You know you’re supposed to plant corn close together in blocks, and not plant different varieties near each other - but why? Short answer: because corn is a weird vegetable, botanically speaking. The way it’s pollinated works differently than any other plant in your garden. Let's dive into the science behind how corn kernels form, and why cross-pollination matters.
How is corn pollinated?
Corn is monoecious, meaning it produces separate male and female flowers on the same plant. (Like squash, but with a completely different flower type.) Male corn flowers are the tassels at the top of each plant, which produce pollen, and the female flowers are the ears along the stalk, which contain ovaries. Silks will emerge from the female flowers right around the time when the tassels begin releasing pollen from the top of the plant. From that point, pollination usually occurs over the next six days or so as the pollen drifts through the air and falls onto the silks. This reproductive strategy is known as wind pollination.
Image: Mikrolit', FAL, via Wikimedia Commons
Each individual silk on an ear of corn is connected to a kernel - or rather, a female reproductive cell called an ovule that will become a kernel if it is fertilized. At least one pollen grain must fall on every single one of those silks and fertilize each of those ovules in order for a full ear of corn to develop. Many novice corn growers have husked their first ear of homegrown corn only to find a sad smattering of misshapen kernels on a mostly empty ear. That happens because of poor pollination. An ear of corn with hardly any kernels means that many of the silks did not receive a pollen grain. This is why it's important to plant your corn close together in blocks, not spread out in long rows - you want to give the ears the maximum chance of having plenty of pollen fall on them. If you don’t have very many corn plants and want to be sure to get the maximum yield, you can hand-pollinate by cutting off tassels and shaking them directly over the silks.
Image: Rasbak, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Once in a blue moon, you do everything right and still end up with poor corn pollination. What could have gone wrong? One cause for pollination failure is extreme heat. Sustained temperatures over 95 degrees F can prevent pollen formation, so if your area experiences a heat wave around the time your corn starts to tassel, your crop could be in trouble. Another cause of bad pollination is mistiming of pollen release and silk emergence, often caused by drought. Fun fact: the period of time when pollen release coincides with silk emergence is called “nick” in the corn growing industry. A crop that was poorly pollinated because of timing problems between tassels and silks is said to have suffered from “poor nick” or “missing the nick.”
Why should different varieties of corn be planted away from each other?
For most vegetables, which varieties you plant next to each other only matters if you intend to save seeds, but corn is a different story. Planting a paste tomato right next to a cherry tomato won’t have any effect on the fruits of those plants. A paste tomato pollinated by a cherry tomato will still grow a paste tomato (though if you save that plant’s seeds, the next generation will give you some surprises!) In this case, the fruit that we eat has only the genetics of the “mother” plant, and the mix of genes that ends up inside the seeds themselves doesn’t make any difference in the fruit we get this season. With corn on the other hand, the "fruit" we’re eating is the seeds themselves, and because of a quirk in how corn pollination works on the cellular level, the quality and texture of the kernels is significantly affected by the pollen that fertilizes them.
Remember your high school biology lesson on how a human embryo forms? Sperm meets egg, they join together, and the nucleus of the sperm cell (the part that contains DNA) fuses with the nucleus of the egg cell, giving the embryo two sets of genes, one set from mom, one set from dad. Pretty simple. You’d think plant reproduction would be even simpler. You’d be wrong. Each corn pollen grain contains not one, not two, but three nuclei, each containing a set of genetic information for a different purpose. When a pollen grain lands on a silk, it begins to grow a tube, which penetrates into the silk and elongates downward until it reaches all the way to the ovule the silk is connected to. The first nucleus in the pollen just directs the tube formation process, and doesn’t fertilize the ovule. The other two nuclei travel down the pollen tube to the ovule, where two female nuclei are waiting, one in the germ (the embryo that will sprout when the seed is planted), and one in the endosperm (the starchy or sweet part of the kernel that will feed the embryo as germination begins.) The two pollen nuclei fuse with the two female nuclei in a process known as double fertilization. The genes in the pollen nucleus that fertilizes the endosperm actually influence the structure and starch content of the endosperm, which is the key fact here, because the structure and starch content of the endosperm is what determines the texture and sweetness of a corn kernel. To go back to the tomato comparison - the main part of a tomato fruit only contains the genes of the "mother" plant, whereas the main part of a corn kernel has the genes of both parents.
Image via NMSU: https://pubs.nmsu.edu/_h/H232/index.html
So, what happens when your sweet corn gets pollinated by your neighbor’s dent corn? Sweet corn carries genes for low conversion of sugar into starch in the endosperm, whereas dent corn converts nearly all its sugar into starch. If dent corn pollen fertilizes sweet corn, the dent corn genes will be doing half the directing of endosperm formation in the kernels, leading to much higher sugar-to-starch conversion than if all the genes were from sweet corn. You won’t end up with dent corn exactly, but your sweet corn won’t be very sweet either, and the texture might be chewier. It’s a similar scenario vice-versa - if your dent corn gets pollinated by sweet corn, it won’t convert sugars into starch as efficiently as it’s meant to, leaving you with less filled-out kernels with higher sugar content that might not be what you were hoping for in cornmeal. Even different varieties of sweet corn can create crosses that are less than satisfying, particularly if a supersweet variety is crossed with an older, less-sweet heirloom.
How to keep corn from crossing
For most gardeners, it’s fairly easy to ensure that your corn doesn’t get ruined by cross-pollination. Just a barrier like a tall building, or a stand of trees in between one corn patch and another cuts down pollen drift a great deal. If your garden is right next to your neighbor’s, you might try coordinating to grow the same variety, or timing your plantings so that different varieties will not be in flower at the same time. For example, if you plant an early maturing sweet corn two weeks or so before a longer-maturing dent corn, you can be fairly sure the sweet corn will be done pollinating by the time the dent corn tassels. If you’re worried about lingering pollen from the earlier crop, you can remove the tassels from the sweet corn before the dent corn begins silking.
Now, if you're getting deeper into the heirloom corn game and you want to save your own perfectly pure seed, there are further steps you can take. You can bag tassels and shoots (developing ears) to collect pollen and individually pollinate the ears you want to save. We carry specially made tassel bags and shoot bags that make that job easier. This method is helpful when you want to be totally sure there has been no cross-pollination by another variety, even if multiple varieties are growing in close proximity. It can also allow you to make intentional crosses. (And boom! You're an amateur corn breeder.)
Whew! That's all the nerdy corn botany we have to throw at you for now. To get back to basics, check out our article on how to grow corn at home, or if you're ready to get planting, shop our full collection of corn seeds in our online store.
Written by Sow True Seed's Agriculture Director, Leah Smith
Sources: Univ. of Missouri, "Corn Pollination: The Good the Bad, and the Ugly" https://ipm.missouri.edu/cropPest/2012/7/Corn-Pollination-the-Good-the-Bad-and-the-Ugly-Pt-3/
Univ. of Nebraska-Lincoln, "How Extended High Heat Disrupts Corn Pollination" https://cropwatch.unl.edu/2019/how-extended-high-heat-disrupts-corn-pollination#:~:text=Heat%20over%2095%C2%B0F,pollen%20from%20leaving%20anther%20sacs.