Plainville is a city in Rooks County, Kansas, United States. As of the 2020 census, the population of the city was 1,746. It was named from its s…Plainville is a city in Rooks County, Kansas, United States. As of the 2020 census, the population of the city was 1,746. It was named from its setting upon the plains. Washington Irving Griffin settled along a freight trail in an area known as Paradise Flats in 1877. In order to establish a post office, Griffin completed a US Postal Service application that required the signature of the local Justice of the Peace. While reviewing the application, Justice of the Peace Lambert P. Darland suggested the name "Plainville". Griffin operated the Plainville post office and a small store from his sod house, the first structure in what would become the city of Plainville.