From 1916:

This vegetable is also known as “knot celery” and “turnip-rooted celery.” The roots are about the size of a white turnip, and the stalks are eaten.
Pare celeriac, cut in thin, narrow slices, and put into cold water. Drain from this water and drop into boiling water and boil 30 minutes. Drain and rinse with cold water. Celeriac is now ready to be prepared and served same as celery.