Revised 11/05 -- Author Reviewed 11/05 HIL-50
Introduction
In most of the south, sweet corn (Zea mays var. rugosa) can be produced from early spring until
fall. However, sweet corn does have some specific environmental and cultural needs that must be
met for the plant to produce high-marketable yields.
Corn is a warm-season crop that requires high temperatures for optimum germination and rapid
growth. Soil temperature should be between 70° and 85°F for best germination. If the soil
temperature is too low, the seed will not germinate. If, at the same time, the soil is too wet, the
seed may rot before the soil temperatures are adequate for germination. In general, sweet corn
does not tolerate cold weather, and frost will injure sweet corn at any stage of growth. Other
stressful climatic conditions, such as drought or flooding, can reduce yields and cause small,
deformed ears.
Because corn is a short-day plant, some cultivars will not flower when the day length is more
than 13 hours. This is rarely a problem with commercially available sweet corn varieties in the
Southeast, but a grower should be mindful of day length when planting heirloom or tropical corn
varieties.
Corn is wind-pollinated and should be planted in blocks of at least 4 rows for good pollination to
occur. Sweet corn can also cross-pollinate with other types of corn. If sweet corn is planted
downwind of popcorn or field corn, kernels will be starchy instead of sweet. Cross-pollination
between white and yellow cultivars will change the colors of the kernels. Extra-sweet and
standard cultivars also should not be planted near each other or at the same time. To prevent
cross-pollination problems, sweet corn should be separated from different types of corn by at
least 400 yards, different types or cultivars of corn should be planted at least 1 month apart, or
cultivars with different maturity dates should be planted.
Overview of an Organic Production System
Organic agriculture employs a combination of the best methods of traditional agriculture and
modern technology. Present-day organic growers use tried-and-true practices such as crop
rotation, growing a diversity of crops, planting cover crops, and adding organic matter to the soil.
At the same time, most organic production systems include use of modern equipment, improved
cultivars, and new technologies such as drip-irrigation and plastic mulch.
In contrast to "conventional agriculture," organic farming relies on preventive rather
than corrective practices. Instead of depending on synthetic insecticides, fungicides, and
herbicides for pest control, organic growers employ an Integrated Pest Management (IPM)
approach and use beneficial insects and biological products such as Bacillus thuringensis (Bt).
Rather than amending the soil with synthetically derived fertilizers, organic growers build soil
fertility by using natural products such as cover crops, manure, and compost. A successful
organic system provides a grower with an income adequate to maintain a good standard of living
by producing an abundance of high-quality food, while at the same time nurturing the soil,
protecting the environment, and ensuring that the land will be healthy and productive for
generations to come.
Organic Certification
The organic industry uses standards to verify that foods labeled “certified organic”
are produced without the use of synthetic fertilizers or pesticides. If a grower sells $5000 or more
of a product labeled as organic, he or she must be certified by a federally approved certifying
agent. To be certified organic, land must not be treated with any prohibited substances for 3
years. During that 3-year period, a farm is considered transitional. In addition, a grower must
have and follow an ecological soil management program. Growers in North Carolina can be
certified organic by any organization that is approved by the National Organic Program, a
division of the USDA. The North Carolina Crop Improvement Association is the organization
that certifies many organic growers in North Carolina. More information on organic certification
can be found at the author’s organic agriculture website: www.ncorganic.org , as well as
at the homepage for the National Organic Program: www.ams.usda.gov/nop/ .
Soil Management and Fertility
Sweet corn grows best in a well-drained soil with a pH of 5.5 to 7.0. In choosing a site for corn
production, heavy clay soils with poor drainage and areas subject to flooding should be avoided.
Dry, sandy sites should only be used if irrigation is available. Soil pH can be raised by
incorporating ground limestone. Because the soil reaction with lime is slow, limestone should be
incorporated early, preferably in the fall before a spring planting.
