Miyawaki is primarily a botanist specialized in plant ecology and seeds. He wrote a thesis on this subject in the Department of Biology at the University of Hiroshima. He then conducted field research in various parts of Japan, while working as a research assistant at the Yokohama National University, continuing his studies at the University of Tokyo.
Reinhold Tuexen (1899-1980), who headed the Federal Institute for Vegetation Mapping, invited him to Germany. Miyawaki then worked with him on the concept of "potential natural vegetation" (vegetation which would occur naturally in the absence of human intervention), from 1956 to 1958.
Returning to Japan in 1960, he applied the methods of mapping potential natural vegetation (PNV). He found relics of ancient forests still present in the vicinity of temples and shrines (surrounding Chinju-no-mori sacred groves). Inventorying over 10,000 sites throughout Japan, he was able to identify this potential flora affected by different types of human activity, including in mountainous areas, river banks, rural villages and urban areas.
From these data, he created maps of existing vegetation and maps of potential natural vegetation. His maps are still used as a basis for scientific research and impact studies, and as an effective tool for land use, diagnosis and for mapping biological corridors. These maps of potential natural vegetation serve as a model to restore degraded habitats and native plant environment.
Over a period of ten years, from 1980 to 1990, in cooperation with laboratories of phytoecology and universities, Miyawaki led botanical and phytosociological inventories to map vegetation throughout Japan, compiled into a ten-volume book with more than 6,000 pages of comment.
"Rather than scrap them, I want to bring memory objects back to life as earth resources - for the sake of repose of souls, and for the future."