https://en.wikipedia.org/wiki/Cladium_mariscus

Thesis Statement:
The Old English rune eolh-secg (ᛉ) does not derive from a genitive form of eolh meaning “elk,” as earlier glossators and 19th-century lexicographers speculated, but rather preserves an older, sacral meaning linked to the Proto-Germanic root alh- (“sanctuary, shrine”). Drawing on the protective function of the rune in Anglo-Saxon magic, its semantic association with danger and boundary in the Rune Poem, and its likely cognacy with Gothic alhs (“temple”) and the divine Alcis described by Tacitus, the term eolh in this context is best understood as denoting a protective or warding force, not an animal. The plant referred to—likely Cladium mariscus, a sharp fen sedge—serves as a ritual or symbolic boundary-marker, and eolh-secg should thus be interpreted as “shrine-sedge” or “warding reed”, reinforcing the rune’s use in apotropaic contexts. This reading aligns with the work of R. I. Page, who stressed the symbolic over the phonological function of this rune, and with Warren and Elliott’s reevaluation of algiz as denoting protection rather than “elk.”