Arboreteum, damnit!

I was sure that the L.A. County Arboretum in Arcadia was spelled a-r-b-o-r-e-t-e-u-m. I consulted several dictionaries and could not find arboretum with that other spelling. Yet if one Googles "arboreteum," one can find many examples of places that use the extra "E" in the spelling. I have a feeling that the L.A. Arboretum used that other spelling when I was growing up and that's why it's imprinted on me. However, I can't find evidence that this is the case. How very vexing.