A Taste of Summer: Growing the Best Sweet Corn
By Chris Colby
From planting to harvesting and everything in between, learn how to grow summer’s signature stalk.
For the American farmer, corn is king. Corn and soybeans obviously form a formidable one-two punch for commercial farmers. For American gardeners, though, tomatoes probably take the crown, but perhaps you could say sweet corn is queen.
For many gardeners each summer, nothing tastes as good as fresh sweet corn, brought in straight from the garden and cooked. Maybe a little butter sets it off at your table.
Watching the grass grow
Corn (Zea mays) is a cereal grass, in the same botanical family (Poaceae) as wheat, rice, barley, sorghum, oats, millet, rye, and others. Each kernel on a cob of sweet corn is a seed, which contains the embryo of the corn plant and a reserve of starch that fuels the plant’s early growth.
When a corn seed germinates, its roots begin growing into the soil and its coleoptile — the first tubular leaf-like structure — emerges above the soil. New (true) leaves emerge from the collar of the “tube” one at a time as the plant grows. After a few weeks, when the plant has four or five leaves, it almost looks like a small bush. Later, the distance between the leaves increases, as the plant “telescopes” to grow taller. It also keeps producing new leaves up until sexual maturity. By this time, some of the earliest leaves will have turned yellow and fallen off. Some field corn varieties can grow up to 16 feet tall, although most sweet corn varieties only hit 3 to 5 feet.
Corn produces both male and female reproductive structures on the same plant. It is monoecious (having both male and female reproductive parts), in the lingo of biologists. Eventually, a cluster of male flowers — called the tassel — appears atop the plant. The flowers shed pollen when mature. These flowers lack large, showy petals and don’t look like what we normally think of as flowers. Very shortly after this, the silks emerge from the presumptive corn ears. Each silk is an elongated part (style) of a female corn flower. Like the male flowers, female flowers don’t have showy petals. The silks capture pollen and direct them to the ovary in the growing ear. The fertilized ovaries become corn kernels and soon fill with a clear liquid. The corn embryo also appears at this time. This is called the blister stage.
Next, in the milk stage, the liquid inside the kernels turns a milky white. The color is due to the presence of sugars. These sugars are progressively converted into starch — through the dough and dent stages — and then they dry out. The embryo continues to grow through all these stages. Most varieties of field corn take over 110 days to mature. Sweet corn varieties take less time, because they have been selected for quicker maturation and are harvested in the late milk stage. Typical sweet corn maturation time is 60 to 90 days.
The dried kernels from field corn can be ground into cornmeal, corn grits, or corn flour (masa harina) and used to make many dishes, including corn bread and tortillas. Often the corn is treated with an alkaline solution, a process called nixtamalization, before it is ground. Corn in some form makes its way into many processed foods, even those that do not traditionally use corn as an ingredient. The starch in corn kernels can also be converted by enzymes into corn syrup, or high fructose corn syrup. Sugar from corn can additionally be fermented and distilled into alcoholic beverages or a fuel additive. Corn is also used for animal feed. Most gardeners, however, grow sweet corn to be eaten fresh or canned.
Sweet corn (Zea mays var. saccharata) is derived from field corn. Ordinary sweet corn is the result of a corn plant with two su alleles at the location of the su1 (sugary1) gene. (An allele is one of the alternate forms of a gene.) The dominant allele at this locus is the Su1 allele (with a capital “S”). A recessive allele at this locus in sweet corn is the su1 (small “s”) allele. Plants with two Su1 alleles, or plants with one Su1 allele and one su1 allele, produce dent corn or flint corn, i.e. ordinary field corn. In contrast, corn plants with two copies of the su1 allele have sweeter, creamier kernels.
The Su1 allele produces a “debranching enzyme,” an enzyme that removes side branches of starch from a growing starch molecule. The su1 allele does not yield a functioning enzyme. As such, ordinary sweet corn has higher levels of water soluble polysaccharide (WSP). WSP is a highly branched starch molecule that, as the name states, is more soluble in water than regular starch. Ordinary sweet corn is also higher in a compound called phytoglycogen. When this type of sweet corn is harvested during the milk stage, it can be cooked and eaten like a vegetable. Native Americans discovered sweet corn growing as natural mutants in their corn fields and introduced Europeans to the food when they arrived.
Sweet corn tastes great when picked and cooked immediately. If picked and stored, it loses some flavor and texture within 24 hours.
Another gene, the se gene, contributes to sweetness in some sweet corn varieties. It’s not known what the se gene does. However, plants that have one or two copies of se, along with two copies of su1, yield corn that is sweeter than normal sweet corn. This is due to actual differences in the level of sugar (mostly sucrose) in the corn. Ordinary (su1) sweet corn kernels contain 5 to 15 percent sucrose. Se sweet corn has levels in the 10 to 25 percent range.
