Morphological characters and statistical analyses
A total of 34 morphological characters were selected for the analyses and scored for all specimens (Table S2). Characters were selected to cover all parts of the plant generally available on herbarium specimens (stem, leaves, calyces, flowers, and sexual organs) and care was taken to avoid intrinsically dependent characters as well as characters directly related to the size/vigour of the plant. Indumentum characters are widely used, and generally considered important, in Menthataxonomy (Šarić-Kundalić et al., 2009; Yu et al., 2018) and ten characters referring to various types of trichomes were therefore included (Table S2). All characters were treated as continuous traits, although some were scored as multiple categories (Table S2). Cases of missing data were treated by mean value imputations. The morphometric data was analyzed using Principal Component Analyses (PCA) conducted in R v.3.6.3 (R Core Team, 2021) and sub-clusters of specimens were visually identified. To illustrate and assess the importance of indumentum characters, separate PCAs were conducted first excluding all characters referring to the density or structure of trichomes and secondly only including the characters of indumentum (Table S2). In addition, detailed analyses of the distributions of indumentum characters between different taxa were conducted.