Sweet Corn Garden Guide

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Sweet Corn Care At A Glance

  • Sweet corn is a popular warm season, heavy-feeding summer annual crop.
  • Sweet corn seeds require warm soil to germinate well.
  • The type of corn will dictate what soil temp. seeds need for optimal germination.
  • Sweet corn is wind-pollinated; therefore, different varieties must be isolated from one another to avoid disappointing quality due to unwanted cross-pollination.
  • Corn requires well-drained, nitrogen-rich soil with consistent moisture for the best results.

Sweet Corn Types

  • Su = Sugary or Normal sweet corn varieties. Think of heirloom, traditional sweet corn flavor. These must be isolated from Sh2, and Synergistic types. In Su kernels, desirable sugars convert to starch quickly after peak ripening, which means ears should go right from the field to the pot or grill with little to no delay.
  • Se or Se+ = Sugary-enhanced varieties have genes that enhance sweetness as well as tenderness. These must be isolated from Sh2 varieties. Se and Se+ types have a slightly longer harvest period than Su varieties (about 7-10 days) but will still offer the best quality if consumed shortly after harvest.
  • Sh2 = Shrunken or Supersweets. Super sweet genes provide higher sweetness in kernels that are crisp and juicy. Their enhanced genetics also offer good holding capacity for longer storage or shipping. Market growers favor Sh2 types to get them picked and to market without sacrificing quality. Improper cross-pollination from other sweet or field corn varieties can ruin crop quality.
  • Se/Sh2 = Synergistics. These varieties have beneficial qualities from both sugary enhanced and supersweet types. These offer natural corn flavor with enhanced sweetness along with a tender texture and good holding capacity like Sh2 types. Isolate from Su (Sugary) and Sh2 (Supersweet) for best quality.
  • Quad Sweets. These are the latest generation of sweet corn hybrids that offer heirloom flavor with all the modern sweetness. Quad sweet corn must be isolated by time or distance from any Sh2 varieties.

Isolation Guidelines

  • For home gardens separate Sh2 varieties from others by at least 25-40 feet.
  • For larger acreage plantings 250+ feet of separation are recommended.
  • Fields of more than 20 acres, 660 feet distance should be used. If the isolation distance is less than this, the removal of 8-12 border rows may be necessary before harvesting.
  • For isolation by time, plant incompatible varieties so they mature at least 14 days apart.

Days To Maturity

  • Most sweet corn varieties mature in about 60-90 days.
  • The average number of days to maturity is calculated from seedling emergence, not from planting.
  • To estimate time to maturity, mark the calendar at 85-90% field emergence.
  • From that date, calculate the average number of days which will give you an estimated day of maturity.
  • But then monitor closely for silk development, which will provide even closer guidance to maturity. (See Harvest & Storage below)
  • Early maturing varieties do best in short, cool growing seasons, and tend to produce smaller ears with less sugar.
  • Later maturing varieties perform best in longer and warmer growing seasons.
  • Because seeds require warm soils for germination, waiting until later in the spring to plant usually yields better results than rushing to plant too early.
  • Weather conditions, which growers have the least amount of control over, have the greatest effect on crop yields, maturity, and quality. Plant stress from drought, cold, heavy rainfall, hail, or windy conditions can all wreak havoc on sweet corn crop development.

Planting Instructions

  • Soil temperature is critical for sweet corn seed germination. If the soil temperature is too low, poor or inconsistent germination will occur.
  • Optimal soil temperatures for different genotypes.
    • For Su, Se, Se+, Synergistics, or Quad Sweets soil temp. should be 60°F
    • Seeds pre-treated with fungicide coating may still germ with min. at 55°F for those varieties that indicate "good cold soil emergence".
    • All Supersweet types (Sh2) require 65°F soil temp.
  • Soil moisture capacity is the next most critical factor for sweet corn seed germination.
    • All varieties must have adequate moisture for long enough for that water to soak through the seed coat to initiate germination. Supersweets with their shrunken germplasm require twice the moisture as the other types for proper germination. Therefore, follow rules for proper seeding depth to have the best success.
    • Less than adequate soil moisture leads to low, erratic, or no germination.

For more information relating to this topic please review our Garden Guide: Sweet Corn Seed Germination Issues

  • All types of corn are heavy feeders. Months prior to planting, have a soil test done to assess current soil nutrient levels, then add nitrogen according to recommendations provided by the test results. Adding needed organic amendments can happen any time prior to planting as well.
  • If you choose not to have a soil test done, before planting apply 5 pounds of all-purpose 10-10-10 per 100 square feet and blend that in with ample amounts of compost or aged manure. Or apply 3-5 pounds of 5-3-3 organic fertilizer per 100 square feet.
  • Once fertilizer and amendments have been added and soil temperatures are optimal, then sow corn seeds 1-1.5 inches deep, 8-10 inches apart in rows 3 feet apart. Seeds sown closer together should be thinned to 12 inches spacing once seedlings are about 4 inches tall.
  • Seeds can also be planted using the hill method. Mound soil 3-4 inches high, 10 inches wide, and plant 4-5 seeds in each hill, spacing each hill about 12 inches apart. Once seedlings emerge, thin to the 3 strongest seedlings by simply cutting two off at ground level.
  • In home gardens, a minimum of 4 rows must be planted for proper cross-pollination. The more plants the better for good wind pollination. Sweet corn can also be planted in a block or grid pattern with several plants clustered together, then separate that cluster by a 3-4 feet path, then plant another block, and so on to fill the space you have at least 3-6 blocks in each grid.
  • The soil should be kept consistently moist with regular supplemental watering as needed.
  • Once seedlings are 2-3 feet tall, hilling or mounding soil around the base of the stalks will help suppress weeds and keep root tops covered and moist.
  • At the same time, top dress with organic 5-3-3 or 3-5-3 fertilizer, applying 8 ounces per 10 feet of row starting about 3 inches back from the base of the stalks. Water thoroughly after application.

Harvest & Storage

  • Sweet corn ears mature in about 18-22 days from silk emergence, depending on conditions.
  • Ears are typically fully mature when silks dry and turn brown.
  • Tip kernels will have good color, look plump and show milky-colored juice when punctured.
    • Ears picked too early will have small kernels, lack color, and have watery flavor.
    • Ears picked past their peak maturity stage will have doughy, pithy, and undesirable kernels.
  • Once peak maturity has been reached, ears can be harvested for about a week depending on the variety, but for the best quality, ears should be harvested as soon as possible after maturity. Starch accumulation happens quickly if ears are not harvested at peak ripeness.
  • Ears on outer rows will typically ripen sooner than those of inner rows. When checking a larger field, check ears several rows in to get a true assessment of total field ripeness.
  • Sweet corn can be stored for several days with refrigeration. Leave husks on until use.