Sweet corn is a fairly heavy feeder, and proper soil fertility is critical for high yields and good
growth. Once stunted by lack of nutrients, sweet corn may never fully recover. Over the growing
season, sweet corn needs approximately 150 lbs of nitrogen (N) per acre. A grower should ensure that 50 to 60 lbs N per acre are available at the time of planting. It will probably be necessary to side-dress with a natural fertilizer
source with rapidly available N starting when plants are about 2 ft tall. Additional nutrient needs
should be determined from soil tests but usually consist of about 50 lbs per acre of potash and 75
lbs per acre of phosphate.
Nitrogen deficiency is fairly common in sweet corn, particularly in cold, wet soils; flooded soils;
or dry, sandy soils. Nitrogen deficiency in young plants causes the whole plant to be pale with
spindly stalks and yellow leaf tips. In older plants, nitrogen stress is often expressed by shriveling
of tip kernels.
Phosphorus-deficient plants are usually dark green with reddish-purple leaf tips and margins. At
low pH or in sandy soils, magnesium deficiency may occur. Magnesium deficiency in corn
appears as yellow to white striping between veins of leaves. Older leaves become reddish-purple
and leaf tips may die.
Commonly used organic sources of nitrogen, phosphorous, and potassium are bloodmeal (~15%
N), bonemeal (~4% N and 21% P2O5), cottonseed meal (7% N, 2.5% P2O5, and 1.5% K2O),
and soybean meal (7% N and 2.3% K2O). Many local growers use poultry litter or commercially
available poultry-litter based products. For example, a 3-2-3 or 3-2-2 poultry litter product can be
applied before planting at a rate of 700 to 1000 lbs per acre. A blended fertilizer or cottonseed
meal/bonemeal mix (10-10-10) can also be used. Natural sources for micronutrients include rock
phosphate, greensand marl, and limestone rock. A liquid fish-emulsion and seaweed product
(average analysis of 4-1-1) may then be used for sidedressing.
Adding organic matter such as feedlot manure and compost to the soil increases the level of
nutrients, improves soil microbial activity, and increases water-holding and nutrient-holding
capacity. Organic matter also improves the physical condition of the
soil for cultivation and improves soil structure so the surface of the soil does not crust. Any soil
can be improved through the addition of organic matter. A vast array of organic
materials are available in the Southeast, including manures from poultry and livestock
operations, wood by-products from mills, and crop residues from a wide variety of farming
operations. Tobacco stalks, which are readily available in much of the state, are approximately
4% N and 4% K2O. All of these materials may be used fresh or composted. Cover crops are also
an important, inexpensive way to add organic matter to the soil, and much of sweet corn’s
N needs can be met via cover cropping.
Cover Cropping Systems
Most organic sweet corn growers use cover crops to increase organic matter, improve soil tilth,
and reduce erosion. In North Carolina, the most popular cover crops used are rye, rye with hairy
vetch, crimson clover, and wheat. Legumes, such as hairy vetch, clover, and alfalfa, fix nitrogen
from the atmosphere and can supply more than 100 lbs N per acre. To ensure the nitrogen-fixing
capabilities of the legume, a grower should inoculate the legumes with the proper bacteria before
seeding. A study on legume cover crops for no-till corn, conducted at the Mountain Horticultural
Crops Research Station in Fletcher, NC, by Dr. Greg Hoyt, showed that no one specific legume
residue was observed to be superior as a cover crop, though hairy vetch consistently survived
Western North Carolina winters. Grasses, such as wheat and rye, produce large amounts of
biomass and develop long roots which can bring nitrogen up from deep within the soil.
In North Carolina, winter cover cropping is the most commonly used system in both organic and
conventional farming. The cover crop is drilled or broadcast-seeded in the early fall. In spring,
the cover crop is disked under for a clean culture system.
