Career
From 1849 to 1851 he collected plants, especially cacti, along the United States.-Mexico border, and in the process took part in the United States and Mexican Boundary Survey. His collections of cacti were sent to the herbarium at Jena. He was a resident of Berlin, where he maintained an impressive collection of succulents.
He is the taxonomic authority of numerous species within the family Cactaceae, and has a number of cacti species bearing his name, a few examples being Coryphantha poselgeriana, Echinocereus poselgeri and Mammillaria poselgeri.