I've seen it translate to both in different sources when reading the manga. I think in the Anime it was "Aogiri Tree" but I can't quite remember.
1In English localization the often used variant is Aogiri Tree or just Aogiri.– ParawesMar 21, 2018 at 19:01
1@AkiTanaka not quite, see: japanese.stackexchange.com/a/23346/925– кяαzєя ♦Mar 24, 2018 at 20:18
1In the root A anime it was called Aogiri Tree. In the manga I think it was translated both ways.– CallatMar 26, 2018 at 16:29
The name is Aogiri Tree, also referenced to several times as just Aogiri. The name of the organization is Aogiri Tree because the manga and anime translated it into English as Aogiri Tree.
1Welcome to Anime & Manga. Is this the official translation from the licensed manga/anime, or from fansub/scanlation?– Aki Tanaka ♦Mar 23, 2018 at 16:44