Montana Field Guide

PLANTS: A herbaceous annual that grows from a taproot. Solitary stems are ascending to erect, 20-76 cm tall, with both sparse and coarse hairs (strigose). Sources: Berry et al. in Flora of North America [FNA] 2016; Hansen personal communication.

LEAVES: Stem leaves are opposite. Blades are narrowly to broadly elliptic, sometimes lance-elliptic, 10-100 mm long and 5-35 mm wide. Leaf base is cuneate to attenuate. Margins are coarsely toothed (crenate-dentate), and revolute to almost flat. Apex is broadly acute to acuminate. Hairs on the upper and lower surfaces and margins are straight, appressed, and point in the same direction (strigose); hairs may be sparse and also stiff. Veins are pinnate with a prominent mid-vein. Sources: Giblin et al. [eds.] 2018; Berry et al. in FNA 2016

INFLORESCENCE: A terminal umbel or congested cyme that is 3-rayed, dense, flat-topped to slightly rounded, 1-3 cm broad, with 10-25 cyanthia per umbel. Principal umbel bracts (‘leaves’) are green with pale (green-white to mauve) bases, whorled, and similar in size and shape to the nearest leaves. Above the bracts is the cup-shaped involucre that contains inconspicuous flowers, which lack petals and sepals (cyathium). Cyathia (plural) have short peduncles, 0.5-1.0 mm long. Involucre is cylindric, 2.5-3.0 x 1.3-1.8 mm, glabrous. Involucral lobes divided into 5-7 linear, micropapillate lobes. Gland is singular, green, sessile, broadly attached, 0.9 x 1.3 mm, opening oblong, and glabrous. Sources: Mayfield 1997; Berry et al. in FNA 2016

Flowering and fruiting from Summer to Fall (Berry et al. in FNA 2016). Observations in Montana have found fruiting plants in late July with speculations that flowering occurs by July 1 (Hansen personal communication).

David's Spurge - Euphorbia davidii, invasive exotic
* Annual with a taproot; not rhizomatous.
* Stem leaves all opposite.
* Capsules and seeds are larger. Seeds are essential for identification.
* Leaves are more elliptic, thicker with toothed margins, and generally lacks a red blotch.
* Hairs are shorter and stiffer.

Leafy Spurge - Euphorbia virgata, exotic and Noxious
* A rhizomatous perennial.
* Stems are blue-green to pale green.
* Stem leaves are alternate below the inflorescence. Leaf blades are linear, linear-oblanceolate, or linear-oblong, 6-15 times longer than wide, margins are (almost) parallel at the middle, leaf apex is acute, and leaf base is truncate to abruptly attenuate.
* Leaf margins smooth (entire).

Euphorbia dentata, not documented in Montana
* Euphorbia dentata and Euphorbia davidii are most similar in morphology. Consult Mayfield (1997) and Berry et al. in FNA (2016) to identify and distinguish these species.
* Capsules and seeds are smaller. Seeds are essential for identification.
* Leaves are less elliptic.
* Stems and leaf undersides have scattered hairs that are longer and less stiff.

TAXONOMY & NOMENCLATURE
The genus Euphorbia was named after Euphorbus, the physician to King Juba II (Giblin et al. [eds.] 2018).

Populations of what is now called Euphorbia davidiiSubils have long been recognized taxonomically in North America as being included with Euphorbia dentata var. cuphosperma or Euphorbia cuphosperma. In the early 1980s Rosa Subils described Euphorbia davidii as a distinct species based on specimens collected in Argentina by David Anderson; hence the specific epithet of ‘davidii’. As part of his dissertation, Mark Mayfield (1997) examined members of the complex Euphorbia dentata var. cuphosperma and Euphorbia cuphosperma. Mayfield’s treatment linked the name Euphorbia davidii (South America) with the native North American plants and also recognized that Euphorbia cuphosperma is a unique species (found in Arizona to New Mexico south to Mexico and Honduras). Further he determined that the Argentinian material (including the Type specimen) was introduced into Argentina from North American populations, where it can contaminate wheat fields (Mayfield 1997; Mayfield personal communication). Misinterpretations of the taxonomy presented in Mayfield’s 1997 dissertation by BONAP (2014) and the USDA PLANTS database has led to the erroneous concept that all Euphorbia davidii in North America are non-native.

Euphorbia davidii Subils is classified within Euphorbia section Poinsettia which also includes the Christmas Poinsettia (Euphorbia pulcherrima) (Berry et al. in FNA 2016).

In North America, David’s Spurge has been found in forests, stream and river banks, prairies, roadsides, and open disturbed areas (Berry et al. in FNA 2016). In Montana plants are establishing along roadsides at interstate exchanges and in city parks.

David’s Spurge can vary greatly in size with plants growing larger in well-watered areas (Mayfield personal communication).

Plants reproduce by seed.

FLOWER [Sources: Mayfield 1997; Berry et al. in FNA 2016]
Flowers have peduncles, 0.5-1.0mm long within a small, cup-like involucre (cyathium). Each cyathium bears 5-8 stamens (male flowers) that surround a single pistil (female flower). The pedicel of the pistillate flower elongates in fruit, to 3mm long. The pistil has a glabrous or sparsely strigose ovary and 3 styles that are partially united and splits into 1 mm long branches.

In the Cited Literature section consult Stein (2011) to learn about Euphorbia ‘flowers’, cyathia.

FRUIT [Sources: Mayfield 1997; Berry et al. in FNA 2016]
Fruit is a capsule. Capsule is ovoid, 2.9-3.3 x 4-4.8 mm, 3-lobed, glabrous. Columella is 2.2-2.7 mm long. Seeds are black to brown or pale gray, ovoid to triangular ovoid, angular in cross section, 2.4-2.9 x 2.2-2.9 mm. Surface of the seed has low tubercles that are irregularly arranged or occurring in faint, transverse rows. Seeds easily roll due to their shape. Caruncle 0.9-1.1 mm.

GENETICS
Chromosome number is 2n=56, making this plant species a tetraploid among the Euphorbia section Pointsettia species (X=14).

The likely primary vector for long-distance dispersal is presumed to be as a contaminant in wheat seed (Mayfield personal communication). Humans definitively have aided its dispersal into Wyoming and Montana (Mayfield personal communication). David’s Spurge has become an agricultural weed where it has been introduced into places within North America, South America, Australia, and Russia (Berry et al. in FNA 2016), specifically into wheat-growing regions (Mayfield personal communication).

David’s Spruge does not compete well in established native grasslands unless vegetation is sparse (Mayfield personal communication).

David’s Spurge has become an agricultural weed where it has been introduced into places within North America, South America, Australia, and Russia (Berry et al. in FNA 2016).