Common Buckwheat belongs to the same family of plants as Knotweed and Rhubarb.  Not a grain but a psuedocereal, the name Buckwheat comes from the Dutch "boekweit," meaning Beech Wheat, a reference to the shape of the seed.  that has been in cultivation for 8000 years.  It's precusors were foraged and then domesticated by peoples in Southeast Asia where it found it's way to Europe 7300 or more years ago, and wild ancestors of the plant are still found in south-central China.  Though this seed is intended to be used as a cover crop, there's no reason the seed couldn't be milled for flour.  Because the plant is so extensively foraged by bees and other pollinators and needs cross pollination, there are few cultivars available mostly developed in the late 20th century.

Buckwheat makes an excellent cover crop.  Planted at a rate of 16 seeds per square foot, the stand will surpress weeds, build a mat of roots, and increase organic matter in the soil like no other plant.  Buckwheat germinates very fast, and grows extremely fast, and in the best conditions can produce flowers within 21 days.  When the plant is well underway and blooming, but before seeding, the crop can be mowed and the residue left on the beds as an organic mulch.  The material will decompose quickly, adding nutrients to the growing beds.