Southern Seaside Goldenrod
Solidago mexicana L. is native to coastal marshes and dunes from Maryland to Texas and into Mexico south to Tabasco state at 0-30 m ele. The species's range is included in the map for S. sempervirens.; the gaps in distribution along the east coast of Tamaulipas and Veracruz and Tabasco states in Mexico may be an artifact of the small sample size. The species is characterized by its usually elongated narrow secund conical inflorescences, numerous thick stem leaves that are reduced in size upward, and heads with 7–11 Ray florests and 10-16 disc florets (Semple & Cook 2006 FNA; Semple et al. 2016). Inflorescence shape ranges from ones similar to inflorescences of S. sempervirens to ones similar to S. virgata (= S. stricta sensu authors not Ait.) but with slightly secund apices. Head size varies due to several ploidy levels; diploid 2n=18 and tetraploid 2n=36. Some plants in Texas and Louisiana can be very similar to S. virgata; these are tetraploids. Hybridization of diploids with diploid S. virgata seems likely along coastal Florida.
The species was treated as S. sempervirens subsp. mexicana (L.) Semple in FNA, but is treated here as a species based on a multivariate study of the S. sempervirens complex (Semple et al. 2016). While it has always been viewed as a smaller headed relative of S. sempervirens with generally fewer florets, S. mexicana has much more reduced upper stem leaves than S. sempervirens and is more likely to be confused with S. virgata over much of its range. It also shows similarities with S. maya (Semple 2016), which has fewer more widely spaced upper stem leaves and occurrence at 460-2200 m ele. It might be confused with S. paniculata from areas around Mexico City and west across Michoacán state in boggy and marshy areas.