In most low-till or no-till systems, the cover crop is sown in the fall, killed in the spring, and left
on the surface of the soil as a mulch. For an organic grower, the options for killing the cover crop
include mowing or undercutting. In a no-till system, the crop is planted through the cover crop. In
a low-till system, a narrow strip is cultivated for planting the crop seed or transplant.
Research on sweet corn production in low-till or no-till systems is still in progress in North
Carolina. In Connecticut (DeGregorio et al.), however, sweet corn grew well in a no-till system
with hairy vetch. Marketable yields were three or more times higher with hairy vetch than with
crimson clover or winter peas. Inoculated vetch seeds were planted in early August and
flail-mowed to a stubble-height of 6 inches in mid-June. Corn was then planted in 30-inch rows
with a conservation planter. For weed control, the vetch strips were mowed again to about 4
inches tall in late July.
Results may be site- or region-specific, however, because, in Oregon, sweet corn growth and
yields were reduced 16% with strip-till and 31% with no-till compared to a conventional tillage
system (Peterson, et al. 1986). The reductions were attributed to lower soil temperatures, wetter
soils, and more slugs found under the reduced tillage systems. Thus, in our region a low-till or
no-till system for sweet corn may work best in the lower elevations of the Piedmont and Coastal
Plains where soils warm up early in the spring.
Growing a cover crop as a living mulch between rows of corn is an alternative to conventional
cultivation and fertilization. A perennial living mulch provides a year-round groundcover which
protects against erosion and soil compaction. Growing the corn in narrow rows with wide mulch
strips improves access to the corn and facilitates mechanical control of the mulch crop. Even
under very wet conditions, with a living mulch in place a grower can still move equipment
through the field. A living mulch increases soil nitrogen availability and reduces annual weed
populations. Legumes, such as alfalfa, clover, and hairy vetch, add fixed nitrogen and organic
matter to the soil. To avoid competition with the corn, a living mulch may need to be
suppressed or controlled by mowing or light tilling during the growing season.
Grubinger and Minotti (1990) in New York tested white clover as a living mulch for sweet corn.
The clover was broadcast seeded in late April. In early June, a wheel-hoe (a hoe blade on a
wheeled frame) was used to open 15-inch strips for sowing the corn. They found the most
effective means of clover suppression was partial rototilling with a multivator (multiple-row unit
rototiller) when the clover was well established, about 2 weeks after corn emergence. As long as
a strip of clover roots passed intact between the tiller tines, regrowth of the clover was extensive
providing an effective, yet non-competitive, living mulch. Corn yields were reduced when the
clover was mowed rather than tilled. Conversely, in an Oregon study by Fischer and Burrill
(1993), mowing the clover did not seriously reduce corn yields. A grower might want to
experiment with both methods in this region. An interesting side note from the Oregon study was
that when corn smut was present, corn grown with suppressed clover suffered less damage than
clean, cultivated corn. Growers should note that a living mulch system for vegetables can be
risky if irrigation is not available, because the mulch may out-compete the cash crop for limited
soil moisture.
A living mulch system can be difficult to manage during the learning phase. Anyone considering
a living-mulch system should start small. Some problems that may be encountered include
increased numbers of rodents and slugs, as well as cooler soils, which may slow germination.
According to Burrill et al.(1987), the most predictable and least complex system to start with
involves annual seeding of clover after the corn is well established. The clover then has no
chance to compete with the corn and makes most of its growth after the corn is harvested. The
clover then protects the soil during the winter and grows in the following spring. The clover can
then be disked under and another crop planted.
Crop Rotations and Intercropping
Crop rotation is the single most important practice in an organic vegetable production system.
Crop rotation is the practice of following one annual crop with another crop that is as different
from the first crop as possible in terms of nutrient needs, rooting patterns, disease and insect
pests, and growth habit. Many growers have detailed, complicated crop-rotation plans that range
from 3 to 7 years before the same crop is replanted on a plot of land. For example, sweet corn
may be planted in a 3-year rotation with pumpkins and beans.
