Sweet corn, a garden favorite, presents a unique challenge to gardeners who wish to grow multiple types simultaneously. Unlike most garden vegetables where cross-pollination only affects the seeds saved for the next season, corn kernels themselves are immediately impacted by the pollen they receive. Planting incompatible varieties close together can lead to a disappointing harvest, resulting in tough, starchy kernels instead of the intended sweet ones. Understanding the genetic differences between sweet corn varieties is the first step in successfully managing a mixed corn patch and preventing flavor loss.
The Genetic Families of Sweet Corn
Modern sweet corn varieties are categorized into four primary genetic families, each defined by a specific gene that controls the conversion of sugar into starch. The oldest type is Standard Sugary, labeled “Su,” which is known for its traditional, creamy texture and classic corn flavor. Because the sugar in Su varieties rapidly converts to starch after harvest, they must be eaten immediately for the best quality.
The next major group is Sugary Enhanced, or “Se,” which possesses a modified gene that increases the sugar content and tenderness compared to the standard type. Se varieties maintain their sweetness for a longer period after picking. Supersweet corn, designated “Sh2,” carries the shrunken-2 gene, which drastically slows the starch conversion process. These varieties boast the highest sugar levels and feature a distinctively crisp, crunchy texture, allowing them to store well for up to a week.
The final category, Synergistic or “Sy” types, combines the beneficial traits of the Sh2 and Se genes. Sy varieties aim to balance the high sweetness of Sh2 with the tender kernel texture of Se and Su types.
Why Cross-Pollination Affects Kernel Quality
The immediate change in kernel quality due to cross-pollination is a phenomenon known as Xenia, meaning the pollen directly influences the characteristics of the resulting kernel. Corn kernels are the seeds of the plant, and the endosperm, which is the part we eat, is genetically determined by both the parent plant and the incoming pollen.
The genes that create the sweet, tender kernels are recessive. This means the kernel must receive the sweet gene from both the silk (female) and the pollen (male) to express the sweetness. If a high-sugar Sh2 kernel is fertilized by pollen from a standard Su or field corn, the dominant starch-producing gene carried by the foreign pollen will be expressed immediately. This genetic mixing causes the developing kernel to become tough, dry, and starchy, ruining the intended eating quality.
Isolation Requirements for Different Corn Types
Because of the direct effect of Xenia, certain sweet corn types require strict isolation to maintain their quality. The Supersweet (Sh2) varieties are the most sensitive and must be isolated from all other types, including Standard Sugary (Su), Sugary Enhanced (Se), Synergistic (Sy), field corn, and popcorn. Cross-pollination from any of these other groups will destroy the Sh2’s characteristic high sweetness and tender texture.
The Standard Sugary (Su) and Sugary Enhanced (Se) groups are generally compatible and can be planted together without significant negative flavor effects. These two types share similar genetic mechanisms for sugar production, allowing them to be grown in the same block. Synergistic (Sy) varieties should be kept separate from the Sh2 group to prevent the kernels from becoming starchy.
Techniques for Successful Mixed Planting
Gardeners who wish to grow incompatible sweet corn varieties in a limited space must utilize strategies that prevent the pollen of one type from reaching the silks of another.
Isolation by Distance
The most straightforward method is isolation by distance, where a minimum separation of 250 to 400 feet is recommended between incompatible types. This large distance is necessary because corn is wind-pollinated, and its pollen can travel significant distances, especially in open areas.
Isolation by Time
When physical distance is not possible, the most reliable technique is isolation by time, which involves staggering planting dates. The goal is to ensure that the pollen-shedding period of one variety does not overlap with the silking period of the other. Planting varieties so their maturation times are separated by at least 14 days is typically sufficient to prevent cross-pollination. Gardeners can achieve this by either planting varieties with different “days to maturity” at the same time or by planting the same variety in succession two weeks apart.
Using Physical Barriers
A physical barrier can be used as a secondary defense, though it is not a complete substitute for distance or timing. Planting a tall, dense crop like sunflowers or using a solid fence line between varieties can help disrupt the wind patterns that carry corn pollen. However, for highly sensitive types like Sh2, combining a slight distance separation with the timing method provides the most reliable insurance against cross-pollination.