Sweet Corn

There has been a lot of change in the corn world over recent decades, and while it has made growing sweet corn easier and more rewarding, it has also made choosing the right variety more difficult. There are both more kinds of sweet corn, and more cultivars within the established kinds, each with important advantages, disadvantages, and cultural requirements.

In my grandfather’s there was simply field corn, often called “cattle corn,” sweet corn for fresh eating and popcorn for, well, popping. These days almost all field corn is hybridized, and a lot of it is genetically engineered, a topic discussed elsewhere. This all gets pretty complicated so we will leave it alone here; follow the links if you find it as interesting as I do. Note: home garden varieties of popcorn have been pretty much left alone.

Types of Sweet Corn

So we now have at least four types of sweet corn, each with its own specific genetics. Breeders have worked long and hard to control the triggers that balance of starch and sugar in sweet corn kernels, in order to bring gardeners new sweet corn cultivars which are not only sweeter and more tender, but hold those qualities longer after harvest.

The standard sweet corn, picked at its peak of ripeness and transported immediately to any already boiling pot of water for a quick—not too lengthy—dip, is the paragon of all summer produce, surpassing in my childhood memories even the taste of that first ripe, red tomato. That taste, to me, says sweet corn. And this is the only group in which you will find non-hybrid, heirloom sweet corn varieties.

Next up the ladder is sugar-enhanced sweet corn, which has a variant gene that adds new tenderness to the species, and often extra sweetness as well; but what is most important about this “sugar corn,” as I call it, is that it holds its sweetness much longer, and doesn’t have to be cooked within five minutes of picking to have good old-fashioned sweet corn flavor. Within this group there are two types: one  that was created by crossing standard sweet corn with the enhanced type, and one that was produced by crossing two enhanced types. In seed catalogs, the first is called “se” corn and the second usually has a denotation as “se+” corn.

Then there is super-sweet corn, the product of a slightly different genetic arrangement that nearly doubles the sugar content of the kernels, as well as slowing conversion of that sugar to starch. This perhaps the best kind for freezing, and is noted in the catalogs as “sh2″ because the dry seed has shrunken kernels. BUT: in order to get the benefits of all this breeding work, supersweet corn needs to be complete isolated from any other type of corn, and since corn is wind pollinated this can be an insurmountable problem in either urban community garden, or rural field corn growing areas.

Finally, there is a new “synergistic class” which is a cross between the “se” and the “sh2″ types, which combines the best of both. These kinds also need to be isolated.

Fortunately, there is a solution to the isolation problem, at least for rural gardeners: temporal succession. Pay attention to when the corn is tasseling in your area (that is when the pollen will be floating on the air) and time your planting to tassel before or after that…then there will be no cross pollination and  you can get a good crop of supersweet corn for freezing. The difficulty of doing this in urban and suburban areas should be immediately apparent, as you don’t necessarily even know who is growing what kinds of corn within the half-mile or so range of wind-blown corn pollen.

Growing Sweet Corn

The process of growth and maturation in corn involves three stages. First, the vegetative growth of the plant takes place. This is the growth of the stalks and the leaves.

Then the plant tassels and ears begin to form; plant sugars that have accumulated in the stalk of the plant are rapidly translocated to the developing ears, and peak at a stage that gardeners call “full milk.” If you have ever traveled through corn country in mid-summer, with the windows down you have probably smelled the ripening corn.

Finally, growth slows and the ears begin their final maturation; from that point on the sugars in the kernel begin a one-way, irreversible conversion to starch that will allow the kernels, which are, after all, the plant’s seeds, to store the carbohydrate energy they’ll need to germinate when the times comes.

It is this last step that breeders have tried to change, slowing down the conversion of sugar to starch as much as possible so that the eating quality of the kernels lasts longer.

As noted above, one of the differences between the different kinds of modern sweet corn is critically important for home gardeners, especially those with limited space: what I call sugar corn (se and se+ types) can be grown right next to traditional sweet corn without worry, but the supersweet (sh2 types) cultivars must be isolated, or they will cross with other types and the ears will revert to a tough, starchy texture without the flavor that makes them super.

Corn is the only cross- pollinated crop where the current season’s seeds (the kernels) are the part we eat, and since it is wind-pollinated, the isolation from incompatible types needs to be at least twenty-five feet, and, if the garden is in a windy area, fifty or even one hundred feet is best. You can limit yourself to growing only one cultivar, or at least only one type of corn each year to avoid crossing, but if you live in a densely populated area with neighbors who also garden close by on both sides, or are a member of a community garden, their planting plans may affect you, whether you like it or not.

There are other differences as well. Both the sugar corns and the supersweets have a different texture than traditional sweet corn. Some people consider them crisp, but others watery, while just about everyone agrees that traditional sweet corn, picked at its peak, is creamy or milky. This is apparently due to a lower level of polysaccharides in the juice of the new kinds. Which type you prefer is strictly a matter of taste, but the difference between types does affect garden performance.

