Euphorbia herbstii (syn. Chamaesyce herbstii) is a rare species of flowering plant in the family Euphorbiaceae known by the common name Herbst's sandmat. It is endemic to Oʻahu, Hawaii, where it is rapidly disappearing.[1] Like other Hawaiian euphorbs, this plant is known locally as ʻakoko. It is a federally listed endangered species of the United States.
This plant is usually a tree which can grow up to 8 meters tall. The leaf blades are long and narrow, and the branches bleed a milky sap. The inflorescence is a cluster of cyathia.
This species is known from perhaps four populations[2] in mesic forests on the Waiʻanae Range.[1] Since the year 2000 the plant has undergone a severe decline, with populations extirpated and remaining ones becoming smaller.[1] The exact number of remaining plants is unknown because each survey tallies fewer individuals, while more have been planted in appropriate habitat.[1]
Threats to the species include feral pigs, which uproot and trample vegetation, leading to loss of plant cover and root networks, which then leads to erosion.[1] They pick up the seeds of invasive plant species and transport them to new habitats.[1] Fences have been erected in critical areas to exclude the pigs.[1] Non-native plants affecting the euphorb include silky oak (Grevillea robusta), corky-stem passionflower (Passiflora suberosa), and strawberry guava (Psidium littorale).[1]