During the British occupation 1795-1803 he gave drawing classes and with the Dutch reoccupation 1803-1806 he re-entered army service. With the British reoccupation of the Cape he settled at Plettenberg Bay in the Eastern Cape collecting botanical specimens for Joseph Mackrill 1762-1820 a practitioner with a special interest in medicinal plants. Mackrill established a garden and trial ground for crops in Somerset East occasionally sending herbarium material to the British Museum.
Wehdemanne developed an interest in indigenous trees and produced a number of sets of annotated paintings some of species new to science. Two of these ended up in the libraries of the Natural History Museum and the Royal Botanic Gardens Kew one with the National Botanical Institute in Pretoria and one with the Parliamentary Library in Cape Town. In addition to these he prepared hinged book-like boxes the wood and bark forming the box and its contents all being from the same tree species the contents consisting of fruit and seeds pressed leaves and flowers together with drawings or paintings and a description. Similar boxes were to be found only in Japan and Europe.
Wehdemann died destitute on the Baviaans River farm Lichtenstein belonging to his friend Ludwig Krebs the noted Prussian collector. He was commemorated in Ichthyosma wehdemanni Schltdl. in the Balanophoraceae which turned out to be a synonym of Sarcophyte sanguinea Sparrm.
