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Sweet corn is a summertime tradition, and there is a plethora of sweet corn types available to gardeners. Besides standard or open-pollinated types, there are normal sugary (su), sugary enhanced (se), shrunken (sh2), and synergistic (75% se and 25% sh2), types. Normal sugary types have moderate amounts of sugar (8-18%) that convert to starch rapidly after harvest. Sugary enhanced types have slightly increased sugar levels (30-35%) and tenderness, and slower conversion of sugars to starch after harvest. No isolation is necessary from su types. Shrunken types, also known as “super sweet” have 2-3 times the amount of sugars (40-50%) of standard corn varieties, and crispy textures. Seeds are smaller and lighter in weight, and look shrunken when dry. Synergistic types have the tenderness of sugary enhanced types with enhanced sugar content. Sugar content in synergistic types take longer to accumulate than in sugary enhanced types, and kernels can be watery if picked too early. Isolation from shrunken types is necessary to avoid tough, starchy kernels in both types.
Sweet corn also comes in white, yellow, and bicolor kernel types. White types should be isolated from yellow and bicolor types when planted.
Sweet corn is direct seeded. Varieties differ greatly in vigor, especially in cool, wet, or compacted soils. To isolate different types from each other, plant at different distances or times. It is easier to isolate by distance, because isolating by time depends not on the calendar, but on growth stage and varietal maturity characteristics. To isolate by distance, plant different genetic types at least 250 feet from other types or white varieties 500 feet or more from other colored varieties, and plant the variety most affected upwind of the other types. However, many gardeners don’t have the space in their garden to effectively isolate by distance. To isolate by time, plant at approximately 12-14 day intervals (depending on the relative maturities of varieties) to avoid cross pollination.
For su and se types, planting can start as soon as soil reaches 50°F. Seeds germinate best when soil temperatures are 55-60°F. Plant seed 1 inch deep, and 8-12 inches apart, with rows 30-36 inches apart. Plant in blocks of at least 4 rows rather than a long single rows for proper pollination. Shrunken types need to absorb twice the moisture of normal types to germinate.
Sweet corn grows best in soil that is well drained and well supplied with organic matter, with a pH of 5.8 - 7.0. Nearly all garden soils benefit from the incorporation of well-rotted manure or compost before planting. Addition of manure or compost can add micronutrients and organic matter to soil (see the University of Minnesota Extension publication Composting and Mulching). A garden fertilizer can be applied at planting and once or twice during the growing season.
Continuous use of high phosphorus fertilizer such as 10-10-10 or 15-30-15, or high rates of manure or manure compost results in phosphorus buildup in the soil. Although phosphate fertilizer applied to soil is bound tightly and resistant to movement in the soil, some runoff may occur. It can then become a major pollution concern in our lakes, rivers and streams. High levels of phosphorus support over-production of algae, which causes significant reduction in water quality (see Preventing Pollution Problems from Lawn and Garden Fertilizers). If your soil tests high in phosphorus, use a low phosphorus (such as 32-3-10, 27-3-3, or 25-3-12) or no phosphorus (such as 30-0-10 or 24-0-15) fertilizer.
Proper watering will enhance good production. Sweet corn has a relatively shallow rooting depth. Soak the soil thoroughly when watering, to a depth of at least one inch each week during the growing season. There is little or no value in light watering that only wets the soil surface. Sandy soils may require more frequent watering.
Frequent, shallow cultivation will kill weeds before they become a problem. Do not cultivate too deeply. Cultivate just deeply enough to cut the weeds off below the surface of the soil. Be careful not to damage the plants when cultivating.
Harvest ears when kernels in the center of the ear are full and “milky” when squeezed (thumb nail test). Silks will generally be drying and browning at this stage, usually 18-24 days after silk first become apparent or when half the plants show silk, although this timing will vary based on weather conditions.
For assistance in diagnosing unknown problems visit the University of Minnesota Extension diagnostic site 'What’s wrong with my plant?'
Two insect pests you may encounter are corn earworm and European corn borer. You should base your need to manage these pests on their activity in your garden. If you find they are damaging your corn more years than not, anticipate that they will be a problem. If you generally do not see them attacking your corn, you can assume they will not be a problem in a given year.
You can generally avoid problems from these insects by planting corn early (roughly early May) and making sure the maturity of the hybrid is less than 80 days. If you harvest prior to August 1, there is little risk of infestation from corn earworm in most of Minnesota and no control is needed as corn earworm moths have not migrated into the state yet (extreme southern Minnesota may be a little earlier). Likewise for European corn borer, if you harvest in late July or early August, that puts the susceptible silk stage between the two generations of European corn borer and typically there is little to no risk of infestation.
If you have an ongoing problem with these insects, apply control measures just as sweet corn begins to silk. Treat the foliage and silk with a labeled insecticide, such as products that contain Bacillus thurngiensis, permethrin, or carbaryl. Make applications every 4-7 days (shorter interval with hotter temps) to make sure all silk is treated as it emerges. Silk can emerge 1-2 inches per day under August temps. Roughly 7-10 days from the beginning of silking on the first plant, all silk emergence and elongation should be complete. Once either pest enters the ear there are few control options except to pull the husk back on the ear tip and hand pick the larvae.
Common diseases that affect sweet corn include common smut and leaf rust. Smut causes unusual firm tumor like growths on leaves, stems, ears and tassels. These start out light green but become purplish gray with age and eventually rupture and release large quantities of black powdery fungal spores. Look for smut galls throughout the season and cut them out before they produce spores. Remove these galls from the garden and do not compost them. Leaf rust appears as rusty orange streaks on leaves that release an abundance of powdery orange spores. Rust resistant varieties are available and are the best form of control. In addition keep leaves as dry as possible by avoiding sprinkler irrigation.
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