“Knee High by the 4th of July”
In Indiana, and especially in the source’s hometown near Indianapolis, cornfields surround most of the neighborhoods, and this is where all of the neighborhood kids play during the summer. In fact, when the source moved to Los Angeles, he said he was surprised to find that no one else knew how tall corn was supposed be during different parts of the year. He understood “Knee High by the 4th of July” to be common knowledge all over the country.
People in Indiana mark time, especially in the summertime, by how tall the corn is. It is accepted there that if a corn crop is doing well, it will be “Knee high by the 4th of July”. The height of the corn is also indicative of how hot the summer has been, or how much rain has fallen. He said that people use the cornfields in Indiana to gauge the weather conditions, much like people in Malibu watch the ocean for seasonal changes and weather patterns.
The source explained that the saying is used often, mostly in cars as people pass various cornfields and discuss how the crop is coming along that summer.