[1] He has conducted botanical expeditions in both Africa (Cameroons, Republic of Congo, Ethiopia, Kenya, Madagascar, Tanzania and Uganda) and the Americas (Costa Rica, Guatemala and Mexico).
Gereau has specialised in the Floristics and phytogeography of eastern Africa, he is also engaged in collaborative studies between ethnobotany and archeology.
Gereau has publications to his name in journals such as Annals of the Missouri Botanical Garden, Economic Botany and Novon.
Along with Jon C. Lovett, Chris K. Ruffo and James RD Taplin, he is the author of Field Guide to the Moist Forest Trees of Tanzania, which was published in 2006.
He participates in the Flora Mesoamericana, a collaborative project that is aimed at identifying and describing the vascular plants of Mesoamerica.