Euphorbia species of the United States's Journal

Here, I have listed a lot of the resources I use and have created to identify and understand members of the genus Euphorbia in the United States. If you're new to Euphorbia and/or this project, I recommend starting with the following links:

Basic explaination of the Christmas Poinsettia
About the genus Euphorbia (Euphorbia PBI)
Euphorbia, What to Photograph?

The basic US subgroups:
Subgenus Chamaesyce section Alectoroctonum:
The eastern members of sect. Alectoroctonum
iNaturalist photos
Subgenus Chamaesyce section Anisophyllum:
Section Anisophyllum explained
iNaturalist photos
Subgenus Chamaesyce section Poinsettia:
Basic explaination of the Christmas Poinsettia
iNaturalist photos
Subgenus Esula:
Subgenus Esula explained
iNaturalist photos
Subgenus Euphorbia section Nummulariopsis:
Section Nummulariopsis
iNaturalist photos
Subgenus Euphorbia section Crepidaria:
Dressler, 1957. The Genus Pedilanthus (the illustrations, in particular, are very informative; take this as an example)
iNaturalist photos

These will give you some idea of what a Euphorbia is and what makes it different from every other plant. The following is organized by subject. For identification guides, check the "Identification resources by subgroup" or "State specific resources" near the end.

iNaturalist projects:
Euphorbia species of the United States
Euphorbia of Mexico
Sandmats of the World

iNaturalist journal posts (general):

Lists and project info:
Species list for the United States
List of species that have not been observed on iNaturalist yet
Project observation fields explained
Tracked statistics

Tips:
Euphorbia, What to Photograph?
Tips on Harvesting and Photographing Seeds

Identification and taxonomy information:
Cyathium explained (Euphorbia PBI)
Cyathium explained in detail (journal post) and tips on cyathium dissection
Advanced Seed Morphology
Euhorbia PBI data portal (for finding species information including subgeneric taxa, nomenclatural information, and more)
Species commonly identified as Euphorbias
Euphorbia subgroups explained
Euphorbia marginata (Snow-on-the-Mountain) and E. bicolor (Snow-on-the-Prairie)
It's that time of year again: The spots of Spotted Spurge (Euphorbia maculata)
Section Nummulariopsis
Euphorbia albomarginata (Whitemargin Sandmat) and E. polycarpa (Smallseed Sandmat)
Euphorbia leaf colors and patterns

Euphorbia esula/virgata information (leafy spurges)
Detailed discussion
Discussion on E. saratoi
Flora of the Southern and Mid-Atlantic States (Weakley, draft 2015) (see pg. 675)
FNA treatment

General recomended external links:
Flora of North America
BONAP (for maps)
Euphorbia PBI
Euphorbia PBI species search
Tropicos (great way to find primary literature sources)
Biodiversity Heritage Library (great way to find primary literature sources)
GBIF (great way to find herbarium records)
Encyclopedia of life (often useful if you can find a good global map)
SEINet (great way to find herbarium records and photos)
Index herbariorum (useful in understanding what the herbarium abbreviations refer to)
Flora of the Southern and Mid-Atlantic States (Weakley, draft 2015) (see pg. 669)

Identification resources by Euphorbia subgroup

Section Alectoroctonum:
The eastern members of sect. Alectoroctonum
Euphorbia marginata (Snow-on-the-Mountain) and E. bicolor (Snow-on-the-Prairie)

Section Anisophyllum:
Section Anisophyllum explained
Euphorbia albomarginata (Whitemargin Sandmat) and E. polycarpa (Smallseed Sandmat)
It's that time of year again: The spots of Spotted Spurge (Euphorbia maculata)
The Weedy Species of Sandmats (Euphorbia sect. Anisophyllum) in Texas
Nathan Taylor's thesis: Explorations into Euphorbia sect. Anisophyllum (Euphorbiaceae) in the trans-Pecos region of Texas with a focus on the Fendleri Clade (I know, the title is very long)
Varieties of E. deltoidea

Section Crepidaria:

Subgenus Esula:
Subgenus Esula explained
California Euphorbs of subgenus Esula
Texas Euphorbias, Subgenus Esula
Notes on cyathia with 5 glands
A few members of subgenus Esula sect. Helioscopia

Section Poinsettia:
Basic explaination of the Christmas Poinsettia
Poinsettia cyathia explaination

