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Introduction

Sweet corn (Zea mays convar. saccharata var. rugosa) is a warm-season crop and one of the major vegetables grown in New England.  It is an extremely popular crop for roadside stand and farmers' market sales as well as for wholesale markets.

Sweet  corn grows on a wide range of soil types.  Early planting should be on light, well-drained soil in a warm, sheltered location.  Heavier soils are best for the main crop.  Plowing under corn stalks and cover crops to maintain high levels of organic matter in soils is recommended for best crop production.  Irrigation may only be necessary in dry weather, particularly on light soils, and especially as silking starts.

Types and Varieties

Sweet corn varieties are categorized by their genotype.  The most common types are normal/sugary (su), sugar enhanced (se), and supersweet/shrunken (sh2).  Other newer varieties include sweet breeds, synergistic, sweet genes, augmented, or improved supersweets which have various combinations of these genes.

su           "Normal" or "Sugary"

This is the original standard hybrid sweet corn, which is a mutant type of corn that differs from field or dent corn by a mutation at the sugary (su) locus. This type accumulates about twice as much sugar than field corn. Because of its short shelf life compared to newer genotypes, it is no longer widely grown commercially, and very few varieties are still available through seed companies.

se          Heterozygous Sugary Enhanced or "Modified Sugar" or "Everlasting Heritage (EH)"

se+        Homozygous Sugary Enhanced  

There are two distinct groups within the cultivars containing the "se" gene, which increases sugar levels above those found in "su" varieties.  The "homozygous se" or se+ cultivars have higher sugar levels in 100% of their kernels.  Cultivars that are "heterozygous se" have higher sugar levels in 25% of their kernels; the other 75% contain the normal "su" gene with lower sugar levels.  Thus, "se+" cultivars are usually sweeter than "se" cultivars.

  • Sugar conversion to starch (shelf life) is about the same rate as standard (su) sweet corn.  Refrigeration is essential to maintain quality.
  • Isolation not required, but desirable. Note directions on seed tag label.
  • Kernels tender and creamy.
  • Tip cover weak on some varieties

sh2           "Super Sweet" or "Extra Sweet" 

  • Sugar content twice as high as standard sweet corn.
  • Slow sugar conversion to starch so quality will hold 7-10 days. Refrigeration suggested to extend quality.
  • Isolation is required from all other types of corn. If pollinated by other genotypes, the kernels will be starchy, not sweet. Note directions on label.
  • Kernels can be somewhat tough or crunchy due to a thicker outer layer (pericarp).
  • Does not germinate well in cold, wet soil.  Plant after soil temperatures exceed 65° F and be sure there is adequate soil moisture.  Adjust planting depth for soil conditions and soil type.  Seed of the "sh2" type is light-weight, wrinkled, and fragile. It is easily broken by some mechanical planters and should be handled gently to prevent damage.

syn          "Synergistic"

These include synergistic or improved supersweets such as Sweet Breed and Sweet Genes.  These varieties have combinations of su, se, se+, or sh2 genes.  Synergistic varieties having the sh2 gene may be grown with other "Super Sweet" varieties, but must be isolated from "se" and "su" types to achieve best quality.

aug          "Augumented Shrunkens"

Augmented Shrunken sweet corns contain the "sh2" gene, but kernels of the augmented shrunken type are more tender, like "se" varieties.  Augmented shrunken varieties should be hand harvested because their tender kernels will likely be damaged by machine picking. These varieties need to be isolated from normal, "se" and synergistic hybrids. Additionally, they have the same seed vigor problems as supersweet varieties and should only be planted under optimal conditions.

MiraiTM    

MiraiTM sweet corn varieties carry two copies of all three sweet corn genes: su, se and sh2. They have all tender supersweet kernels and, like the augmented shrunken varieties, need to be hand-harvested. Mirai varieties also need optimal soil conditions for seedling establishment. Mirai types do not require isolation to avoid starchy kernels, but isolation from other sweet corn types will assure best quality.

Bt Hybrids (AttributeTM, PerformanceTM, etc.)

