Corn Isolation Requirements

“Dear Horticulture Talk Expert, I am interested in growing Polka Bicolor Corn and Honey Select Triplesweet Corn OR the Honey Select with Mirai 131Y.  It says that I am supposed to isolate these.  Why?  Also, can you tell me where I can purchase these?  Thanks, R.C.”


Thank you for your question regarding corn isolation.  Sweet corn is a naturally occuring genetic mutation of field corn, producing kernels consisting mostly of sugar rather than starch.  However, sugar in the kernels rapidly converts to starch after its prime harvest stage. Recent sweet corn hybrids have been bred for even higher sugar concentrations and slower conversion of sugar to starch. Several different types of mutations and gene combinations can result in sweet corn. The following types are most commonly available:

1. Standard Sweet – su or su*(Silver Queen Hybrid, Peaches & Cream, etc.)

2. Partially modified types – at least 25% of the kernels are modified as follows:

a. Synergistic or Sugary Supersweets – su sh2* (Honeycomb, golden Nectar, Sugar Time, and Sugar Loaf;

b. Sugar Enhanced or EH – su se* (Kandy Korn EH, Earliglow EH, and Tendertreat EH)

3. Fully Modified types – su se* on all kernels (Miracle)

4. Single gene replacements for su – usually sh2* (Illini Xtra Sweet, Early Xtra Sweet, Northern Xtra Sweet)

5. Multiple gene replacements for su  are combined to replace su (Mirai varieties)

6.   A relatively new type of sweet corn known as “triplesweet” has both sugar enhanced (se) and supersweet (sh2) kernels on the same ear (Honey Select, Serendipity, Avalon, Polka)

Corn pollen is carried by the wind from the tassels to the silks. Different types of corn can cross-pollinate and contaminate one another. All sweet corn types must be isolated from other types of corn including field corn, popcorn, and ornamental corn because their pollen will turn sweet corn starchy. The shriveled characteristic of sweet corn is dominant, so popcorn pollinated by sweet corn will be sweeter, have less popping potential, and probably shriveled. The color yellow is also dominant, so yellow corn that is pollinated by white corn will remain yellow. However, white corn that is pollinated by yellow will turn yellow.

Cross-pollination among some of the genetically  different types of sweet corn can have undesirable results. For example, sweet corn types 4 and 5 must be isolated from each other and from all other types of sweet corn because pollen from the other types will make the kernels starchy like field corn. In addition, pollen from types 4 and 5 can make standard sweet corn starchy. Types 2a and 2b will regress to normal sweetness when pollinated by standard type pollen. Type 6 (Polka, Honey Select) does not require isolation from most sweet corn types, except for Type 4 and 5. In order to preserve the intended sweet quality of the corn you are planting, isolation is recommended to prevent cross-pollination with other types.  Isolation can be achieved in several ways.

–Distance. Since pollen is carried by the wind rather than insects, distance can be used as an effective barrier. A distance of 250 feet between different types will result in some contamination, but not enough to materially affect the quality of the produce. A distance of 700 feet should give complete isolation; however, complete isolation is only necessary for scientific and plant breeding purposes.

–Maturity. The number of days to maturity can be used to prevent different types from being at a pollinating stage at the same time. Maturity isolation can be achieved by staggering planting dates or by selecting cultivars that mature at different times. A minimum of 14 days should separate the tasseling time of the different types.

–Barrier/Border Rows. A considerable amount of contaminating pollen can be diluted by planting two to five border rows between different types. Most of the cross-pollination would occur in these border rows so that isolation distances could be reduced.

–Wind Direction.  Isolation can be enhanced, although not fully achieved, by avoiding the prevailing wind direction.

As for a place to purchase these varieties, it depends on whether you are interested in treated seed (I recommend this for those planting in spring and live in the northern regions) or untreated seed.

Treated seed:

Untreated Seed:



© Mertie Mae Botanics LLC and Horticulture Talk!, 2011. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Mertie Mae Botanics LLC and Horticulture Talk! with appropriate and specific direction to the original content.


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