Section Nummulariopsis:
Section Nummulariopsis

State specific resources (not comprehensive and in progress)
Alabama:
Alabama Euphorbia species
Arizona:
City Spurges - Tucson
California:
Jepson eFlora
California Euphorbs of subgenus Esula
Calflora
City Spurges - San Diego
Florida:
Florida Euphorbia species
Atlas of Florida Plants
Section Nummulariopsis
Varieties of E. deltoidea
New Mexico:
The status of the genus Chamaesyce in New Mexico
Texas:
Texas Euphorbia species list
The Weedy Species of Sandmats (Euphorbia sect. Anisophyllum) in Texas
City Spurges - DFW area
Sandmats (Euphorbia sect. Anisophyllum, previously Chamaesyce) of the Llano Estacado
Texas Euphorbias, the Tithymaloids
Nathan Taylor's thesis: Explorations into Euphorbia sect. Anisophyllum (Euphorbiaceae) in the trans-Pecos region of Texas with a focus on the Fendleri Clade (I know, the title is very long)

Extralimital
New Zealand Plant Conservation Network
European Euphorbia checklist - found here
British members of subg. Esula
Some European Spurges

Provided below is a list of the Euphorbia species of Texas. I have provided a little additional information on the species outside of section Anisophyllum and sect. Alectoroctonum. I may write a separate journal post for these sections and have already written one for subgenus Esula (which can be found here). Links: iNaturalist Texas species list (not up to date), BONAP, Flora of North America.

Subgenus Esula

E. lathyris - Leaves clearly opposite, forming an X shaped pattern going up the stem; plants bluish-green
E. spathulata - Serrated leaves, no horn-like appendages on the oval glands, and warty fruits
E. texana - Just like E. spathulata but with smooth fruits
E. helioscopia - Like E. texana but typically with around 5 pleiochasial branches instead of 3 (the main branches of the inflorescence)
E. peplus - Winged fruits
E. roemeriana - Partially fused dichasial bracts (the bracts that are held in pairs)
E. brachycera - Large perennial plants usually with some triangular-ovate bracts (hard to separate unless you actually see the plants or a picture of the plants)
E. peplidion - Dichasial bracts +/- broadly lanceolate with acute apices
E. longicruris - Dichasial bracts imbricate with rounded apices, not mucronate
E. austrotexana - Leaves narrow, linear to oblanceolate or narrowly lanceolate
E. tetrapora - Dichasial bracts mucronate; stems erect, unbranched at the base
E. helleri - Dichasial bracts mucronate; stems ascending, branched at the base; lower leaves notably emarginate, leaves except dichasial bracts spathulate or oblanceolate.

Section Poinsettia

Glands appendaged
E. bifurcata - Plants annual, bracts not linear, broadest at the apex; glands with petaloid appendages, 1(-3).
E. exstipulata - Plants annual, bracts linear to lanceolate to nearly ovate or rhombic, typically broadest at the middle or base; glands with petaloid appendages, 4 per cyathium.
E. eriantha - Plants perennial, bracts green and linear; glands obscured by fuzzy appendages, 4 per cyathium.
Glands unappendaged. Lower leaves alternate, stems glabrous
E. cyathophora - Plants annual, tall in bloom, bracts typically red basally (often completely green in central Texas), linear or broader; glands oblong, 1 per cyathium.
E. heterophylla - Plants annual, tall in bloom, bracts green (sometimes whitish basally); glands circular.
E. radians - Plants perennial, very short in bloom, bracts pink; glands oblong, 1-5 per cyathium.
Glands unappendaged. Lower leaves opposite, stems hairy
E. davidii - Seeds slightly angled, tubercles unevenly distributed.
E. dentata - Seeds essentially round in crossection, tubercles evenly distributed.

Section Alectoroctonum

E. antisyphilitica
E. bicolor
E. bilobata
E. corollata
E. discoidalis
E. graminea
E. hexagona
E. innocua
E. marginata
E. strictior
E. wrightii

Section Anisophyllum

E. abramsiana
E. acuta
E. albomarginata
E. arizonica
E. astyla
E. bombensis
E. capitellata
E. carunculata
E. chaetocalyx
E. chaetocalyx triligulata
E. cinerascens
E. cordifolia
E. cryptorubra
E. fendleri
E. geyeri
E. geyeri wheeleriana
E. glyptosperma
E. golondrina
E. hirta
E. humistrata
E. hypericifolia
E. hyssopifolia
E. indivisa
E. jejuna
E. laredana
E. lata
E. maculata
E. micromera
E. missurica
E. meganoesos
E. nutans
E. ophthalmica
E. parryi
E. perennans
E. prostrata
E. revoluta
E. serpens
E. serpillifolia
E. serrula
E. setiloba
E. simulans
E. stictospora
E. theriaca
E. theriaca spurca
E. velleriflora
E. vermiculata
E. villifera