These are genetically-modified (GM) varieties that protect plants from corn rootworm, European corn borer, and to a lesser extent, fall armyworm and corn earworm caterpillars because they express the protein exotoxin of the bacteria Bacillus thuringiensis. They are currently available as super sweet (sh2) or as synergistic "TripleSweet" varieties, composed of 75% "se" and 25% "sh2" kernels. In the synergistic varieties, the Bt toxin is expressed in the "se" kernels, the stalks, leaves, tassels and fresh silk. Bt varieties are still susceptible to attack from flea beetles, cutworms, aphids, sap beetles, high levels of corn earworm and soil-dwelling insects. Bt seed costs more than the seed of other sweet corn varieties and currently comes with a list of resistance management strategies with which to comply. These varieties may reduce the time, energy and expense associated with insect control and may simplify sweet corn pest management. There are many other less obvious potential risks and benefits that may be associated with the use of this technology. GM sweet corn varieties, including all Bt varieties, are not allowed in organic production. Growers in Maine must be certified to grow Bt corn. Contact the Maine Board of Pesticides Control for more information. 

Sweet Corn Varieties
Variety Type Color Wilt Rust MDMV
Sweet Chorus (67) syn Bi L M M
Sweetness (68) syn Bi - - -
Latte (68) syn Bi M M -
Vitality (68) se Bi - - -
Espresso (70) syn Bi M M -
Mirai 308BC (70) mirai Bi - - -
Harris 1001 (72) se Bi M - -
Temptation (72) se+ Bi - - -
Reflection (72) syn Bi - M -
Cuppa Joe (73) syn Bi M M -
XTH 2170 (73) sh2 Bi M - -
Mirai 336BC (73) mirai Bi - M M
Sweet Rhythym (73) syn Bi M L -
Anthem XR (73) sh2 Bi L H -
Charisma (74) se Bi M M -
Fanstastic (74) sh2 Bi - - -
XTH 274A (74) sh2 Bi M - -
Sweet Ice (74) syn W M L L
Kristine (74) syn Bi M M -
Mystique (75) se+ Bi H M L
SC1102 (75) syn Bi - - -
Sparkler (75) syn Bi - - -
Mirai 301BC (76) sh2 Bi M M -
Awesome (76) sh2 Bi M - -
Luscious (77) se Bi - - -
Accord (78) se Bi - - -
Legion (78) sh2 Bi M M M
Pow Pow (78) syn Bi - - -
Essence (78) syn Bi M M -
Providence (79) syn Bi - M -
Montauk (79) syn Bi M - -
Delectable (80) se+ Bi M H L
Precious Gem (80) se+ Bi M M L
Primus (81) sh2 Bi M M -
Obsession II (82) * sh2 Bi M M -
BC 805 (82) * se Bi - H -
Jackpot (82) se+ Bi M M -
Silver King (82) se+ W M M -
Argent (83) se W H M L
Incredible (85) se+ Y H - -
Cameo (84) se Bi M M -
Silver Queen (94) su W H L L

The number in parentheses is the approximate number of days to maturity from seeding.

H, M, L indicates high, medium or low resistance or tolerance to the following diseases - Wilt: Stewart's wilt, Rust: Common rust, MDMV: Maize dwarf mosaic virus.

Other codes: mirai: mirai, su: normal sugary, se: sugar enhanced, se+: sugar enhanced homozygous, sh2: supersweet/shrunken, syn: has some combination of su, se and sh2 genes, Y: yellow kernels, W: white kernels, Bi: bicolor, *: Bt hybrid

Soil Fertility

Apply lime according to soil test to maintain soil pH between 6.5 and 6.8.

Less nitrogen fertilizer will be needed if legume sod was plowed down or if manure was applied (see Table 1 and Table 7). Banding starter fertilizer (usually 1-3-1 or 1-3-3) 2" to the side and 2" below the seed with 30-80 lbs N/A depending on field history (cover crop, manure, etc.). Higher amounts of nitrogen and potash applied as a concentrated band may damage corn seed and young plants. Side dress at late whorl stage with 50-80 lbs N/A (see table below). If broadcasting and incorporating fertilizer preplant, apply 0-50 lbs N/A, depending on the N status of the soil.

Plant Nutrient Recommendation According to Soil Test Results for Sweet Corn
Soil Test Results Nitrogen (N) Lbs per acre

   Phosphorus (P) Lbs P2O5 per acre

   Potassium (K) Lbs K2O per acre

Very Low Low  Optimum Above Optimum Very Low Low  Optimum Above Optimum
Broadcast and Incorporate 0 100 40 0 0 150 90 0 0
Band Placement at Planting 40 40 40 40 0-40* 30 30 0-30 0
Sidedress 60-90** 0 0 0 0 0 0 0 0
TOTAL RECOMMENDED 100-130 140 80 40 0-40* 180 120 0-30 0

*Phosphorus applications are recommended only for early-season corn sown in cold soils.
**Before sidedressing use a nitrate test to determine the need for additional N

Planting

Effective isolation is required in some cases with the present types of sweet corn on the market. Isolation is affected by distance, wind direction, time of pollen shed and silking, and blocking. From a practical standpoint, the same methods are needed as when separating white and yellow varieties. Field experience indicates isolation can be managed more easily than initially expected.