Soils deteriorate because of erosion, continuous cropping, deep plowing, and compaction.
Rotating fields with soil-improving crops maintains long-term soil fertility. According to Eliot
Coleman (1995), a well-thought-out crop rotation is worth 75% of everything else that might be
done, including fertilization, tillage, and pest control. Diversity is the key to stability in a
biological system.
Intercropping is the practice of planting several crops together in one area. Increasing diversity of
crops and reducing the amount of land in a monoculture discourages many pests. To
conserve space, sweet corn is often planted at the same time in a field with vine crops, such as
cucumbers, pumpkins, and muskmelons. The vines can be trained to grow between the corn
plants. In western North Carolina, it is a time-honored tradition for farmers to grow
greasy beans in their corn. An alternative to intercropping within the same field is strip-cropping.
In this system, two or three different crops are grown in strips, commonly 2 to 6 rows wide.
Variety Selection
Four types of sweet corn are available: standard (su), sugary-enhanced (se), and supersweet
(sh2), and synergistic or triplesweet (sy). The su type is the old-fashioned sweet corn with which
we are all familiar. It must be consumed quickly after harvest, or the sugars rapidly turn to starch.
The se types contain more sugar than the su type and, if cooled, will remain sweet for several
days after harvest. The sh2 type also contains more sugar than the su type but converts very little
sugar to starch. If properly cooled, an sh2 variety will remain sweet for 7 to 10 days after harvest.
The sy type is a hybrid comprised of 75% se and 25% sh2 kernels, resulting in an ear with the
sweet and tender characteristics of the se type but with the shelf- life of the sh2.
Replicated organic sweet corn variety trials have not been conducted in North Carolina. Many
North Carolina organic growers, however, prefer the following cultivars. 'Platinum Lady' (se) and
‘Bodacious’ (se) are good early-season sweet corn varieties.
‘Silvarado’ (se), ‘Kandy Corn’ (sh2), ‘Argent’ (se),
and ‘Snowbelle’ (se) are excellent mid-season varieties, and
‘Delectable’ (se), 'Pegasus' (sh2), and 'Silver Queen'(su) are good late-season
white corns. In a crop-rotation study (J.M. Davis, unpublished), 'Silver Queen' exhibited very
little earworm damage, probably because of the tight tips on the husks. All of these varieties are
among those recommended for all North Carolina commercial vegetable growers.
During the early 1990s, an organic growers network in New Jersey, Connecticut, Massachusetts,
and Vermont recommended 'Silver Queen' (su) (because of consumer name recognition and
earworm resistance) and 'Snowbelle' (se) as good white corn varieties for organic growers. For an
early-season yellow corn, they suggested 'Sugar Buns' (se). 'Bodacious' (se) is a favorite early to
mid-season yellow, and 'Clockwork’ (se) is an excellent mid-season bicolor in that part of
the country. For wholesale shipping of sh2 varieties, the Northeastern group recommends the
varieties ‘Aloha’, ‘Skyline’, ‘Diablo’,
‘Starstruck’, and ‘Escalade’.
They caution that results in another location or market might be different. Varieties for the new
sy type include ‘Renaissance’ and ‘Nantasket (early season),
‘Montauk’ and Bojangles’ (midseason), and ‘Providence’,
‘Cameo’ and ‘Charmed’ (late season). In addition, improved sweet
corn hybrids are being introduced every year, especially se, sh2, and sy varieties, with
improved disease resistance and seed germination.
Please note that you should be careful to avoid Genetically Modified varieties of sweet corn.
These varieties are transgenetic, for example, containing the Bt gene. The use of GMO corn is
not approved for organic production.
Planting
Fresh corn seed should always be used, especially for supersweet cultivars. Standard and
sugary-enhanced corn seeds should be planted about 1 inch deep in moist, heavy soil; 1 to 2
inches deep in very light, sandy soils. Supersweets need to be planted shallow -only about 1 inch
deep. Minimum soil temperatures for germination are 50°F for su varieties and 60°F
for se and sh2 varieties. Seed planted in moist soil below these temperatures will often rot. The
optimum, but rarely obtained, soil temperature for sweet-corn germination is 85°F.
