A primer on decoding the sweet corn section of your seed catalog
Before you can plant sweet corn, you have to order seeds, and to order seeds you have to wade through seed catalogs. Use this primer to understand abbreviations, evaluate the claims and make the right picks for your farm.
The arrival of seed catalogs each winter is an exciting time— there are new hybrids to see, big claims made and an interesting assortment of variety names. The sheer number of things available is enough to make your head spin, and once you get into the Su’s, Se’s and Syn’s of the sweet corn section, it can be difficult to decipher what variety is best for your operation. Understanding the genetics and biology behind these varieties can help you sort through the flashy pictures and alluring names to find the best fit for your farm and markets.
The origin of the sugars
If you learned about Mendel and his pea plants back in biology class, you have basis for understanding sweet corn genetics. Field, ornamental, pop and sweet corn are all one species (Zea mays), and a set of dominant and recessive genes control the kernel characteristics. The genes that code for starchiness is dominant, so plants and ears with these genes present will produce tough kernels.
Sweet corn is produced by isolating the plants so the recessive genes, which code for sugar and tenderness, shine through. There are three recessive genes that code for different kinds of sweetness and kernel toughness, which can be stacked to create different characteristics. The combinations of the genes are referred to as genotypes (genetic types).
Su: normal or standard sweet corn
Sweet corn referred to as normal or standard contain primarily the su gene for sweetness. This genotype of sweet corn was the primary type available until the last century (think Silver Queen). These varieties have a sugar content of about 9 percent and the sugars convert quickly into starch, which means narrow harvest windows and limited storage ability.
These varieties are best suited to local markets with quick turnaround. The creamy texture and mild sweetness are preferred by some customers. They are capable of germinating in soil temperatures ranging from 55 to 60 degrees Fahrenheit, and young plants are vigorous.
Se: sugar enhanced, sugary enhancer or EH sweet corn
Varieties categorized by sugar enhanced or sugary enhancer contain se genes. This genotype has increased (16 to 18 percent), more stable sugar levels than normal sugary types. They have the ability to convert sugar to starch; it just takes longer because they start with a higher level. These varieties have a good flavor and hold better than the previously discussed genotype. However, their pericarp (skin of the kernel) is quite tender and care must be taken in harvesting and handling so kernels are not damaged.
This genotype is best suited to direct retail sales and localized wholesaling where it will be moved quickly from the producer to the consumer. They are capable of germinating in soil temperatures ranging from 55 to 60 F.
Sh2: supersweet, ultrasweet, extra sweet or shrunken-2 sweet corn
Most often referred to as supersweet or shrunken-2, these varieties possess the sh2 gene. These genotypes provide kernels with high sugar content (about 35 percent) and a crunchy—as opposed to creamy—texture. This type stores due to a couple of characteristics, the first being that this genotype does not have the ability to convert sugar into starch. The thick kernel is the other reason. The kernel (pericarp) of this type is thicker than the previously discussed types, which is good because it increases the storage life and shipping capability, but bad because it can lead to consumer complaints about kernel toughness.
This genotype is sometimes referred to as “shrunken,” referring to this seed’s small size, lightweight and overall “shriveled” appearance. Sh2 varieties require warm (60 F at 2 inches) soils when planting. These types are suited for a wide range of markets, from direct local markets to wholesale, long distance shipping markets.
From these three main types, newer genotypes have been bred, which aim to combine the best characteristics of su, se and sh2 genes.
This is a newer genetic type that contains normal sugary (su) and supersweet (sh2) genetic components, with the exact ratio dependent on the variety. This combination balances a sweet taste, creamy texture, tenderness and shippability. Some of these varieties are sold under trade names such as Triplesweet or Sweet Breeds. Its su origins help with vigor, though these varieties are better suited towards local markets as opposed to shipping or processing.
Augmented or augmented shrunkens (sh2)
These varieties are a modified version of sh2 genotypes, taking the sweetness of sh2 varieties while leaving behind the kernel toughness. The genes at play here are the se and sh2. This tenderness is due to a reduced seed coat, and the trade-off for improved eating comes during planting—these varieties need warmer soil temperature and precise depth at planting in order to germinate (60 F at 2-inch soil depth). Some of these varieties are sold under trade names like Multisweet, Gourmet Sweet and Xtra-Tender Brand.
Knowing your customer: color, insect packages and beyond
Once you’ve picked a type that matches the flow on your farm, seed catalogs often break the sweet corn catalog down by color and packages of genetically engineered traits.