Provided below is a list of the Euphorbia species of Alabama. I have provided a little additional information on the species outside of section Anisophyllum and section Nummulariopsis. I may write a separate journal post for section Anisophyllum and have already written one for section Nummulariopsis (which can be found here). Links: iNaturalist Alabama species list (not updated), BONAP, Flora of North America.

Subgenus Esula

E. commutata - Angles of fruits essentially smooth; native.
E. peplus- Angles of fruit with two wings; introduced.
E. spathulata - Angles of fruits warty; native.

Section Poinsettia

Lower leaves alternate, stems glabrous
E. cyathophora - Bracts typically red basally, linear or broader; glands oblong, 1 per cyathium.
E. heterophylla - Bracts green (sometimes whitish basally); glands circular.
Lower leaves opposite, stems hairy
E. dentata - Seeds essentially round in crossection, tubercles evenly distributed.

Section Alectoroctonum

Most petaloid appendages as long as the glands are wide or shorter
E. mercurialina
Petaloid appendages at least 2-3 times as long as the glands are wide or longer
E. corollata - Leaves lanceolate or wider, not revolute; appendages longer than 2.2 mm long.
E. pubentissima - Leaves lanceolate or wider, not revolute; appendages shorter than 2.2 mm long.
E. discoidalis - Leaves linear to ocassionally ovate, revolute; appendages shorter than 2.2 mm long.

Section Nummulariopsis

E. inundata var. inundata
E. floridana

Section Anisophyllum (11)

E. bombensis
E. cordifolia
E. hirta
E. humistrata
E. hypericiolia
E. hyssopifolia
E. maculata
E. nutans
E. ophthalmica
E. prostrata
E. serpens

Subgenus Esula

Euphorbia peplus

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): BJ Stacey (click here for observation).

Euphorbia terracina

Photo credit (upper left): Jay Keller (click here for observation). Photo credit (upper right): Jay Keller (click here for observation). Photo credit (lower left): Jay Keller (click here for observation). Photo credit (lower right): Jay Keller (click here for observation).

Euphorbia lathyris

Photo credit (left): madge (click here for observation). Photo credit (right): Carol Blaney (click here for observation).

Euphorbia rigida

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): Jay Keller (click here for observation).

Euphorbia helioscopia

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): Jay Keller (click here for observation).

Euphorbia segetalis

Photo credit: Jay Keller (click here for observation).

Section Alectoroctonum

Euphorbia misera

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): Jay Keller (click here for observation).

Section Anisophyllum

Euphorbia polycarpa

Photo credit (upper left): katgrom (click here for observation). Photo credit (upper right): Jay Keller (click here for observation). Photo credit (lower left): Millie Basden (click here for observation).

Euphorbia maculata

Photo credit (upper left): Jay Keller (click here for observation). Photo credit (upper right): BJ Stacey (click here for observation). Photo credit (lower left): Jay Keller (click here for observation). Photo credit (lower right): Jay Keller (click here for observation).

Euphorbia serpens

Photo credit (upper left): Jay Keller (click here for observation). Photo credit (upper right): Jay Keller (click here for observation). Photo credit (lower left): Jesse Rorabaugh (click here for observation).

Euphorbia hypericifolia

Photo credit (upper left): Jay Keller (click here for observation). Photo credit (upper right): Jay Keller (click here for observation). Photo credit (lower left): Jay Keller (click here for observation).

Other groups

Euphorbia tirucalli

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): Jay Keller (click here for observation).

Subgenus Esula has some confusing terminology associated with it because of its odd morphology. In a key like the one in Flora of North America, you're bound to stumble on terms like pleiochasial and dichasial. This is designed to explain those terms and help you understand the overall morphology.