When planting small acreages of sweet corn, be sure to plant the corn in blocks of at least 4 rows
to insure good pollination. If large acreages of sweet corn are being planted, especially se and sh2
varieties, purchase of a precision seeder might be cost worthy to reduce seed costs and labor
costs for thinning. No-till systems require no-till planters such as those used for field corn.
Depending on the cultivation equipment available, seeds are usually planted 5 to 6 inches apart in
rows 30 to 42 inches apart. Once the plants are well established, they should be thinned to stand
8 to 12 inches apart in the row.
Because sweet corn is wind pollinated, quality is best if the stands are isolated from field corn,
popcorn, and ornamental corn, as well as from other types of sweet corn. Isolation, in this sense,
means either planting the crops at least 250 feet apart, or staggering your plantings so that there is
a 10 to 14 day difference in maturity between crops. If possible, sweet corn should be planted
upwind of field corn.
Successive plantings can be made to provide a continuous supply of sweet corn. A second
planting can be made when the first plants contain 3 or 4 leaves. Early and late-season cultivars
may be planted at the same time for a longer harvest. Many growers, however, try to time their
harvests before August when armyworm and earworm populations are highest.
Weed Control
Weed control is one of the most difficult management problems facing organic vegetable
growers. Before planting, weed populations can be reduced through use of crop rotations and
cover crops. Try to rotate crops with different growth habits, warm and cool season crops, and
crops grown in wide and narrow rows. On small plantings, organic mulches, such as straw or
grass clippings, can help shade out weeds between the rows. Growing corn in strip-till, no-till, or
living mulch systems can also reduce weed problems.
In a clean-culture system, cultivation is a common and effective method used to control weeds
before and after the crop is planted. Prior to planting, till the soil several times to expose weed
seeds and stimulate their germination. Conduct the last tillage just before sowing the crop. After
the crop has emerged, cultivate frequently, getting as close to the corn plants as possible without
damaging the roots. Spring-tooth harrows and finger weeders work especially well for this
purpose. When the corn is 12 to 18 inches tall, till for the final time, throwing soil against the
base of the plant. Commonly used equipment to cultivate between rows include multi-row
rototillers, coil-tine harrows, and rolling cultivators. For small-scale production, a grower may
walk the field frequently with a hand-held hoe.
Soil solarization is a relatively new method for weed control that is still being tested. Solarization
kills weed seeds, perennial weeds, soil pathogens, and nematodes by heat treating the soil.
Solarization requires long-range planning because a field must be solarized the summer prior to
planting the corn. The system involves laying clear plastic over tilled, moist soil and sealing
around the edges with ridges of soil. The plastic is left in place, usually during mid-summer, for
6 to 8 weeks. After the plastic is removed the soil surface should be disturbed as little as possible
to prevent bringing weed seeds to the surface. It is better to plant a fall vegetable crop or winter
cover crop than to leave the solarized soil fallow for the winter.
Flame weeding by propane torches is another method used by some farmers. Small, inexpensive, hand-held propane units and large, tractor-mounted units
are available. Weeds are best controlled by flame weeding when the weeds are 1 to 2 inches tall
or at the 3- to 5-leaf stage. Flame weeding can be used before a cash crop emerges or as a
directed flame after the crop is 2 inches tall.
Pest Management
Pest management in an organic system is based on prevention. The goal is to have a healthy,
balanced plant and soil system in which pest populations will be stay within tolerable limits. In a
conventional system, synthetic pesticides may help a grower save the current crop from an
immediate pest problem; however, in many cases, the problem recurs or another develops. The
organic approach is based on the theory that major pest problems usually occur when something
is out of balance in the system. Are the plants undernourished or stressed from growing too
quickly? Is there a nutrient imbalance? Is the soil too wet or too dry? Has a good crop rotation
been followed? Is there a diversity of plants to support beneficial insects? Thus, studying the
problem and trying to determine why it occurred should help prevent similar problems in the
future. This will, of course, take time to learn and develop. Unless growers refuse to use any
pesticides, they may at times choose to apply some organic pesticides to save a specific crop.