A separate path to sweet corn began with a University of Illinois professor named John Laughnan who was researching the sh2 (shrunken2) gene in corn. The recessive allele sh2, when present in two copies, produces corn ears with shriveled kernels. Normally, the sh2 gene product (the enzyme ADP-glucose pyrophosphorylase) catalyzed a reaction that added glucose residues to a growing starch molecule. The sh2 allele, however, produces an inoperative enzyme, so sugar is not converted into starch. As such, sugar builds up in the kernel at the expense of starch. As such, sh2 sweet corn kernels contain 25 to 35 percent sucrose. In addition, sh2 sweet corn will retain its sweetness for weeks if stored in the refrigerator.
Still sweeter corn
There are now even newer sweet corn varieties, such as Augmented sh2
and Synergistic varieties. Augmented sh2 varieties combine sh2 plants
with plants carrying su and se alleles. Synergistic (sometimes
abbreviated sy) are se plants that produce some sh2 kernels.
Growing sweet corn
There are challenges to growing the best sweet corn, but these can easily be met if the gardener is aware of them. The first two choices a gardener needs to make are where and what.
Sweet corn needs full sun. Even areas that only get sporadic shade — where some garden vegetables grow fine — won’t do for sweet corn. Without full sun, the cobs will not grow to full size, although they will taste just fine. In addition to full sun, corn requires rich soil with good drainage. If you’re planting sweet corn in an established garden, with decent soil, that has grown healthy vegetable plants the previous year, amending the soil with compost will generally make it suitable. Digging a spade full of soil, mixing it with roughly half that volume of composted manure, and returning the mix to the hole provides a nice little hill for each plant to start in. If you dig a new garden site, your best bet is to get the soil analyzed before.
Corn is wind pollinated, so you need to designate a space large enough to plant a sizable block of it. A single plant, or corn grown in a single row, will not pollinate completely and you’ll end up with ears of corn that are not completely filled in with kernels. Most garden guides give the minimum planting as a 4-by-4 block. For small gardens, you can arrange the plants in a grid. The grid would be composed of 14- to 16-inch squares, with a seed planted at each corner. If you stagger the rows, you can tighten the spacing up by a couple inches. For larger plantings, plant the corn with 12-inch spacing in rows 2 to 21⁄2 feet apart. You should have at least four rows for complete pollination. If you’re pressed for space, you can get away with a smaller grid, but you’ll have to pay attention to when the plants are shedding pollen and hand pollinate each one. Most sweet corn varieties should be planted to a depth of 2 inches.
While you want to make sure your sweet corn is pollinated, you also need to be sure it is pollinated by the right type of corn. That means they need to be pollinated by another plant that shares their genetics (as far as sweetness goes). Sweet corn pollinated by field corn will not be sweet. Su sweet corn pollinated by sh2 sweet corn will not be sweet. As such, you need to plant blocks of sweet corn with only one variety or with compatible varieties. Fortunately, seed catalogs will list the sweet corn type and tell you what other varieties are compatible with it. If you plant more than one different type of sweet corn, you’ll need the blocks to be separated by 250 feet, or 14 days between the time they tassel.
Sweet corn, and especially some of the newer types, are very sensitive to temperature and won’t germinate if it is too cold. With its relatively long growing time, many gardeners want to plant their corn as soon as possible to get a head start on the growing season. If you want to plant early, watch the weather reports closely and only do so if the weather permits. For a small enough garden, planting the seeds in 4-inch seed-starter planters indoors works well as long as the plant is transferred to the garden before the second true leaf appears. You can use a seedling heating mat to keep the planters warm, if needed. Most seed catalogs will list the required temperatures, especially for the more finicky varieties of sweet corn. Your best practice most years is to wait until the weather is suitable, or — for small blocks of corn — plant inside a week to 10 days ahead of when you normally would.
In a rotation
If your garden spot remains the same every year, it pays to plant the corn in a space that did not grow corn (or another cereal) the previous year. In one common crop rotation pattern, corn would be the fourth crop in a spot after root vegetables such as onions or carrots (but not potatoes, turnips, or rutabaga); legumes (beans), and brassica vegetables (cabbage, broccoli, cauliflower, turnips, etc.). However, growing corn after any other garden vegetable — and especially vegetables that are “light feeders” — will work. Given corn’s need for nitrogen, planting corn after beans (or other legumes) is an obvious choice. If your garden is big enough that you can leave parts of it fallow each year, you can plant a cover crop (such as hairy vetch) one year and corn the next.
Corn can be grown in containers, but you’ll need many large containers. To successfully grow corn in containers, you’ll need half-barrel planters or something similar in size, around 24 inches in diameter. And, you’ll need at least 16 of them for a 4-by-4 block of corn plants, unless you plan to pollinate the silks by hand. The planters should have excellent drainage and you should feed individual plants with a dilute fertilizer solution frequently.
Sweet corn has relatively shallow roots, and therefore it needs to be watered during periods when rainfall is scarce and the top layers of soil dry out. If corn plants are significantly water-stressed, the leaves will roll. Simply checking on the garden every few days and watering the plants as needed is all that’s required. Water stress is exacerbated at temperatures above 86 degrees Fahrenheit, so you may need to check your garden at least every other day during heat waves.
