Next to tomatoes, sweet corn is probably the most anticipated vegetable harvested in the garden. Sweet corn differs from field corn because it possesses a recessive sugary (su) gene. Kernels carrying this gene develop nearly twice as much sugar as starchy types. In older sweet corn varieties, the sugar levels are unstable and they change to starch rapidly after harvest. The main interest of sweet corn plant breeders has been the identification of genes that promote high sugar levels and improve post harvest storage properties. Over a dozen genes influencing kernel quality have been identified, but only two are being used extensively. These supersweet corn varieties include the shrunken-2 (sh2) and the sugary enhancer (se) genes.
The sh2 varieties can produce peak sugar levels nearly double that of normal sweet corn. In addition, the conversion of sugar to starch occurs at a much slower rate providing flexibility in harvest and storage. Some varieties will keep 5 to 10 days after harvest. Kernels are very sweet and 'crunchy' (which many people dislike). The newer hybrids have eliminated the skin toughness problem.
These varieties will produce kernels with sugar levels and storage characteristics intermediate between those of normal sweet corn and the best shrunken-2's. The presence of starch contributes to their creamy texture and flavor. The kernels have tender skin, almost too tender for mechanical harvest and extended handling.
Pollination of sweet corn is done by wind. To ensure pollination, plant in short blocks of 3 or 4 rows rather than a single long row. Do not interplant different types of corn or cross-pollination will occur. Dent corn will make sweet corn kernels starchy and less sweet. Popcorn will cross-pollinate with sweet corn unless it is more than 100 feet away. Standard sweet corn varieties will reduce the quality of the supersweets. There are several ways to isolate corn to reduce or eliminate pollination problems. A distance of 700 feet will give complete isolation, however, in most situations this is impractical. A distance of 250 feet will give some contamination, but not enough to materially affect quality. If planting upwind, a distance of 100 to 150 feet is adequate. Isolation can also be provided by time of maturity. There should be a minimum of 14 days between maturities in order to receive isolation. In some cases (cool air and soil temperatures) more than 14 days is necessary between plantings because maturity depends upon the total amount of heat the corn receives. Wait until the first planting is about knee-high before planting the second variety. July 1 would be the latest date for planting early maturing varieties. A final way of isolating corn is through the use of barriers and border rows. A large amount of contaminating pollen can be diluted by using 2 to 5 border rows for protection. Isolation distances can be slightly reduced with these barriers.
Other helpful hints to guarantee sweet corn success:
- Make sure soil temperatures are above 60 F especially with the supersweet varieties. Seed rots easily i in cold, wet soil.
- Space plants properly. A final stand will have plants 8 to 10 inches apart and rows spaced 3 feet apart. Closer spacings result in yield loss due to poorly filled ears.
- Provide adequate moisture. Water needs are greatest from tasseling to harvest but do not neglect watering during the early stages of growth.
- Prepare a smooth seed bed and plant shallow, only 3/4 to 1 inch deep.
- When plants tassel, watch for and record the date that about half the plants show silking. The crop will be ready for harvest in 15 to 21 days depending on temperatures during this time. To check maturity, open the husk and press a kernel. If it spurts milky juice, it is at the peak of ripeness. Many varieties produce 2 ears per plant. The top ear usually ripens a day or two ahead of the lower one.
- Use immediately or refrigerate extra corn following harvest. The supersweets will keep longer due to a slower sugar-to-starch conversion. Freeze or can excess as soon as possible.
- Suckers are common on many sweet corn varieties. They do not take strength away from the main stalk and should be left in place.
- Corn requires a moderate amount of fertilization. At planting, band fertilize with a 5-10-10, side-dress when the plant is 8 inches tall and again when it reaches 18 inches with the same fertilizer.
This article originally appeared in the May 24, 1996 issue, p. 90.