An integrated pest management (IPM) approach is well-suited for organic production. IPM is a
system in which insects, diseases, and weeds are closely monitored, and different methods are
used to keep pest populations at levels that are not economically damaging with minimal adverse
environmental effects. IPM encompasses use of cultural and biological control methods, use of
resistant varieties, and judicious use of pesticides. When pesticides must be used, an effort
is made to select ones with low toxicity, non-persistent residues, narrow spectrum of
control, and low environmental impact. There is not enough room here to cover IPM thoroughly.
Several good books are available and should be consulted, such as Vegetable Insect Management
(1995).
There is no guarantee that, once an organic system is established, there will never be a disease,
weed, or insect problem. Stressful conditions that a grower cannot control will occur, such
as weeks of endless rains, droughts, periods of extremely high temperatures, hurricanes, plagues
of grasshoppers, or hail. Likewise, if an airborne disease invades your area, your plants will
probably be infected. However, with careful observation and preparation, an
organic system should progressively have fewer pest problems as years go by.
Disease Management
Although there are several corn diseases of concern in the Southeast, there are many
sweet-corn cultivars available with resistance to the major diseases. Whenever possible, a grower
should select marketable cultivars with disease resistance to fit specific needs and conditions.
Corn smut, which appears particularly on white cultivars, is characterized by large, fleshy,
grey-black galls on the stalks, tassels, or ears. It is important to remove and destroy the first galls
before they open. To control smut, avoid injuring plantings and avoid areas where smut occurred
before.
Root rot frequently occurs on seedlings planted in cool, moist soil. Planting should be delayed
until soil temperatures are adequate to allow for rapid germination.
Stewart's bacterial wilt is sometimes a problem in the mid-South. Early-infected plants wilt and
die. Later-infected plants are stunted and contain yellow streaks on the leaves. The disease is
spread by the corn flea beetle, especially after mild winters. New cultivars resistant to Stewart's
wilt should be used if early sweet corn is to be planted where flea beetle populations were high
the previous year.
Rust and leaf blights can be a problem in extended periods of warm, moist weather or areas of
heavy dew. Rust blown from field corn planted upwind nearby can threaten sweet-corn crops.
Cultivars resistant to rust are recommended in threatening conditions.
Insect Management
Insects which attack sweet corn during its early growth include southern corn rootworm,
cutworm, white grub, wireworm, and flea beetle.
Wireworms and white grubs are often abundant in soils previously planted in alfalfa or sod. After
the soil has been worked, large numbers of birds feeding in the field indicate a high pest problem.
To avoid pest damage to the crop, a farmer can delay planting to let exposed worms and grubs
starve or be eaten by birds. Another strategy is to till the field and plant it with another legume
cover crop, such as crimson clover or Austrian winter peas, for a season before planting a crop
susceptible to wireworm or grub damage.
Corn earworms are a major pest on corn. The night-flying, light brown- or buff-colored moth lays
eggs on the corn silks. In North Carolina, the moth can emerge as early as late March. These tiny,
dome-shaped eggs hatch in 1 to 2 days in warm weather or 10 days in cool weather. The small
caterpillars move down the silk into the ear, where they feed on the tip. After 12 to 13 days, the
caterpillars leave the ears by boring out the side or crawling out the tip. They burrow 3 to 5
inches into the soil to pupate. After about 12 more days, they can emerge to start the cycle again.
There can be at least three generations per year, and the pupal stage can also overwinter.
