In the early 1860s, Crépin’s life changed dramatically.
It began with the publication of his Manuel de la Flore de Belgique in 1860. This flora - a book detailing the plants of a particular region or habitat - contained identification keys and short descriptions of all the plant species found in the wild in Belgium. It was the result of his botanical exploration and study from the previous decade and was an immediate success. Five editions were published, with numerous reprints. Until the mid-1930s, it remained the Belgian standard flora for both amateurs and academics.
In 1861, the self-taught botanist was nominated to teach botany at the Ecole d’Horticulture de l’Etat in Gentbrugge. When, in 1862, the Société royale de Botanique de Belgique came into existence, Crépin was immediately elected as a member of the board. Along with the society’s president, Barthélemy Dumortier (1797-1878), he was before long recognised as the leading figure of the Société. In its bulletin, Crépin published papers on the flora of Belgium, which revealed his special interest in plant geography and roses.
During the 1860s, Crépin’s research goals became ever more focused on roses.
In 1866, the expanded species list of roses in the second edition of the Manuel included Rosa coronata, a species from the Belgian Ardennes described (or ‘created’ as some preferred to say) by Crépin himself. Almost certainly influenced by Dumortier, who had published a monograph of the genus Rosa in Belgium in 1867, lifelong bachelor Crépin took up the major challenge of writing a world monograph of the genus Rosa.
This was not a spontaneous notion, as the idea had been fermenting in his mind for years. As early as 1859, he had told Alexandre Boreau that he counted on the support of other rose aficionados in order to broaden his field of study to include all European roses.
In 1869, he published the first part of Primitiae Monographiae Rosarum, a series of building blocks for the world monograph which he intended to finish within a period of ten to twelve years. In it, he told his readers that he had given up his resistance to ‘transformism’, a term used in those days to describe the slow process of gradual evolution of plant and animal species through geological time. He had come to the conclusion that any endeavour to achieve a natural classification of roses only made sense within an evolutionary framework.