Phyllotaxy (leaf arrangement):
Let's start with the basics. A bract is a modified leaf subtending (directly below) a reproductive structure. This can be an inflorescence, a flower or a cyathium (you can read all about the cyathium here). With some exceptions (like E. trichotoma), the plants have three sets of leaves (two sets of bracts): alternate (alternating; not grouped or paired at the same node) non-flowering leaves, whorled bracts subtending the entire inflorescence (pleiochasial bracts), and opposite (paired) bracts of the cyathia after the first one (dichasial bracts). If you want to get technical about it, the plants actually have three sets of bracts considering the cyathial bracts, but we won't get into that here. In the United States, there are two exceptions. One is E. trichotoma which has alternate dichasial bracts and the other is E. lathyris which has opposite leaves.

Left: Euphorbia helleri. Right: Euphorbia longicruris.

Cyathial glands:
There is a decent amount of diversity in the glands of these plants. In some, the outer edges are roughened (technically crenulate). In others, the glands are completely smooth. In most species in the United States, the glands have horn-like projections at the edges. These horns can be about the same color as the gland or white.

Left: Euphorbia spathulata, glands (yellow structures) without horns. Right: Euphorbia peplus, glands with horns.

Fruit texture:
There are three major categories of fruit texture: smooth, winged, and rough. In the United States, only E. peplus has winged fruits. These "wings" occur along the angles of the fruit where the fruit splits at maturity (along three lines of dehiscence). Rough fruits are a bit more complicated but most extreme in the case of warty fruits (most commonly seen in E. spathulata). Some are simply granulate (with many tiny bumps) as in E. cyparissias.

Left: Euphorbia spathulata (warty fruits) and E. texana (smooth fruits). Right: Euphorbia peplus (winged fruits).

Related posts:
Texas Euphorbias, Subgenus Esula
California Euphorbs of subgenus Esula

Observations from the area.

Subgenus Esula - More identification information here.

E. spathulata - Leaves serrated; no horn-like appendages on the oval glands; fruits warty.

Photo credit (left): Nathan Taylor (click here for observation). Photo credit (right): Nathan Taylor (click here for observation).

E. longicruris - Leaf edges smooth (entire); paired (dichasial) bracts imbricate with rounded tips (apices), not mucronate; glands with horn-like appendages; fruits smooth.

Photo credit (left and middle): Nathan Taylor (click here for observation). Photo credit (right): Bob O'Kennon (click here for observation).

E. tetrapora - Leaf edges smooth (entire); paired (dichasial) bracts with short, abrupt point (mucronate); stems upright, unbranched at the base; glands with horn-like appendages; fruits smooth.

Photo credit (left): Sam Kieschnick (click here for observation). Photo credit (right): markg (click here for observation).

Section Poinsettia

Lower leaves alternate, stems glabrous
E. cyathophora - Bracts sometimes red basally, but often completely green in the DFW area, they may be linear or quite broad.

Photo credit (left): Sam Kieschnick (click here for observation). Photo credit (right): marytmn (click here for observation).

Lower leaves opposite, stems hairy
E. davidii - Seeds slightly angled, tubercles unevenly distributed.

Photo credit: Mark Mayfield (click here for observation).

E. dentata - Seeds essentially round in crossection, tubercles evenly distributed.

Photo credit: Mark Mayfield (click here for observation).

Section Alectoroctonum

Showy bracts with white edges; showiest part of the plant the leaves and bracts.
E. bicolor - Uppersides of leaves and bracts hairy; more information here.

Photo credit (left): Sam Kieschnick (click here for observation). Photo credit (right): mmkimberly (click here for observation).

E. marginata - Uppersides of leaves and bracts glabrous; more information here.

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): niamora (click here for observation).

Bracts small or obsolete, not showy; showiest parts of the plant the petal-like appendages of the flower-like cyathia.
E. corollata

Photo credit (left): sehnature (click here for observation). Photo credit (right): Annika Lindqvist (click here for observation).

Section Anisophyllum
Also read here.

E. hypericifolia - Identification information here.

Photo credit (left): Meghan Cassidy (click here for observation). Photo credit (right): Nathan Taylor (click here for observation).

E. hyssopifolia - Identification information here.

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): Nathan Taylor (click here for observation). Photo credit (right): Nathan Taylor (click here for observation).

E. nutans - Identification information here.

Photo credit (left): Chuck Sexton (click here for observation). Photo credit (right): Nathan Taylor (click here for observation).

E. hirta - Identification information in the comments section here.

Photo credit (left): Jed Aplaca (click here for observation). Photo credit (right): Nathan Taylor (click here for observation).

E. ophthalmica - Identification information here.

Photo credit (left): Jay Keller (click here for observation). Photo credit (right): howardhorne (click here for observation).