Once earworm caterpillars have worked their way inside the ear, they cannot be controlled. Early
plantings often are not affected by earworms, but late plantings can be under serious pressure.
Low infestations are often handled by simply removing the damaged ear tip of the corn after
harvest. Some growers provide a free "de-silking" of the ears at sale, so the consumer
never sees the worms.
Earworm adult moths should be monitored by pheromone traps placed near the corn field. In
Oklahoma, Kuepper and colleagues (1991) used oils and oil-pesticide blends to control corn
earworm on small acreages. They injected oil into the neck of the ear (where the silk emerges
from the husk), with a standard oiling can. To be effective, oil must be applied 2 to 3 days past
the full-brush stage (when silks are at maximum extension from the eartip). Addition of Bt or
pyrethrum is more effective than the oils alone. Be aware that using Bt with oil is difficult, time
consuming, and not always effective. Estimates obtained for costs of these methods, however, are
competitive with the conventional pesticide methods.
Fall armyworms can also be a major pest in North Carolina. They overwinter further south and
migrate north each year. The female moths are about 1.5 inches across the wings with
grayish-white hind wings and dark gray forewings with splotches and white spots near the
tips. They deposit clumps of up to 150 pinkish-white eggs on the leaves. In warm weather, these
eggs hatch in 3 to 4 days. In 2 to 3 weeks, the armyworms are about 1.5 inches long, light tan or
green to almost black with yellow and dark stripes running the length of the body. Their
distinctive characteristic is an inverted Y on the front of the head. Fall armyworms will feed on
just about any plant, but damage is especially severe in late sweet corn and field corn. They will
eat all above-ground parts of the plant and are very messy eaters. The best defense is to plant
early so that corn matures in mid-August before armyworms peak in the fall.
Pheromone traps have not been very reliable, so blacklight traps are recommended for
monitoring armyworms. Traps should be checked daily and both male and female moths counted.
Less than 10 moths per night do not indicate an immediate problem. More than 10 moths per
night for 3 consecutive nights indicate that moths are laying eggs.
Mature European corn borer larvae overwinter in stalks and ears left in the field. The larvae are
.75 to 1 inch long, cream colored with small, round, brown spots on the back. Early in the spring
the larvae pupate, and in June adults emerge and lay eggs. The adult female moth is pale
yellowish-brown, with wavy bands running across the wings. The males are darker, with olive
brown markings on the wings. Both have a wing span of about one inch. The females lay white
eggs in masses of five to fifty, usually on leaf undersides. In less than a week, the eggs hatch and
the young larvae feed on, but not through, the leaf surface. They move into the whorl and within
2 to 5 days enter the midribs of leaves and continue boring into the stalk or ear. The stalks
become weakened and may fall over. Nutrient movement in the corn plant is also disrupted.
European corn borers usually have two generations per year.
Corn is vulnerable to corn borers when tassels, silk, and pollen are present; preventative action is
required or damage will occur. The size of any year's European corn borer population is greatly
dependent upon the synchrony of the pest population and the development stage of field corn in
the region. If field corn is planted early and provides the first generation moths with plenty of
corn to lay their eggs on, large populations will probably be available to attack sweet corn later.
Field corn planted nearby is the source of most European corn borer problems for sweet corn.
In most cases, a grower should plow under corn debris at the end of the season to help destroy
overwintering stages of some pests. Tilling, however, will also destroy many beneficial insects.
Growers in North Carolina are experimenting with beneficial insects, although to date there are
few true success stories. Trichogramma wasps may provide some control of European corn
borers, beneficial nematodes sprayed on the corn plant and silk may reduce earworm damage,
and ladybugs will help control aphid populations.
