Corn, Sweet

Introduction

Sweet corn (Zea mays convar. saccharata var. rugosa) is a warm-season crop and one of the most important 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-season crop. Plowing under corn stalks and cover crops to maintain high levels of organic matter in soils is recommended for best crop production. In much of New England, irrigation is needed regularly in dry weather and on light soils, especially as silking starts.

Types

Sweet corn varieties are categorized by their genotype. The most common types are sugary/normal (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 the su, se, and sh2 genes. In some cases, pollination of some varieties by certain other varieties will result in the formation of starchy kernels in the ear. Thus, certain varieties must be isolated from others; these isolation requirements are listed below. The shelf life of sweet corn is largely determined by the rate that sugars in the kernels are converted to starches. The slower this conversion, the longer the shelf life of the variety. Refrigeration will slow the conversion of sugars to starches and is essential for some sweet corn types. Some sweet corn types have a gene that slows down the sugar-to-starch conversion, making refrigeration less essential.

su (Sugary, "Normal")

This is the original standard hybrid sweet corn, which differs from field or dent corn by a mutation at the sugary (su) locus. This type accumulates about twice as much sugar as 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.

• Kernels are sweet but the conversion of sugar to starch is rapid and ears must be harvested and eaten within a narrow window of time. Refrigeration is essential to maintain quality.
• Isolation from field corn, popcorn, and supersweets is essential.

se (Heterozygous Sugary Enhanced) and se+ (Homozygous Sugary Enhanced)

Se varieties contain the sugary enhanced gene that increases sugar levels above those found in su varieties. When one parent carries the se gene the resulting variety will be heterozygous, and 25% of the kernels will be extra sweet and 25% will be “normal” su kernels. When both parents carry the se gene the resulting variety will be homozygous and 100% of the kernels will be sweeter.

• Kernels are sweeter than su types but the sugar-to-starch conversion rate is similarly rapid. The harvest window is a bit longer than for su types due to the higher sugar content. Refrigeration is essential to maintain quality.
• Isolation: Heterozygous se varieties can be planted with su varieties but must be isolated from all other corn. Homozygous se+ varieties must be isolated from su varieties as well as all other corn.

sh2 (Shrunken, also known as "Supersweets")

• Kernels are extra sweet and tender.
• Sugar conversion to starch is slow, extending field holding and eating quality. Refrigeration is suggested to extend quality.
• Isolation from all other types of corn is required.
• sh2 seed is light-weight, wrinkled, and fragile. It is easily broken by some mechanical planters and should be handled gently to prevent damage. It does not germinate well in cold, wet soil. Plant after soil temperatures exceed 60°F and be sure there is adequate soil moisture. Adjust planting depth for soil conditions and soil type.

syn ("Synergistic")

These are se or se+ varieties that have some sh2 kernels in the ear. For example, “Sweet Breed" varieties have about 50% su kernels, 25% se kernels, and 25% sh2 kernels. These carry some of the vigor of the older su types with improved sweetness. “Triplesweet” varieties have 100% se kernels and 25% also have the sh2 gene, making them as vigorous as an se type but sweeter. 

• Isolation is required from sh2 and aug types but can be grown with se and su types.

aug ("Augumented Shrunkens")

Augmented shrunken sweet corns contain both the sh2 and se genes. Augmented shrunken varieties should be hand-harvested because their tender kernels will likely be damaged by machine picking. 

• Isolation: Required from su, se, and syn. Can be grown with sh2.
• aug varieties have the same seed vigor issues as sh2 varieties and should only be planted under optimal conditions.

Biotech Varieties (Attribute^TM, Performance^TM, etc.)

These varieties are genetically engineered (GE) to protect plants from insect pests and/or allow for use with systemic herbicides. Insect pests are controlled via expression of a protein produced by an exotoxin of the bacteria Bacillus thuringiensis, which is toxic to caterpillars. This "Bt" toxin is expressed in the corn kernels, stalks, leaves, tassels, and fresh silk; thus, control of European corn borer, corn earworm, fall armyworm, and corn rootworm can be achieved with little or no insecticide sprays. Other insect pests such as aphids and sap beetles still need to be controlled in this "Bt corn". There are five different versions of the Bt protein that may be expressed through different genetic transformation events—four versions of the Cry protein and one Vip3a protein. These may be found on their own or in combinations, and may also be stacked with mutations conferring tolerance to the herbicides glyphosate or glufosinate, depending on the variety. Region-wide studies have demonstrated that corn earworm has developed resistance to all of the Cry proteins, so for reliable control of corn earworm without the need for additional insecticide sprays, especially during the late-season when pressure is high, growers should select varieties that contain the Vip3a protein (e.g. Attribute II and Attribute Plus series). For the most recently updated table describing GE corn varieties available in the US see: https://www.texasinsects.org/bt-corn-trait-table.html. 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. Genetically engineered varieties are not allowed in organic production. Growers in Maine must be certified to grow GE corn; contact the Maine Board of Pesticide Control for more information. 

Sweet Corn Varieties

Soil Fertility

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

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

Plant Nutrient Recommendation According to Soil Test Results for Sweet Corn

Planting

Effective isolation is often required between plantings to prevent cross-pollination that can affect kernel sweetness (see below). Isolation can be achieved with distance (500 ft minimum), wind direction, time of pollen shed and silking, and blocking. The same methods are needed when separating white and yellow varieties. 

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. Closer spacing may reduce ear size and increase the number of earless ("blind") stalks. 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 the appropriate seed plate on the planter. Read seed packages 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 earlier plantings, often producing 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 5-ft-wide strip of plastic. To avoid plant injury, do not let temperatures beneath the plastic exceed 90°F. 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 placed over early corn plantings offer three potential benefits: maturity is increased by 5–10 days, yields are generally increased by 15%–20%, and the first generation European corn borer can be controlled if the covers are left on until after peak 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. Covers should be removed just prior to tasseling but should be removed earlier if plant growth is constrained by the tension of the covers.

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 and costly and requires attention to detail in order to be successful. Transplants usually require irrigation after planting in the field.

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 1 or 2 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 nights. Transplants should be ready in about 14–18 days and should be hardened-off by placing the trays outdoors and limiting water for a few days. 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 just be touching the sides of the cell at transplanting. Plant seedlings once the soil temperature reaches 60ºF–65ºF, by hand or mechanical transplanter. Fertilizer should be banded prior to planting, or applied as a liquid at planting. 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 2–3 weeks after planting. These can be pulled back to allow cultivation. Transplanting and row covers should bring corn to maturity about 2 weeks earlier than direct-seeding. Not all varieties perform well in this system. Test your favorite early to 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 32ºF–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.