Buckwheat is the great impostor of the grain bin, for it is not a grain at all. The cereals of the world, wheat, barley, rice, maize, and the rest, are all grasses, members of the family Poaceae; buckwheat belongs instead to the Polygonaceae, the knotweed family, and its nearest kin in the kitchen are rhubarb and sorrel. What it produces is not a true grain but a small, hard, three-cornered fruit, an achene, whose floury inside is milled and cooked exactly as though it were a cereal. It is therefore a pseudocereal, like quinoa and amaranth, and like them it carries no gluten, a fact that has remade its fortunes in the present century.

The plant itself is a quick, undemanding annual, a sprawling thing of reddish stems and broad arrow-shaped leaves that throws up clouds of small white or pink flowers beloved of bees. Its virtues are the virtues of poor and difficult country. It races from sowing to harvest in ten or twelve weeks, faster than any cereal; it asks nothing of the soil and will crop on thin, acid, exhausted ground where wheat would fail; and it shoulders out weeds and needs no coaxing. These are the gifts of a mountain crop, and they explain why buckwheat became, again and again across the northern world, the grain of the highlands, the heath, and the hungry season, the thing that grew where nothing better would.

Two species matter at the table. Common buckwheat (Fagopyrum esculentum), the sweeter and milder of the two, was taken into cultivation in the warm-temperate valleys of southwestern China and is the buckwheat of Japanese soba, Russian kasha, the Breton galette, and the American griddle cake. Tartary buckwheat (F. tataricum), darker, more bitter, and far hardier still, was domesticated separately on the high, cold ground of the eastern Himalaya and the Tibetan plateau margin, and it is the buckwheat of the roof of the world: the staple of the Yi people of the Liangshan mountains, of Bhutan, Nepal, and Ladakh, and the source of the rutin-rich bitter buckwheat tea now drunk across China and beyond.

Two further plants are sometimes counted, loosely, amongst the 'types' of buckwheat, though neither is a cultivated grain and neither is the ancestor of the two that are. Golden buckwheat (Fagopyrum dibotrys, syn. F. cymosum), the jinqiaomai of China, is a hardy perennial of the same south-western mountains, but it is grown chiefly as a medicinal plant, its rhizome long used in Chinese medicine, and eaten only marginally, as a leafy potherb in parts of south-western China and the Himalaya; it is a wild relative of the cultivated species, not their forebear. 'Wild buckwheat', meanwhile, is a name attached to several quite different plants: the weedy black bindweed (Fallopia convolvulus) of the same family, whose seeds were gathered in prehistoric Europe but are no longer eaten; the large and unrelated North American genus Eriogonum, the true 'wild buckwheats' of that continent; and, in the strict sense, the wild progenitors from which common and Tartary buckwheat were each separately raised. It is those two cultivated species alone that carry the culinary story that follows.

From its mountain cradle in southwestern China buckwheat travelled out along two quite different roads. The first ran east and north, the road of common buckwheat. It spread down into the loess highlands of northern China, where on the thin, dry soils of Shanxi and Shaanxi it became a noodle grain of the poor, pressed into the chewy strands of qiaomian héle. From China it passed into Korea, where as memil it became the cold-noodle and the buckwheat-jelly grain, and on to Japan, where buckwheat had grown since prehistory but only in the Edo period gave rise to soba-kiri, the cut noodle that became the soul food of old Tokyo.

The second road ran west, and it ran for thousands of miles across the grasslands of Inner Asia. Carried along the steppe corridor and quickened, by most accounts, by the movements of the Mongol age, buckwheat reached the lands of the Rus, where it found its truest second home. The East Slavs called it grechka, the Greek grain, after the Byzantine Greeks through whom they believed it had come, and made of its toasted groats the porridge, kasha, that is as close to a national dish as any grain has ever been to a people; from its dark flour they raised the blini of the pre-Lenten feast of Maslenitsa. Through the East Slavic lands buckwheat passed to Poland and the wider Ashkenazi world, where the toasted groat married the noodle as kasha varnishkes.

In the later Middle Ages buckwheat at last entered the rest of Europe, and Europe, baffled by a dark grain out of the east, named it for the heathen: blé sarrasin and grano saraceno, the corn of the Saracens. It took hold first on the poor sandy heathlands of the Low Countries and northern Germany, where the Dutch christened it boekweit, the beech-wheat, the name from which the English buckwheat descends. From that northern belt it spread to the acid soils of Brittany, the kingdom of the galette de blé noir; into Slovenia, where ajda became a matter of national feeling; and across the Alps to the Valtellina of Lombardy, the home of pizzoccheri and of polenta taragna. The Dutch then carried boekweit across the Atlantic to their colony of New Netherland, and the buckwheat cake, dark and sour and risen with yeast, became a fixture of the American winter breakfast from the Hudson Valley to the hills of Appalachia.

The high road of Tartary buckwheat kept to the mountains. It remained the bread grain of the eastern Himalaya and the Tibetan borderlands: the staple of the Yi of Liangshan, who eat it as cakes and brew it as tea, and of the Himalayan kingdoms, above all the Bumthang valleys of Bhutan, where buckwheat pancakes (khule) and buckwheat noodles (puta) are the everyday fare, and the fields of Nepal and Ladakh besides.

Buckwheat's modern geography is a map of the cold and the high. In Japan it is soba, one of the two great noodle traditions of the country and a marker of the seasons and the calendar alike: chilled zaru soba dipped in soy-and-dashi tsuyu in the heat of summer, steaming bowls in winter, and, on the last night of the year, the slithering strands of toshikoshi soba eaten to cut off the old year and pass into the new. In Korea the same grain is the cold noodle naengmyeon and the trembling grey jelly memilmuk. In northern China it is a highland noodle, and in the southwest mountains the bitter Tartary buckwheat is a true staple still, eaten as the buckwheat cakes of the Yi and drunk, increasingly across the whole country and abroad, as a fragrant, rutin-rich health tea.

Across the Slavic world buckwheat keeps a place no Western grain quite matches. Grechka, the toasted-groat porridge, is the everyday comfort food of Russia, Ukraine, and their neighbours, the side dish to a thousand meals and a grain so freighted with feeling that its sudden absence from the shops is read as a national alarm; the buckwheat blini of Maslenitsa carry the same weight of tradition. In the Ashkenazi kitchen the groats live on as kasha, and as kasha varnishkes. Western Europe keeps its regional strongholds: the savoury galette complète of Brittany, folded round ham, cheese, and an egg; the pizzoccheri and the buckwheat-and-maize polenta taragna of the Valtellina; the ajdovi žganci of Slovenia; and the Dutch and Flemish buckwheat pancake. In North America the buckwheat cake survives as a regional and heritage food of the north-eastern hills, celebrated each autumn at the buckwheat festivals of West Virginia and Pennsylvania.

And everywhere, in the last two decades, buckwheat has been reborn. Naturally free of gluten, rich in the flavonoid rutin and in a uniquely complete plant protein, it has moved from the peasant table and the health-food shelf to the centre of the contemporary kitchen, milled for gluten-free baking, sprouted, toasted as a crunchy garnish, and prized once more as one of the oldest and most nourishing of the foods of the poor.