Ronald Koes studies the genetic networks underlying the development of higher plants (plants that reproduce by seeds or spores), exploring the way in which genes control the development of a small group of undifferentiated cells into complex organisms consisting of many different cells. Koes's research focuses in particular on the flowers of higher plants. The initiation and development of flowers is key to the production of fruits and seeds – and thus plant reproduction, the yields of fruit and seed crops (including grains) and the beauty of ornamental plants. Flowers also serve as an effective model for the study of fundamental biological, cellular and molecular-genetic development processes.

Kroes mainly studies the genetic regulation of the initial stages of flower development, such as the formation of cell groups that eventually evolve into flowers and flower organs, and late processes such as the (terminal) differentiation of pigmented cells in the corolla epidermis. In addition to the Arabidopsis thaliana – a plant widely studied around the world – he conducts the majority of his research on petunias. Over the course of his research on so-called petunia lines – which feature hyperactive 'jumping' DNA strands – Koes has amassed a collection of mutants with genetic and other defects in their early and late processes, and identified the corresponding genes.  

For years, research on flower pigmentation mutants focused on the regulatory and other genes that trigger flower pigmentation synthesis. Kroes currently focuses on analysing the flower pigment and other genes involved in fundamental cell biology processes. This research requires expertise from various disciplines: Kroes's group has been working with a large number of research groups in the Netherlands and abroad – including several at the UvA's Faculty of Science – for many years now.

Koes has been affiliated with VU University Amsterdam (VU) since 1983. After earning his PhD in 1988, he worked as a postdoctoral researcher, university lecturer and senior university lecturer in the field of plant genetics. In 2006 he was named professor of Developmental Genetics at VU. He has also served as head of the Genetics Department and as board member of the Molecular Cell Biology Department since 2008. Koes is further a board member at the Dutch Experimental Plant Sciences (EPS) Graduate School, an inter-university collaboration between the participating universities' plant labs. He is also the founder and co-organiser of the International Petunia Network. Koes has published extensively in scientific journals such as Nature Cell Biology, Developmental Cell, Plant Cell, and Cell Reports.