and bloom in late summer and early fall. Some species produce abundant nectar when moisture is plentiful, or when the weather is warm and sunny. The section Ptarmicoidei is sometimes treated as a separate genus Oligoneuron, and is dropped by flat-topped to rounded corymbiform flowerheads. Taxonomy Solidago is in the family Asteraceae (formerly known as Compositae), a diverse and widespread clade containing approximately 23,000 species and 12 tribes, which inhabit all continents except Antarctica. Within Asteraceae, Solidago is in the tribe Astereae and the subtribe Solidagininaeae. The genus Solidago is monophyletic as indicated by morphological characters and molecular evidence. All Solidago species are herbaceous perennials, growing from approximately 2 cm to 2.5 m tall. Yellow to white, pistillate ray flowers and yellow, perfect disc florets are characteristic of Solidago inflorescences, which have a wide range of shapes. Molecular studies using nuclear rDNA have hypothesized boundaries on the genus Solidago, but there have been difficulties in parsing out evolutionary relationships at the sub-genus scale and defining which should be included and separated from Solidago. Solidago and related taxa Related Asteraceae genera, such as Chrysoma, Euthamia, and Oreochrysum, have been included within Solidago at one point or another, but morphological evidence has suggested otherwise. In a study comparing morphological characters of Solidago and related subgroups, the authors consider the subjectivity of classifying a genus and how to define it within broader tendencies concerning the taxonomy of North American Asteraceae. Little to no differences were observed between Solidago and the subgroups in terms of karyotype. However, external morphological characters such as habit, or the general appearance of the plant and how a suite of traits contribute to its phenotype; pappus size; and the point of freeing of stamen filaments from the corolla tube, are useful classification schemes for Solidago, since they are applied to differentiating between Asteraceae taxa. One school of Asteraceae taxonomy thought unites all taxa sharing similar floral head structure and subsequently ignores deviation from this morphology, while another places greater weight on these morphological deviations. The authors argue that the latter opinion should be applied. Since there is no theoretical foundation for relative taxonomic importance of traits, they assert that habit should be a central trait when defining taxa, and subsequently that all the subgroups considered in their study (Brachychaeta, Chrysoma, Euthamia, Oligoneuron, and Petradoria) should be segregated from Solidago. Results from a leaf anatomy study comparing differences in mesophyll,