Head of department Prof Katharine Dickinson said the fact it was now New Zealand's only stand-alone Department of Botany was testament to Otago University's continued support.
The department had an amazing heritage and had a strong future, which was shown by the inclusion of a new botany building on the university's priority development plan, Prof Dickinson said.
Events marking the 90th include a Botany symposium held today and a Moss, Liverwort and Lichen Walks and Workshops on Saturday.