Growing sweet corn in home gardens

Three genes control sweetness in corn: Sugary (su), Sugary Enhancer (se) and Shrunken-2 (sh).

Newer varieties contain either the Sugary Enhancer gene, the Shrunken-2 gene or a combination of the two. You will see code letters on packets of sweet corn seed or in catalog descriptions that give information about growing each variety: su, se, sh, sy.

Classic plant breeding techniques produced these hybrids. These varieties are not products of genetic modification or genetic engineering.

• Has an average level of sugar (8% to 18%) that becomes starch quickly after harvest.
• Both hybrid and open-pollinated types are available.
• It is the traditional type of sweet corn.
• Some describe sugary corn as having “old-fashioned” flavor.
• Its kernels are tender and creamy. They can taste chewy as the sugar becomes starch.
• It is important that you pick these varieties the same day that you plan to eat them.
• Gardeners who want to freeze or can corn prefer this type, as the flavorful, creamy kernels hold up to processing.  
• Isolate from Shrunken (sh) and Synergistic (sy) types.

• Sugary enhanced has an increased sugar level (30% to 35%), and the kernels are tenderer.
• The sugar is slower to become starch after harvest.
• Fully Sugary Enhanced (se+) types are even sweeter. They will still be sweet after three or more days in the refrigerator.
• Isolate from Shrunken (sh).

• Also known as "Supersweet."
• Shrunken have two to three times the amount of sugar (40% to 50%) compared to Sugary varieties.
• They have a crispy texture.
• They can keep their sweetness for a week in the refrigerator.
• Some people find shrunken corn lacking in “corn” flavor, and they are less creamy.
• Seeds are smaller and lighter in weight, and look shrunken when dry.
• Isolate from all other sweet corn types.
• Cross-pollination between Shrunken (sh) and any other types will result in starchy, tough kernels.

• They have a combination of sweetness genes.
• All have at least one (se) gene.
• They may be combinations with (sh) or (su), or both.
• These varieties are very sweet, crisp, tender and creamy.
• Some also have longer storage life, lasting more than a week in the refrigerator.
• Sugar content in synergistic types take longer to build up than in sugary enhanced types, and kernels can be watery if picked too early.

Sweet corn must self-pollinate or pollinate by a similar variety. Wind moves corn pollen from the tassel at the top of the plant to the silks of the ears, and to lower parts of the plant.

Always plant corn in blocks of at least four rows. Corn planted in a single row will have much of its pollen blown out of the row, and will produce ears that have blank areas where kernels did not form.

Isolation is necessary to avoid mixing of kernel color, genetic types and other kinds of corn. Cross-pollination can lead to poorer flavor and texture.

Isolate by space and time

The simplest way to isolate is putting distance between the plants.

If you garden in an agricultural area, try to plant your sweet corn 300 feet from the nearest cornfield. Large acreages of field corn will produce so much pollen that you could spoil your crop unless you plant your sweet corn far enough away.

The home garden may not be big enough to plant different sweet corn types far enough apart.

Different varieties produce pollen at different times, so you can isolate them by time. Since different varieties will respond individually to growing conditions, do not isolate them with less than two weeks until the given “days to maturity” in the variety description.

You can also plant one variety earlier than another variety to achieve the needed difference in pollen production.

Finally, you could choose to grow just one type of sweet corn, and not worry about isolation.

Direct seed sweet corn.

Each variety differs in growth strength, especially in cool, wet or compacted soils.

Seeds germinate best when soil temperatures are close to 60°F. Use a soil thermometer to be sure the soil is warm enough.

Plant seeds one inch deep, and eight to 12 inches apart, with rows 30 to 36 inches apart. Plant in blocks of at least four rows rather than a long single row for proper pollination.

Small, lightweight Shrunken (sh) kernels demand the most care at planting. Do not plant them deeper than three-fourths to one inch. They will absorb up to twice as much water as other types before they germinate, so keep the seedbed moist until the shoots emerge.

In cooler soil, diseases are more likely to infect seeds. Most gardeners plant fungicide-treated seed. Check the seed packet or catalog description to know whether treated or untreated seeds are in stores. Even treated seed can fail to germinate under poor conditions.

If you use untreated seed, be sure that your garden soil temperature is at least 65°F. Some gardeners use plastic mulch to speed soil warming.

• One inch of rainfall per week is good for your corn.
• If your soil is sandy, it is better to water more often than once a week.
• An inch of water will wet a sandy soil to a depth of ten inches, a heavy clay soil to six inches.
• Use a trowel to see how far down the soil is wet. If it is only an inch or two, keep the water running.
• Most garden plants stop active growth when temperatures rise above 85°F, but all types of corn will continue to grow in very hot weather.
• Sweet corn can become drought-stressed in hot dry spells if you do not irrigate it.
• Sweet corn has a shallow rooting depth. Although you will see many new roots developing just above the soil at the base of the plant, these roots do not absorb much water or nutrients. Their function is to stabilize the plant.

• Frequent, shallow cultivation with a hoe or other tool will kill weeds before they become a problem.
• Hoe just deeply enough to cut the weeds off below the surface of the soil. Be careful not to damage the plants when cultivating.
• Once the corn plants have established, they will form a canopy of leaves that can discourage new weeds from growing.
• If you use the herbicide trifluralin (sold as Treflan, Preen and many other trade names) in your vegetable garden, note that you cannot use it near sweet corn.

Two insect pests that feed on the developing ears of corn are corn earworm and European corn borer.

It is common to husk an ear of sweet corn and find a caterpillar near the tip of the ear, and a chewed-up area of kernels. If this happens in your kitchen, cut the affected part of the ear off with a knife and cook the rest of the ear.

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.

You can avoid problems from these insects by planting corn around early May, and making sure the variety will mature in less than 80 days.

But if you grow later maturing varieties, or if your soil does not warm up early enough to plant in early May, and you have had trouble with either or both of these pests in the past, you will want to take steps to control them.

Crows and other birds will eat corn seeds before they emerge from the soil. A floating row cover left in place until the plants emerge will discourage them. 

Raccoons will eat the ears as they mature, often harming the plot overnight. It is difficult to fence out raccoons, but an electric fence with two strands about four inches apart, starting five inches above the ground, may keep them away. Keep weeds and grass away from the wire.

• Smut causes firm, tumor-like growths on leaves, stems, ears and tassels. Look for smut galls throughout the season and cut them out before they produce spores. Remove these galls from the garden and bury them. 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. 
• Use good cultural control practices to reduce disease problems to a good level and allow for a successful harvest.

For assistance in diagnosing unknown problems, visit the University of Minnesota Extension diagnostic site “What’s wrong with my plant?”

• Harvest ears when kernels in the center of the ear are full and "milky" when squeezed.
• Silks will be drying and browning at harvest time, usually 18 to 24 days after silk first becomes visible. This timing will vary based on weather conditions.
• Depending on genetic type, freshly harvested corn may retain its quality for one to seven or more days in the refrigerator.
• You may can sweet corn only if processed using a pressure canner. 
• Freezing is an excellent way to preserve sweet corn.