"Corn" in English is used for a variety of grains depending on the locale. This term "Welschkorn" is used by Carl Schurz to describe a field of grain in the Palatine. It is not critical to following the narrative but I am just curious and I have not found an answer on the Internet.
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The German Wikipedia for corn (zea mays):
Note that for buckwheat typically the adjective "black" is added, so check your sources.
Taking your (probable) source into account
and double-checking with the Palatine and Lorraine dictionary of the University Trier and the Southern Hessian dictionary of the Hessisches Landesamt für geschichtliche Landeskunde, I'm quite sure that Schurz describes corn.
Apart from that, hiding in a buckwheat field is very difficult:
Generally, the term "Welsch-" denotes foreign origin, typically referring to the geographically closest "Roman tribe or nation" - for the Palatine that would be France, for (Southern) Tyroleans Italy. It can often be understood as a somewhat generic term for "foreign".
In southern German dialects, "Welschkorn" is simply corn (maize). Buckwheat is "schwarzes Welschkorn".
Being from Lorraine, I have heard in Alsace, people saying "Turki" not for wheat but for corn. It might have had the general meaning of "foreign" as you say about things that are given the name "Welsh", but in fact there is another attested explanation. Some corn seeds, coming from America have found their way to the big market of Constantinople-Istanbul ; from that place, corn has been introduced progressively through the Danube and its affluents in central Europe. It arrived very late (19th century) in Western Europe.
J. M. B.