Reading a few Wikipedia articles I come across different ways to spell buckwheat. One is Гречка (which is what I use when talking about buckwheat) and Гречиха (which I have never heard of before). What is the difference between the two words?
Buckwheat plant is called гречиха. It is cultivated for its grain-like seeds called греча or гречка (гречневая крупа) which we cook. The cooked buckwheat is called гречка, греча, гречневая каша. There are also гречневый продел (which is grated buckwheat grain) and гречневая мука (buckwheat flour) used for cooking.