deform ears.
Remove and destroy galls and severely infected plants. Plant resistant varieties.
Wilt Diseases
Wilting leaves, streaking and drying of leaves, stalk rotting, and plant lodging may occur. Plants often die.
Remove infected plants. Maintain clean garden practices.
Sweet corn ears mature in 17-24 days from silk emergence depending on the temperature. Ears are mature when silks are dry and brown. The husks should appear moist and green. Kernels in the tip of the ear should be plump and release milky juice when punctured. Ears can be harvested over a 5-7 day period. For best quality and flavor, harvest and use immediately. To harvest, grasp the ear, snap downward while twisting the ear. Sweet corn can be stored for several days if refrigerated. Do not husk until ready for use.
Expect one ear per plant. Plant 10-15 feet of row per person for fresh use and an additional 30-40 feet of row per person for canning or freezing. Ear size and yield depends on variety grown.
Sweet corn is high in fiber, potassium, folic acid, and vitamin A. One ear contains 80 calories and 20 grams of carbohydrates.
Often, when I plant corn early in the year, I have poor emergence. How can I get a better stand?
It is best not to plant too early. Wait until the soil is warm, preferably above 60ºF. Try sowing more seed and thin to the correct distance after they emerge. Fungicide seed treatments may also be helpful. The sh2 varieties do not germinate well in cold, wet soil. For early plantings, sow su types or plant through/under clear plastic.
Sometimes the ears do not fill out at the tips. What's wrong?
Poor kernel development at the tip is often caused by: 1) hot, dry weather during silking and pollination; 2) plants growing too close together; 3) low soil fertility; and 4) poor natural pollination. Be sure to keep soils moist in dry weather; plant at recommended spacing; apply fertilizer according to soil test recommendations; and plant corn in blocks of three or more rows rather than one long row.
I regularly get lots of secondary shoot growth around the base of my plants. Are these productive or should they be removed?
Extra shoot growth (suckering) is common in sweet corn when plants are spaced too far apart in the rows. Most suckers do not produce usable ears but removal does not increase plant productivity. Plant the seeds closer together to decrease sucker formation and increase ear yield.
How can I keep raccoons out of my sweet corn?
It is hard to keep raccoons and skunks out of sweet corn. If you grow a lot of corn, an electric fence (two or three wires) helps. Space the wires 4 inches apart starting at 4 inches above ground level. Raccoons prefer sweet corn in the early milk stage so have the fence operating before the corn is ripe. If you have a dog, kennel it close to the corn. The barking may help, but can also annoy you and your neighbors.
Published May
Utah State University Extension
Peer-reviewed fact sheet
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Daniel Drost, Vegetable Specialist
Former Vegetable Specialist
PSC Dept
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