Corn cross-pollinates readily, which is crucial for home gardeners and commercial growers alike. This phenomenon occurs because corn is a monoecious plant, meaning it has separate male and female reproductive structures on the same plant. Corn is primarily wind-pollinated, making it highly susceptible to genetic mixing with neighboring varieties. The consequence of this cross-pollination is often immediate and noticeable, especially when planting sweet corn near a starchy variety such as field corn. Protecting the desired characteristics of your corn requires understanding the plant’s reproductive mechanism and implementing isolation strategies.
The Mechanism of Corn Pollination
Corn plants separate their male and female reproductive parts, which promotes cross-pollination even within the same field. The male flower is the tassel, the large plume that emerges from the top of the plant, while the female flowers develop into the ears further down the stalk. Each tassel produces millions of microscopic pollen grains, which are released and carried by the wind over a period of 10 to 14 days.
The female flower consists of the developing ear, with each potential kernel connected to a single strand of silk. A grain of pollen must land on an individual silk strand and travel down its length to fertilize the ovule, which then develops into a kernel. Silks are receptive to pollen for approximately ten days before they begin to dry out.
Because the pollen is light and easily airborne, it is transported by the slightest breeze, allowing for wide dispersal. While the majority of pollen grains tend to settle within 20 to 50 feet of the source plant, pollen can travel much farther under favorable wind conditions. This reliance on external pollen flow is what makes corn cross-pollination an inherent challenge for growers trying to maintain varietal purity.
Understanding the Genetic Impact
The most immediate and concerning consequence of corn cross-pollination is known as the xenia effect. This is a unique genetic phenomenon where the characteristics of the foreign pollen immediately impact the physical properties of the developing kernel, which is the endosperm. In most other plants, cross-pollination only affects the seed that will be planted the following year, but with corn, the result is visible in the current year’s harvest.
This effect is most noticeable and problematic when different types of corn are grown close together, such as sweet corn planted near starchy field corn or popcorn. If sweet corn silks are fertilized by starchy field corn pollen, the resulting kernels will develop a hard, starchy endosperm instead of the desired sugary one. This produces an ear with tough, flavorless kernels unsuitable for eating.
The visible changes due to xenia can include shifts in color, texture, and size of the kernels on a single ear. For example, a white corn variety pollinated by a yellow variety may produce an ear with a mix of white and yellow kernels. This immediate impact on the edible portion of the crop is why isolating corn varieties is much more important than it is for most other garden plants.
The degree of impact depends on the dominant traits of the pollen source. Field corn and popcorn varieties often possess dominant genes for starchiness and kernel color that readily override the recessive genes for sweetness found in most sweet corn varieties. This dominance means a small amount of foreign pollen can spoil a large portion of a sweet corn harvest.
Strategies for Preventing Cross-Pollination
Preventing unwanted cross-pollination requires employing various isolation strategies based on space and time.
Distance Isolation
For the home gardener, distance isolation is a primary method, though the required separation can be significant due to wind-driven pollen. While commercial growers often need hundreds of feet—up to 660 feet or more—to ensure near-perfect genetic purity, home gardeners can often achieve acceptable results with a minimum of 100 to 200 feet of separation between different corn types.
The effectiveness of distance isolation drops off rapidly, with cross-pollination levels falling significantly within the first 150 feet from the pollen source. If space is limited, the use of a barrier crop, such as a tall hedge or a row of sunflowers, may help to physically filter some pollen. Planting corn in large blocks rather than long, single rows also helps, as it allows plants within the block to pollinate each other before foreign pollen reaches the inner ears.
Timing Isolation
Timing isolation, or staggered planting, is a highly effective alternative to physical distance. This strategy involves ensuring that the tasseling and silking windows of different corn varieties do not overlap. To achieve this, growers should calculate planting dates so the varieties mature at least two to four weeks apart.
A minimum of a 10 to 14-day difference in maturity time between varieties is necessary to prevent the simultaneous release of pollen and the emergence of receptive silks. This temporal separation ensures that by the time a later-maturing variety is shedding pollen, the silks on the earlier variety have already been fertilized and are no longer receptive. Careful planning of planting dates and knowing the “days to maturity” for each variety is necessary for this method to work.