Flora of North America species comparison

Asclepias viridiflora is one of the milkweeds with spherical, greenish umbels and inconspicuous coronas (see also A. engelmanniana, A. hirtella, A. lanuginosa, A. longifolia, A. rusbyi, A. stenophylla). Prior to close examination, the tight green balls of open flowers appear to be merely in bud. The diversity in leaf morphology among individuals (linear to orbiculate) is remarkable, but has no taxonomic significance—the full range of variation may be found within single populations. This is the most widespread milkweed within the flora area, ranging across most of the United States (absent only from the westernmost states and most of New England) and southern Canada. It is nowhere abundant, but may be regularly encountered in suitable, thin-soiled prairie habitats, especially in the Great Plains. It is rare and considered to be of conservation concern on the margins of its range, in Alberta (Cypress, Forty Mile, and Warner counties), Arizona (Coconino, Gila, and Yavapai counties), Connecticut (New Haven County), Florida (Gadsden and Jackson counties), and New York (Columbia, Nassau, Richmond, and Suffolk counties).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera ca. 460, species ca. 4800 (36 genera, 175 species in the flora).

From the early nineteenth into the latter half of the twentieth centuries, the pollinia- and gynostegium-bearing species were segregated as the family Asclepiadaceae Borkhausen. The structure of the combined androecium and gynoecium and their function in pollination are the most complex among eudicots (P. K. Endress 1994). Ample morphological and molecular phylogenetic evidence provided a sound basis for reuniting these families (M. E. Fallen 1986; H.-E. Wanntorp 1988; B. Sennblad and B. Bremer 1996). As yet, the sister group of Apocynaceae within Gentianales is uncertain, with some analyses indicating Gelsemiaceae and others Gentianaceae (P. F. Stevens 2001 onwards, http://www.mobot.org/MOBOT/research/APweb/). The current classification of Apocynaceae allocates genera to five subfamilies and further into tribes and, in some cases, subtribes (M. E. Endress et al. 2014). Three subfamilies that composed the former Asclepiadaceae (Asclepiadoideae Burnett, Periplocoideae R. Brown ex Endlicher, Secamonoideae Endlicher) have been repeatedly demonstrated to be monophyletic, whereas both of the remaining two subfamilies that composed Apocynaceae in the narrow sense (Apocynoideae Burnett, Rauvolfioideae Kosteletzky) are clearly paraphyletic (T. Livshultz et al. 2007). We follow the current classification here, recognizing that future revisions will be required to obtain monophyletic higher taxa. Genera of Rauvolfioideae (nos. 1–13), Apocynoideae (nos. 14–23), Periplocoideae (no. 24), and Asclepiadoideae (nos. 25–36) are present in the flora area.

Apocynaceae species possess diverse secondary compounds, some of which are potent toxins or are pharmacologically active. The indole alkaloids vincristine and vinblastine present in Catharanthus roseus are used in the treatment of Hodgkin’s lymphoma, leukemias, and other cancers. Toxic cardiac glycosides in Asclepias are sequestered by several species of milkweed-feeding insects for use in their own defense, most famously by the monarch butterfly (S. B. Malcolm 1991). Species of Vinca have long been cultivated as evergreen ground covers with showy flowers; species of Amsonia and Asclepias are also widely cultivated, especially for the nectar-rich flowers that are attractive to butterflies and other insects. Cultivated Vinca and Vincetoxicum species readily naturalize, are considered invasive, and present serious management concerns.

The following species have been reported as naturalized or escaped from cultivation, but are excluded from this treatment because convincing proof of persisting populations is lacking: Calotropis gigantea (Linnaeus) W. T. Aiton, C. procera (Aiton) W. T. Aiton, Gomphocarpus fruticosus (Linnaeus) W. T. Aiton, G. physocarpus E. Meyer, Metaplexis japonica (Thunberg) Makino, Periploca graeca Linnaeus, and Vincetoxicum hirundinaria Medikus.

Calotropis gigantea and C. procera are infrequently cultivated in California and Florida. Two collections of C. procera are known outside of cultivation. In California, the species was collected in an agricultural area in Imperial Valley (Laemmlen s.n., UCR-51373). The single individual was destroyed with no subsequent sign of naturalization in the area (A. C. Sanders, pers. comm.). In Florida, collections have been made from a soil dump (suspected to be from the Bahamas) in Hillsborough County in which numerous exotic species appeared (A. R. Franck, pers. comm.; Dickman s.n., USF-273524).

Gomphocarpus fruticosus [Asclepias fruticosa Linnaeus] and, especially, G. physocarpus [A. physocarpa (E. Meyer) Schlechter] have been cultivated in the flora region; the popularity of G. physocarpus appears to have increased dramatically in recent years because of its rapid growth and the novelty of the inflated, soft-spined follicles. Neither species is winter hardy in most of our region, although they may persist for a few years as far north as USDA Plant Hardiness Zone 7. Nonetheless, these species have not been documented often by herbarium specimens from the region, and in nearly all cases it is certain that they represent cultivated plants. A recent collection of G. physocarpus made in Ventura County, California, in 2016 (Provance 1116-02, UCR-278890) may represent the beginning of an established population. Botanists in California, Florida, and Texas should be cognizant of the possibility that these species may establish here, as they have in subtropical regions around the world.

Metaplexis japonica [Cynanchum rostellatum (Turcz.) Liede & Khanum] was reported from an Iowa State University agricultural research farm in the late 1950s and early 1960s (S. L. Welsh and D. E. Anderson 1962). It was suggested that the species may have been introduced during World War II through investigations of fiber sources. Eradication measures were undertaken, and there is no evidence from collections—or observations (D. A. Lewis, pers. comm.)—that the species persists in the state. An additional collection of the species was made in Canyon County, Idaho, in 1966 (Linford s.n., ID-106861). However, no additional collections have been made since, and western botanists report being unaware of any populations of the species in the state at this time (B. Ertter, pers. comm.; J. Smith, pers. comm.).

Until the end of the last century, Periploca graeca was cultivated as an ornamental subject across the warmer parts of the flora region. It seems to have fallen out of favor in recent times. There are several collections housed at herbaria, all of which pertain to cultivated plants or plants persisting for years to perhaps decades around homesteads. However, P. graeca does not appear to be naturalized anywhere in our region.

Vincetoxicum hirundinaria [synonyms: Cynanchum medium (Decaisne) K. Schumann, not R. Brown, C. vincetoxicum (Linnaeus) Persoon, V. medium Decaisne, V. officinale Moench] has been rarely cultivated in the region but is widely reported as a member of regional floras. Reports of naturalized populations ascribed to V. hirundinaria (or one of its synonyms) pertain to V. rossicum.

(Discussion copyrighted by Flora of North America; reprinted with permission.)