Plant early varieties 8-10" apart within rows with 30-36" between rows. Plant main season varieties 10"-12" apart within rows with 36" between rows. This requires 10-15 lb of seed/A (1-2 oz per 100 feet of row) or 17,500-26,000 seeds/A for early and 14,500-17,500 for main season varieties. Match seed size to seed plate. Read seed container or contact the company regarding appropriate plate sizes. 

First plantings are made in early April in southern New England and in May in northern New England.  Growers planting in cold soils run the risk of erratic germination and poor stands.  Avoid planting while soil temperature is lower than 55°F.  If soil temperature is below 60°F, it is advisable to plant treated seed.  If you are planting untreated seed, wait until the soil temperature is at least 65°F.  Most seed companies perform cold germination tests of their seed.  If in doubt about the suitability of a corn variety for early planting, ask your seed dealer about the low temperature germination and vigor of the seed. 

Clear plastic mulch raises soil temperature and can be used for the earliest plantings.  This can allow for earlier planting and provide corn 7-10 days earlier than non-mulched corn.   Apply fertilizer and herbicides, and plant seed before laying the plastic over seeded rows.  Two rows 18"-24" apart are usually planted under each strip of 5 ft-wide plastic.  To avoid plant injury, do not let temperatures beneath the plastic get too high (90°F or more).  When such conditions occur, or when the plants reach 4" in height, cut slits in the plastic to allow heat to escape and plants to grow through.  Remove the plastic completely by the time the corn is knee high to facilitate removal and permit cultivation.

Spunbonded row covers offer a potential three-way benefit for early sweet corn production.  Maturity is increased by 5-10 days, yields are generally increased by 15-20%, and the first generation corn borer can be controlled if the covers are left on until after the peak of the first generation corn borer flight.  They can also be used to pre-warm the soil before planting.  They can be removed to allow planting and then replaced.  Weight cover edges with soil or sand bags to prevent damage by gusty winds, but leave adequate slack for plant growth to the tassel stage.

Transplanting Sweet Corn

Transplanting sweet corn offers some advantages to growers over direct seeding, including better stands, earlier harvest, and less dependence on pre-emergent herbicides.  However, transplanting is more labor intensive, costly and requires attention to detail in order to be successful.

Corn can be seeded by hand, or a simple, inexpensive drop seeder can be built to fit the size of tray to be used.  98-128 cell trays work best (a 128 cell tray requires 82 trays per acre).  The smaller plug trays require less media but are more easily root-bound. Fill trays with a peat-based potting mix, and plant one or two seeds per cell.  Place trays on tables or benches to prevent plants from rooting in the ground.  Greenhouse temperatures should be set for 65°F days and 60°F night temperatures. Transplants should be ready in about 14-18 days and should be hardened off before planting in the field, by placing the trays outdoors and limiting water for a few days.  Fertilizer should be banded prior to planting, or applied as a liquid at planting.  The plants should have good, cohesive roots that come out of the tray with relative ease. Do not allow transplants to become pot-bound; the roots should kist be touching the sides of the cell at transplanting. Plant seedlings once the soil temperature reaches 60-65ºF. They may be planted by hand or machine.  Plants can be spaced 12-16 inches apart within rows, with about 3 feet between rows.  Floating row covers should be placed over the corn for the first two to three weeks after planting.  These can be pulled back to allow cultivation.  Transplanting and row covers should bring corn to maturity about two weeks earlier than direct seeding.  Not all varieties perform well in this system.  Test your favorite early-mid maturity varieties in small trials before committing large amounts of trays and greenhouse space to them. 

Harvesting and Storage

The sweetness and tenderness of sweet corn will deteriorate rapidly after harvest. Sweet corn should be cooled immediately after harvest and kept at 32-34ºF and 90-95% humidity to retain optimum freshness. The crop is best harvested early in the morning when there is less field heat. Harvesting at the proper stage (milk stage) is critical in maintaining quality. Under warm summer growing conditions, mature sweet corn will only remain at the optimum harvest stage for 1-2 days. It will approach maturity 16-22 days after silking and should be picked daily. As the kernel passes the prime harvest time, sugars convert to starch and the pericarp becomes tough. Supersweet varieties retain their sweetness longer than su and se varieties; extra tender varieties maintain eating quality even longer.