Let’s talk sweet corn

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By Dennis Morgeson


Last week I discussed growing tomatoes, which is one  of my favorite vegetables, so this week I thought why not talk about my favorite homegrown vegetable, sweet corn!  Sweet corn is a favorite among home vegetable gardeners, as well. Improved hybrid cultivars are easy to grow. They yield well, taste sweeter, and store longer than old time cultivars. Sweet corn is best adapted to larger gardens since only one or two ears are produced per plant and several rows are recommended to ensure adequate pollination.  However, even small plantings can be successful if planted in blocks rather than rows.

Sweet corn is available as yellow, white or bicolor ear types. Cultivars vary in their days to maturity. They are classified as early, mid- and late season. Late season cultivars generally are the best quality. Many of the new cultivars are higher in sugar content and retain their sweetness longer. There have been very few people that I have talked to that don’t like the newer sweet corn because they say it is too sweet, just keep that in mind just in case you don’t have a taste for sweet things. As for me, there is no such thing as having sweet corn that is too sweet!

Sweet corn is a warm season crop requiring a minimum soil temperature of 50 degrees F for seed germination. Seed should not be planted earlier than 10 days to two weeks after the average date of the last killing frost. If planted too early, poor stands, retarded growth, or frost-killed seedlings may result. However, it may be worthwhile to risk the chance of frost in order to get an early crop.

Starting out with fresh purchased seed each year is advisable. Sweet corn seed is relatively short-lived even under ideal storage conditions. Saving seed from last year’s hybrid crop is not recommended, since seed from hybrid plants shows considerable variability and usually produces inferior plants and ears.

If poor germination does occur, don’t replant the missing plants. The plants that develop from the replanted kernels will be crowded and shaded by the older plants and then yield poorly. Replants would not be ready for pollination at the same time as the original planting. If the stand is very poor, it is best to replant the entire area.

Plant the kernels one inch deep in heavy soils. Space the rows two to three feet apart.  Plant early cultivars eight to 10 inches apart in the row and late cultivars nine to 12 inches apart. Corn can also be planted in hills or mounds instead of rows. Use five or six seeds per hill and then thin to three strong plants per hill. Space hills about three feet apart.

For a steady supply of sweet corn throughout the season, include early, mid-season and late cultivars in your initial planting. Successive plantings of mid- or late season cultivars about every two weeks will help stretch the harvest season.

Corn is wind pollinated, so plant four or more short rows of sweet corn side-by-side rather than one or two long rows. This will help insure good pollination and ear development. Inadequate pollination results in poorly filled ears. 

Sweet corn is a genetic mutation of field corn, producing kernels consisting mostly of sugar rather than starch. However, sugar in the kernels rapidly converts to starch after its prime harvest stage. Recent sweet corn hybrids have been bred for even higher sugar concentrations and slower conversion of sugar to starch.

Corn pollen is carried by the wind from the tassels to the silks. Different types of corn can cross-pollinate and contaminate one another. All sweet corn types must be isolated from other types of corn including field corn, popcorn and ornamental corn because their pollen will turn sweet corn starchy. The shriveled characteristic of sweet corn is dominant, so popcorn pollinated by sweet corn will be sweeter and probably shriveled.  The color yellow is also dominant, so yellow corn that is pollinated by white corn will remain yellow. However, white corn that is pollinated by yellow will turn yellow.

Cross-pollination, among some of the genetically different types of sweet corn, can have undesirable results. In order to preserve the intended sweet quality of the corn you are planting, isolation is recommended to prevent cross-pollination with other types.  Isolation can be achieved in several ways.

Distance - Since pollen is carried by the wind rather than insects, distance can be used as an effective barrier. A distance of 250 feet between different types will result in some contamination, but not enough to affect the quality of the produce.

Maturity - The number of days to maturity can be used to prevent different types from being at a pollinating stage at the same time. Maturity isolation can be achieved by staggering planting dates or by selecting cultivars that mature at different times. A minimum of 14 days should separate the tasseling time of the different types.

Barrier/Border Rows - A considerable amount of contaminating pollen can be diluted by planting two to five border rows between different types. Most of the cross-pollination would occur in these border rows so that isolation distances could be reduced.

Wind Direction - Isolation can be enhanced, although not fully achieved, by avoiding the prevailing wind direction. 

Corn borer, corn earworm, and flea beetle are the chief insects to be controlled in sweet corn. 

Proper timing in applying insecticides is important if you want worm-free corn. Carbaryl (Sevin) or Dipel (which is organic) can be used to control corn earworm. On early and mid-season varieties, four to seven applications will give 85 to 90 percent worm-free ears. Make the first application when 10 percent of the silks are out, and repeat every other day until 90 percent of the silks have wilted. On late varieties, the earworm may be almost impossible to control with any kind of spray program.

It is very important to harvest sweet corn at the proper stage of maturity. The critical time is the milk stage, a stage when the juice in the kernel appears milky when you puncture the kernel with your thumbnail. Sweet corn remains in the milk stage for a relatively short period, so check the ears frequently. Corn that is too young will ooze a watery material, while ears that are too old will have a tough, doughy kernel. During the milk stage, the unhusked ear should feel firm, have full kernels at the tip of the ear, and have brown, dry silks. Generally, ears should be ready about three weeks from silking time.

When harvesting, break the shank (stem of the ear) close to the ear without breaking the main stock or tearing the shank from the stalk. After picking, use the sweet corn immediately for fresh eating, canning or freezing. If you have any questions about growing sweet corn call the Washington County Extension Office at 859-336-7741.  Happy gardening!