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Soybean not a fan of bright lights/big cities


One of those oddities that I get a question or two about every fall. Why do soybean plants near street/highway lights stay green until frost?

While corn plants love light and heat in their rush to harvest. Soybean plants are night owls. The soybean varieties we grow in Illinois are grouped by Maturity groups with groups II, III and IV being the most common in Central Illinois. Soybean plants biologically measure the length of the night. June 21st is the longest day of the year. After that the nights start getting a little longer every day. When the night length gets to a certain point soybean chemistry signals the plant it needs to switch to reproductive/maturing mode. If a light shines on it all night long it doesn't get this signal. It will put on some flowers and slowly start to set pods but it will also continue to grow stems and leaves. Plants tend to grow tall, be more prone to lodging and pods often stay green until frost. Light wavelengths and intensity will also play a role in the amount of impact from this phenomena. Spraying with a harvest aid like Gramoxone can act like a chemical frost to stop the growth and start the beans drying down in order to harvest the field all at once.



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