Buckwheat is not wheat and therefore not chametz, grain which has risen. However it included in the general class of kitniyot, foods that Ashkenazim (and some Sephardim) may not eat on Passover.

This was enacted because kitniyot dishes are similar to chametz. This prohibition also includes corn, beans and rice. This injunction was unanimously accepted by Ashkenazic Jews; many Sephardic Jews, however, continue to eat kitniyot on Passover, provided that they were carefuly checked to ensure that no grains were mixed in. If you are Sephardic, speak to your rabbi to determine your family and community tradition.

The prohibition is only with regards to consumption of kitniyot; there is no obligation, however, to destroy or sell kitniyot products before Passover.

Learn more about kitniyot