Do what you want, but I don't like it. -folium at the genus level is very rare.
-phyllum is far more common but my Greek is weak.
Ceratophyllum spp.: "Hornleaf" (hornwort)
Hydrophyllum spp.: (waterleaf).
EDIT: It looks like the most common Greek word for peace is eir[e-long:220mp0n4][/e-long:220mp0n4]n[e-long:220mp0n4][/e-long:220mp0n4]; I'm not sure how that would combine with phyllum.
Another Greek word is sp[o-short:220mp0n4][/o-short:220mp0n4]nd[o-short:220mp0n4][/o-short:220mp0n4]ph[o-short:220mp0n4][/o-short:220mp0n4]r[o-short:220mp0n4][/o-short:220mp0n4]s: truce-bringing. It looks a bit like spondee.
Trees tend to be feminine; I see that at the species level (and below), -folia is much more common than -folium: Quercus acerifolia: mapleleaf oak; Quercus oblongifolia: "oblong-leafed oak" (Mexican blue oak).
So I'm revising my suggestion to Olea pacifolia ssp. hiroshimaensis.
-folium is neuter, so if you go with Olea pacifolium, you'd want: ssp. hiroshimaense.
We have Eucalyptus umbra, but umbrosus, -a, -um is far more common. Now that I think about it, the narrowest strain would be around Hiroshima, so let's go with
Olea pacifolia ssp. umbrosa var. hiroshimaensis.
Species level and below get lower case letters; genus and above get capital letters.