Does the name end -iam or -ium?

The final vowel of an English word carries almost no information, so the ear cannot tell u from a here at any speed. Latin can. In podium the u is stressed enough in its own language to be unmistakable, and the spelling of the fern's name follows the Latin word rather than the English mumble.

An unstressed vowel, flattened until it gives the ear no information.