In regards to color, your location will likely determine what color of kernels customers accept. Historically, yellow corns were more popular in northern growing areas while white corn was more popular in the south. Currently, about 75 percent of the sweet corn grown is bicolor, or actually tricolor if you look close. There will be white kernels along with a light yellow and a darker yellow due to the unique pollination biology of corn. In Michigan, customers tend towards bicolor corn, while in the south and east coast, white sweet corn is often still the standard.
Genetic engineered (GE) or genetically modified organisms (GMO) are a controversial topic in the public sphere, and you likely already know where your customers stand on buying GE crops. Genetically engineered varieties will have “built in” tolerance to glyphosate or glufosinate, some insects or both. They can make management easier and reduce the number of sprays in a season, producing worm-free corn with good eating quality.
When selecting a GE package, know what insects are covered or not covered; depending on the types of Bt proteins present, you may not get control of the pests you face on your farm. Many large grocery chains will not purchase GE varieties, but growers serving small, local markets may be able to sell GE varieties. You know your markets best, just listen to customer questions and concerns.
Genotype and color isolation
All corn is wind pollinated, and the characteristics of the kernel will be determined by the parent plants and the pollen that makes its way onto their silks. Field corn, decorative corn and sweet corn are all one species capable of cross-pollination, meaning that for your ears to have the desired characteristics, they should be isolated by distance or pollination time from other types of corn. All sweet corn should have a minimum of 250 feet or 14-day difference in projected tasseling dates from other types of corn. Within sweet corn, some isolation is needed between different genotypes.
Within sweet corn, sh2 types should be isolated from su and se types. Isolating su and se from each other isn’t necessary, but isolating them from each other will allow each varieties characteristics to shine through. Among the newer types, synergistic varieties should be isolated from sh2 and augmented varieties. Augmented varieties should be isolated from su, se and syn varieties.
Isolate these varieties from each other if you are considering growing varieties from multiple colors. White sweet corn should be isolated from bicolor and yellow varieties, as a white variety being pollinated by other types of corn will lead to yellow kernels in an otherwise white ear. If bicolor corn isn’t isolated from yellow corn, the resulting ears will have a higher number of yellow kernels.
Early versus late: utilizing days to maturity
It’s unlikely one variety will match all your needs if you’re doing successive plantings—some varieties perform better earlier in the year while other varieties are better suited to Labor Day and beyond. When skimming a seed catalog, you will see a range of days to maturity listed, ranging from 65 to almost 90 days. You will likely need early (less than 70 days), mid-season (70-84 days) and late (more than 84 days) varieties to round out a full season of sweet corn production.
Having some early varieties in your stable will let you get sweet corn on the market early in the summer, allowing you to start building your customer base just before your main season crops are ready. Site selection can help early-season varieties germinate and mature—look for fields with southern exposure where soil is likely to warm early.
Season extension tools can be used to push your early corn. Some farmers use things like plastic mulches, low hoops, and transplants to get sweet corn earlier in the season. For more information on this type of growing technique, see the University of Kentucky resource, “Transplanted Sweet Corn on Plastic,” by Timothy Coolong.
Sources for evaluation of claims
Once you’ve narrowed in on your sweetness level, desired color and maturation time, you’ll be left with an assortment of varieties. The information provided by the seller is one thing you can use in decision-making, but work done by extension educators around the Great Lakes region can help you see how these varieties have performed under field conditions. The ultimate source for this is the Midwest Vegetable Trial Reports.
Variety trials place different varieties under the same soil, weather and cultural conditions, ideally allowing the true character of that variety to shine. The variables measured vary depending on the goals of the trial and usually start with a basic yield estimate, but can include things like tip fill, ear length and width, ease of harvest, disease tolerance, worm presence and even taste preference by consumers. Numerous variety trials stretching back over 15 years can be accessed on Purdue’s website. Recent trials include:
- Supersweet Sweet Corn Cultivar Evaluation for Northern Indiana, 2017
- Sugar-enhanced and Synergistic Sweet Corn Cultivar Evaluation for Northern Indiana, 2017
- Ohio Sweet Corn Evaluations, 2016
Ultimately, you are already likely trying a couple varieties on your farm. Trying to treat areas in which you are testing new varieties equally can help you decipher varietal differences from other factors.
Growing season updates
For in-season field developments, subscribe to Michigan State University Extension’s Vegetable Production newsletter. Many of the locally-based educators track sweet corn pests, which can help you better manage your crop. You can subscribe to the Vegetable Production newsletter on the MSU Extension Vegetables website.
For a wide range of information related to management of your sweet corn varieties, refer to the “Midwest Vegetable Production Guide for Commercial Growers.”