I searched newspaper articles on buckwheat pollens found from the Yahari-Shimojima site in Nanto, Toyama.
All articles are written in JAPANESE, but they said pollens were found from sediments in the late Jomon to final Jomon.
The Chunichi Shimbun: The Origin of Riga soba (buckwheat)?
The Toyama Shimbun: Buckwheat pollen from a site in Riga: Evident for buckwheat cultivation in Jomon
The Kitanihon Shimbun Nanto News: Did Jomon people eat buckwheat? Pollens from site in Riga