Although circumstances initially prevented him to study in that area, he later returned to natural sciences and eventually rose to a position at the largest herbarium in Germany.

Originally interested in all non-higher plants, he specialised into mushrooms and became particularly versed in tropical species sent from abroad.

He also began issuing exsiccatae, e.g., the series Kryptogamen-Typen,[1][2] and seed collections until Nolte's successor, August W. Eichler, appointed him as an assistant.

A complete autodidact, Hennings rose to become one of the foremost mycologists of his time, and particularly a specialist of tropical fungi thanks to the innumerable collections sent to Berlin from the German colonies and South America.

He had two sons from his wife Mathilde, which he had married in 1876, but lost one to illness in 1907, which, in the words of his obituarist, "paralyzed his energies and stole the pen from [his] busy hand".