When the seeds of sugar and supersweet corn cultivars mature (and their abundant sugars are finally converted to the starch that the seeds require for storage of energy for later germination), the kernels shrink and take on a shriveled appearance. This is especially true of supersweets, and the effect is such that, when supersweet corn seed is planted, it needs to take up much more water than traditional varieties before it reaches the state of “imbibition”—as botanists call it—that triggers germination. That takes time, and in the spring seeds that don’t germinate quickly often rot, which is just what will happen to the seed of these new cultivars if the soil hasn’t fully warmed up before planting.

In most corn cultivars the different stages of growth are triggered by a sort of accumulation of good growing weather, and, once each stage is reached, there is no going back. Scientists and breeders use a measure called heat units (HU) to quantify this accumulation. Heat units are measured by taking the number of hours each day when the temperature is above 50F/10C, then multiplying that by the number of degrees over 50F. The earliest varieties might have a heat unit requirement for harvest of about 1,300 HU, while late-maturing varieties might require as many as 2,000 HU to ripen.

While most of us don’t have the time or interest to measure heat units on a daily basis, understanding how they work makes it clear that corn yields are going to be a matter of how big a plant you can grow by the time the cultivar’s heat unit breakpoint is reached.

So, whichever type of corn you decide to grow, put it in the richest part of your rotation, as corn is a greedy feeder. In some cases it could go in the second part, with an extra bit of rotted manure or compost added to the bed. This would be done if the corn was interplanted with cucumbers, melons, pumpkins, or a trailing squash of some kind. That makes good use of the growing space, though it puts very strong demands on the soil.

The sex life of sweet corn makes one more demand: you need to make sure the wind will be able to move enough pollen from the tassels to the silk protruding from the ears. The standard recommendation is that all types of sweet corn should be planted in hills or in blocks of at least four rows. That way, no matter from which direction the wind blows, it will be able to pick up pollen.

We plant two rows in a three-foot-wide bed, with the plants spaced a foot apart in the rows, and simply plant three adjacent beds. If you’ve got to make the most of a small space you can plant hills of four to six seeds in 2:1:2 hex pattern pattern; but ideally you’d still want to have two beds planted next to each other.

Larger, later cultivars need more room, or so we are led to believe by conventional wisdom. But studies have shown that at, plant densities of two to four square feet per plant, yields were up to one ear per square foot. In our garden , with only one square foot apiece, the average early corn cultivar yields just one ear per stalk, with a second, smaller ear to follow if time allows. Though the ears are smaller, on a square foot basis this is better production.

As noted above, corn, especially the sugar-enhanced and supersweet types, should not be planted until the ground is fully warmed. How warm? The absolute minimum soil temperature is 55F/13C, but 65F/18C is better and 75F/24C is ideal. Here in Vermont we don’t really have the luxury of waiting for the ideal, so we plant just after the frost-free date and hope for the best.

You can actually plant up to seven to ten days earlier if you use a cool-soil–tolerant variety (usually noted in the catalog description). That is because the growing point of a corn plant doesn’t actually break ground until the plants are almost six inches tall; if a late frost knocks back the top of the plant, it just grows up out of the soil and keeps going.

You can stretch the harvest period by planting the same cultivar more than once in temporal succession, or by planting two or more cultivars with different maturity rates in varietal succession. By not growing any supersweet varieties (they are too sweet for my liking anyway) I avoid having to isolate the different plantings, and the first, earliest maturing variety functions as my insurance, the second my main crop, and the third a gamble, since sweet corn does not tolerate frost well. Longer season areas can manage more plantings if space allows.

Corn seed should be sown an inch deep, four seeds per foot of row, and thinned to one seed per foot when the plants are three to four inches tall. If you are planting in hills—which are really just clumps of plants, not actual raised mounds—put four to six seeds per hill and thin to the strongest four.

My grandfather, who planted in hills, always removed a shovelful of soil from each spot and replaced it with a shovelful of compost. If I planted in hills, I would too.

Once the seed is up, the corn should be kept weed-free until it is knee-high. After that point you should stay out of the corn patch until harvest time. Most of a corn plant’s root growth takes place between tasseling and harvest, and you compact the soil whenever you walk on it.

Either mulch the rows, sow a cover crop, or train squash or cucumber plants to grow out into the plot to shade out any later weeds. In terms of moisture, the mulch is probably best; in terms of soil fertility, the cover crop—especially if it is a legume. But in terms of efficiency, the interplanting with squash makes the most out of the space available, and corn is a space hungry as well as nutrient hungry plant.

I’ve tried the squash-corn combination, and there is no question that it works, but I’d only do it if I was planting in beds. If you interplant the two in large plots, getting in to harvest is a real pain. I have no doubt that the rough-and-tumble vines keep out raccoons, as companion planting fans claim: I barely want to thrash my way through the abrasive tangle myself to harvest the corn, and I planted it!


ShepherdSweet Corn

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