Most agricultural experts advise commercial sweet corn farmers to add 130 to 150 pounds of nitrogen per acre. Higher rates actually depress the yield. Scaled down to garden size, this is 5.5 ounces of nitrogen per 100 square feet. All granular fertilizers have an NPK rating, for example 10-10-10. The first number of the three numbers is the percentage of nitrogen in the mixture. So, if you take the amount of nitrogen you need, divided by the nitrogen percentage (expressed as a decimal), you will know how much nitrogen to add. For example, let’s say you’re planting 50 square feet of sweet corn. For that, you would need 2.75 ounces of nitrogen. If you add NPK 5-5-5 fertilizer, you would need 2.75 ounces/0.05 = 55 ounces, or 3.4 pounds of the 5-5-5 fertilizer for the 50-square-foot block.
Corn plants need more nitrogen at some stages than others. It takes up most of its nitrogen after the six-leaf stage through tasseling. As such, farmers typically add 30 pounds per acre before planting, and the rest after the plant has reached the five- to six-leaf stage. For garden applications, you should calculate the total amount of fertilizer required (as above) and add 1⁄5 of this before planting. Or, if you use the compost hilling method described above, you should be fine. Then, when the plants reach the six-leaf stage, around six weeks after planting, side dress the corn plants with the rest of the fertilizer. Keep in mind that most garden soil is richer than most farm soil, and you may require a lower amount of fertilizer.
Composted manure containing about 2 percent nitrogen is a great way to side dress plants. You can apply all the remaining fertilizer in the form of compost when the plants reach the four- to five-leaf stage. This gives the compost time to break down, sink into the soil, and feed the corn.
Like all garden plants, corn also needs potassium and phosphorous — the “P” and “K” in NPK — as well as a variety of micro nutrients. However, in most cases, ordinary garden soil should be rich enough. And if you use compost as a fertilizer, it should contain everything the corn needs. Likewise, a light feeding with an all-purpose liquid fertilizer can rectify any problems.
As with any garden vegetable, corn is going to be a target of bacterial, fungal and insect pests. And raccoons love corn. Three growing problems specific to corn are lodging, smut fungi, and corn borers.
Corn requires a lot of nitrogen, relative to most other garden vegetables, to grow. And, it’s a tall plant. However, its root system is shallow and this can lead to lodging — corn plants toppling in heavy winds. If corn is fed too much nitrogen, it causes the plants to grow taller than their roots can support. When the winds pick up, the corn will topple, pushing the plants next to it down as well. You can partially avoid this problem by “feeding” the plants mostly with compost. It can be easy to overdo nitrogen with liquid fertilizers. In contrast, compost usually delivers all the nutrients the plant can use, without going overboard.
Even when not overstimulated with nitrogen, corn plants can get blown over in a windstorm. Most of the time, the plants will right themselves after a few days. If not, the plants can be staked upright.
Corn smut fungus (Ustilago maydis) can infect corn, often causing an unsightly gall at the end of the ear. Spraying a fungicide does not provide an effective control of this fungus as spores can live in the soil for years. The best practice for gardeners is to simply watch for the fungus. If it appears, remove the infected plant parts and dispose of them away from the garden. (In other words, don’t compost infected corn.) If the galls are not removed, they will dry out and rupture, releasing a cloud of black spores. Subsequent corn crops grown there will be more prone to fungal infection.
In Mexico, young corn smut galls are called huitlacoche and are considered a delicacy. In fact, infected corn sells for a higher price than healthy corn there. The galls contain the compounds sotolon and vanillin — which smell like fenugreek or lovage, and vanilla — and also contain a fair amount of the sugar glucose. They are most often cooked and served in enchiladas or tacos. Attempts to interest American diners in huitlacoche, by renaming it the Mexican truffle, have not been successful.
If you’ve only ever eaten supermarket sweet corn, you might not have had the pleasure of knowing the European Corn Borer (Ostrinia nubilalis). This insect was first observed in the U.S. in 1917, in Massachusetts. It is now a pest of corn — as well as many, many other types of plants, including crops and weeds — all over North America. Corn borer caterpillars are frequently found at the silk end of an ear of corn, although they can infect almost any part of the plant. The damage they do to corn can also provide a path for other plant pathogens (including smut fungus) to enter the plant.
Control of corn borers can be achieved with regular treatments of synthetic insecticides or Bt. If you want to avoid pesticides, planting native plants around your garden and encouraging your local insect predator population will help. Lady beetles and lacewings predate corn borers, and trichogramma wasps are an especially effective biological control.
Minor corn borer infestations are nothing to panic about. You can snap off the unaffected part of the corn ear and eat it, for starters. And, if you remove infected plant parts and dispose of them away from the garden, the caterpillars will not mature into the corn borer moth and go on to make more corn borers.
Sweet corn does take some planning and work to grow well. It requires a lot of space to plant, warm soil to germinate, nitrogen-rich soil to thrive, and is prone to lodging in windy locales. However, the rewards of sweet corn at harvest time make it worth the effort. Once it finds a way into your garden rotation, it will likely stay.
Chris Colby is an avid gardener who lives in Bastrop, Texas, with his wife and cats. His favorite variety of sweet corn is ‘Silver Queen’ (su1). His academic background is in biology, but his main interest is in brewing beer. His latest book, Methods of Modern Homebrewing, (2017, Page Street) was released on December 12.
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