Flora of North America species comparison

Euphorbia virgata has caused significant economic and ecological impact over large portions of the United States and Canada. It is part of a taxonomically complex group of species native to Europe and Asia, and there has been much confusion over the naming of the species that has become widely established in the New World (A. Radcliffe-Smith 1985; P. M. Catling and G. Mitrow 2012). There has been speculation that hybridization and polyploidy have played a role in the weediness of leafy spurge, and it is possible that the widespread occurrence of leafy spurge in North America is at least partly due to multiple introductions in grain imported from Eurasia (Ma J. S. 2010). Nonetheless, a re-evaluation of the leafy spurge complex by Berry et al. (unpubl.) revealed that E. esula Linnaeus and E. virgata are two distinct, albeit related species. The true E. esula is restricted in range to certain parts of Europe and shows little tendency toward weediness where it occurs. In contrast, E. virgata is much more widespread across Europe and temperate Asia, where it shows the same weedy characteristics as leafy spurge in the New World. More importantly, it is morphologically consistent with the North American material of leafy spurge.

According to D. V. Geltman (1998), the best way to distinguish morphologically between Euphorbia virgata and E. esula is by differences in their leaf shape. In E. virgata, the leaf blades are linear to linear-oblanceolate or linear-oblong, 6–15 times longer than wide, with margins that are parallel or almost parallel at the middle of the blade; the apex is usually acute, and the base is truncate or abruptly attenuate. In E. esula, the leaf blades are oblanceolate to obovate-elliptic (distinctly wider toward apex), 3–8(–10) times longer than wide, with margins not parallel at the middle of the leaf; the apex is rounded to subacute, and the base is gradually attenuate to cuneate.

There are some herbarium specimens of Euphorbia esula from North America that indicate it probably occurred sporadically in certain states in the late 1800s and early 1900s, but the authors have no evidence that it has persisted in any of those places. Therefore in this treatment, E. esula is considered to be a waif in the North American flora and, by excluding it here, the authors hope to avoid confusion between it and the widespread E. virgata.

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

Species ca. 2000 (139 in the flora).

Euphorbia is one of the two or three most species-rich angiosperm genera worldwide. Members of the genus occur in almost all habitat types, and many species prefer disturbed areas. Species in the genus are vegetatively highly diverse; growth forms include diminutive ephemerals, tuberous geophytes, taprooted perennial herbs, vines, various types of shrubs, trees to 25 m tall, and many xerophytic stem-succulents. Although succulents are primarily restricted to the Old World, a handful of independently derived succulents are native to the New World (B. L. Dorsey et al. 2013). Within the flora area these include E. antisyphilitica and E. tithymaloides. The striking vegetative similarity between the Old World succulent Euphorbia and New World cacti is one of the most commonly cited examples of convergent evolution.

The most distinctive feature of Euphorbia is its unique pseudanthial inflorescence, the cyathium (G. Prenner and P. J. Rudall 2007). This structure is so similar in appearance to a bisexual flower that many early botanists, including Linnaeus, actually mistook it for one. As with all Euphorbiaceae, the flowers of Euphorbia are unisexual, but in contrast to most other members of the family, they are extremely reduced. The pistillate flower comprises a single, perianth-less ovary, and the staminate flower comprises a single perianth-less stamen. Most species are likely insect-pollinated, but a few species are hummingbird-pollinated (R. L. Dressler 1957).

The latex of all of the species is abundant and in some instances highly caustic, and care should be taken to avoid exposure to it. Nevertheless, some species in the flora area have been used for medicinal purposes (for example, Euphorbia corollata and E. ipecacuanhae, C. F. Millspaugh 1892).

Euphorbia is best known for its ornamental taxa, in particular E. pulcherrima Willdenow ex Klotzsch (the Christmas poinsettia), a native of Mexico. This species is widely grown throughout the flora area but has not become naturalized. Other commonly cultivated species in the flora area include E. milii Des Moulins (crown-of-thorns), E. rigida M. Bieberstein, E. characias (Mediterranean spurge), E. marginata (snow-on-the-mountain), and E. antisyphilitica (candelilla); the last two are native species. Euphorbia “Diamond Frost” has in recent years become a popular cultivar to grow in pots or flowerbeds. Its progenitor, E. graminea, is introduced in the flora area and appears mainly associated with plantings in several southern states. In addition to several herbaceous European species that have become naturalized in North America, several others that have been recorded in the flora area in the past appear not to be persistent. These include species such as E. amygdaloides Linnaeus, E. epithymoides Linnaeus (sometimes treated as E. polychroma Kerner), E. lucida Waldstein & Kitaibel, E. paralias Linnaeus, and E. segetalis Linnaeus. In addition to these leafy taxa, numerous succulent species are commonly cultivated in botanical gardens and by private growers. Among the most popular are E. lactea Haworth, E. neriifolia Linnaeus, E. obesa Hooker, E. tirucalli Linnaeus, and E. trigona Miller. Although some of these species may persist around areas where they were previously cultivated, there is no evidence that they are actually naturalized in the region.

One of the most troublesome noxious weeds in the northern part of the flora area is leafy spurge, which was introduced from Eurasia. This species has been widely treated in North America as Euphorbia esula Linnaeus, but it turns out to be a misapplication of that name. The true leafy spurge in North America is more appropriately treated as E. virgata, a weedy species that is broadly distributed throughout temperate Europe and Asia (D. V. Geltman 1998). The actual E. esula is a related species of more restricted distribution in Europe that lacks the weedy tendencies of E. virgata (see discussion under 124. E. virgata for characters that distinguish the two). As with some of the herbaceous European waif species mentioned above, the real E. esula has been recorded historically in different parts of the flora area, but it does not appear to have persisted. It is therefore excluded here, and this should help to dispel the incorrect application of that name to leafy spurge in North America.

Historically, distinctive clades within Euphorbia were segregated into a number of satellite genera. In the flora area, these include Chamaesyce, Pedilanthus, and Poinsettia. Although these segregate genera are morphologically well-defined and monophyletic assemblages, recent molecular phylogenetic research has demonstrated that they are all nested within a broadly defined Euphorbia (V. W. Steinmann and J. M. Porter 2002; J. W. Horn et al. 2012). These molecular analyses both show Euphorbia as comprising four distinct clades, each of which is treated as a subgenus. Three of these subgenera, subg. Chamaesyce (Gray) Caesalpinius ex Reichenbach, subg. Esula Persoon, and subg. Euphorbia, are represented in the flora area. Subgenus Esula is treated here as a morphologically cohesive unit, whereas subg. Chamaesyce and subg. Euphorbia are divided into well-defined sections, which are keyed out and treated below.

The pleiochasia in Euphorbia are determinate. Each bears a whorl of pleiochasial bracts, which subtends multiple dichasial cymes, termed pleiochasial branches, that arise from a common point. The pleiochasium usually is terminated by a cyathium, but that sometimes aborts.

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