Bird Management
Young corn seedlings are vulnerable to bird damage. A large flock of crows can quickly
devastate a new corn planting. There are several methods to reduce bird problems. The most
common, inexpensive, and easy methods are designed to frighten birds away. A method that is
sometimes effective in western North Carolina is a bird-scare balloon with shiny, mylar
"eyes," or inflatable owls. These should be suspended about 5 ft above the ground
and moved every few days. Reflective bird tape that is silver on one side and red on the other can
be twisted and suspended about 6 inches above the newly seeded rows. A field full of this shiny
tape will disorient bird and grower alike. Noise making cannons are used by some large
producers, but will not make you popular if you have neighbors close by.
Irrigation and General Cultural Practices
Sweet corn needs a continuous supply of moisture to ensure pollination and growth of kernels in
the ear. After the tassels are produced, sweet corn requires 1 to 1.5 inches of water each week.
Overhead irrigation is the most common method. Small-scale growers may use soaker hoses laid
on the soil surface and covered with an organic mulch. Whatever method is used, a grower
should never allow the soil to dry out. It takes consistent, adequate moisture from silking through
kernel fill to ensure high yields of high quality ears.
Common recommendations are that suckers should not be removed from the base of the corn
plant. It has been reported in many vegetable production publications that their removal does not
increase yield or ear size and that removal may actually reduce yield and consume valuable time.
Reports from both Clemson University (2002) and Colorado State University (2004) support this
recommendation. Many growers who feel that sucker removal is beneficial still continue this
practice.
Harvest and Post-harvest Handling
Each sweet-corn plant will produce at least one large ear that should be harvested at prime
maturity, when the silks are dry and brown and the ear has
enlarged to the point that the husks are tight. This stage is usually 17 to 18 days after silking
under warm day and night conditions, or 22 to 24 days after silking during cool weather
conditions. At harvest, kernels should be plump and exude a milky liquid when punctured
(except for most supersweets with the sh2 gene where the liquid is clear). This stage only lasts 4
to 5 days, so the corn must be checked and harvested frequently. Sweet corn is removed from the
plant by simultaneously snapping and twisting the ear away from the stalk.
To retain peak quality, sweet corn should be picked in the early morning and cooled immediately
to 32°F preferably, or at least 40°F. The longer the delay
between harvest and cooling, the more conversion of sugar to starch and subsequent loss of
quality. Prompt cooling is critical for standard varieties, but also helps maintain quality in
supersweets and sugary-enhanced varieties. Hydrocooling involves immersing the corn in cold
water, or less commonly, drenching with ice-cold water. Crushed ice can be
added to the crate or box. Corn must be stored close to 32°F, with high
relative humidity, and transported with ice in a refrigerated truck.
Mesh cabbage bags are frequently used to package corn for local sales. For marketing to
wholesalers and supermarket chains, corn must be properly cooled and packaged. For sales to
chain stores, corn is usually packaged in wirebound wooden crates or waxed cartons which hold
4 to 6 dozen ears.
Yields, Marketing, and Economics of Production
With a final plant population of 12,400 plants per acre and an average yield of
one ear per plant, total yield will be 248 crates per acre. In western North Carolina in 2004, direct
sales of organic sweet corn brought around $0.50 per ear. This translates to a gross return of
$6,200 per acre. In western North Carolina, where most organic farm acreages are small, this is
low compared to gross returns for many other organic vegetable crops. However, many growers
who sell directly to the consumer at farmers' markets, tailgate markets, roadside stands, or have a
CSA (Community Supported Agriculture) subscription farm, report that their customers expect
sweet corn and that sweet corn is necessary for sales of other produce. In contrast, in the
Piedmont of North Carolina, growers with larger acreages report that sweet corn is a profitable
part of the vegetable mix that they grow.
Near urban areas, fresh, organic sweet corn is usually sold directly to the consumer at tailgate and
farmers markets. Wholesalers of organic produce, however, report a need for more locally grown
organic sweet corn.
I gratefully acknowledge the assistance of William McCarthy with information gathering, Susan Schmidt with initial preparation of the manuscript, and Agatha Kaplan for the latest
revision.
References
|
|
Published by the North Carolina Cooperative Extension Service |