Euphorbia humistrata - Like E. maculata but with longer style branches and stems that often root at the nodes closest to the base of the plant (I have seen one E. maculata root from the stems when buried in wet sand and it rooted at the internodes, not the nodes). The leaves are typically broader near the apex, but this is variable. The cyathia and fruits are usually pinker, but don't be misled by this characteristic as E. maculata often has pink cyathia and fruit as can be found here and here. It is worth noting that any late season stressed plant within this section may produce shorter styles. Therefore, if you compare a late season stressed E. humistrata with a non-stressed E. maculata, the styles can be of the same length. If you think a plant is late season stressed and it doesn't root at the nodes, it is possible to distinguish the two by the seeds (see discussion on extracting and photographing seeds here). The seeds of E. maculata have several rounded ridges, while those of E. humistrata have none.

Euphorbia humistrata appears to be restricted to edges of lakes and has currently only been found in two localities: Simonds Lake Park (observations here and here) and Valley Creek Park (observations here and here). It is fairly safe to assume that any weedy plant away from a persistant body of water is E. maculata.

Photo credit: Annika Lindqvist (click here for observation).

Euphorbia maculata - Identification information here. Information about the "spot" here. Upper right photo: E. maculata (left), E. prostrata (center), E. stictospora (right).


Photo credit (upper left): Bob O'Kennon (click here for observation). Photo credit (upper center): Adam Cochran (click here for observation). Photo credit (upper right): Nathan Taylor (click here for observation; photo includes E. maculata, E. prostrata, and E. stictospora). Photo credit (lower left): Nathan Taylor (click here for observation). Photo credit (lower center): taharms48 (click here for observation).Photo credit (lower right): sehnature (click here for observation).

E. stictospora - Identification information here. Lower photo: E. maculata (left), E. prostrata (center), E. stictospora (right).


Photo credit (left): sehnature (click here for observation). Photo credit (right): Sam Kieschnick (click here for observation). Photo credit (lower): Nathan Taylor (click here for observation; photo includes E. maculata, E. prostrata, and E. stictospora).

E. prostrata - Identification information here. Lower right photo: E. maculata (left), E. prostrata (center), E. stictospora (right).


Photo credit (upper left): Sam Kieschnick (click here for observation). Photo credit (upper center): sehnature (click here for observation). Photo credit (upper right): Sam Kieschnick (click here for observation). Photo credit (lower left): Bob O'Kennon (click here for observation). Photo credit (lower right): Nathan Taylor (click here for observation; photo includes E. maculata, E. prostrata, and E. stictospora).

E. serpens - Identification information here.


Photo credit (upper left): Sam Kieschnick (click here for observation). Photo credit (upper right): Sam Kieschnick (click here for observation). Photo credit (lower left): sehnature (click here for observation). Photo credit (lower right): mkc123 (click here for observation).

E. glyptosperma - Identification information here.

Photo credit (both): Nathan Taylor (observatins here and here).

E. cordifolia

Photo credit (both): Sam Kieschnick (click here for observation).

E. missurica

Photo credit (both): Sam Kieschnick (observations here and here).

In the interest of better understanding what species might potentially "escaped" and become naturalized in the United States, it is probably worthwhile to at least try to list the exotic species that are cultivated here. The majority of the non-succulents are in the subgenus Esula and these form all the exotic species that are problematic. The other exotics are primarily succulent and are in the subgenus Euphorbia. I will not list all the plants that are too frost sensitive to be planted outdoors (at least, not at present; species with an "*" are too frost sensitive in most of the US) and will leave out almost all of the indoor succulents.

Subgenus Esula
Euphorbia amygdaloides var. amygdaloides
Euphorbia amygdaloides var. robbiae
Euphorbia characias subsp. characias
Euphorbia characias subsp. wulfenii
E. x martinii (E. amygdaloides var. robbiae x characias)
Euphorbia cornigera
Euphorbia cyparissias
Euphorbia griffithii
Euphorbia myrsinites
Euphorbia palustris
Euphorbia paralias
Euphorbia polychroma
Euphorbia rigida
Euphorbia niciciana (syn. Euphorbia seguieriana subsp. niciciana; like E. cyparissias)
Euphorbia wallichii

Potential species (less likely)
Euphorbia hyberna
Euphorbia mellifera
Euphorbia dendroides
Euphorbia atropurpurea
Euphorbia coralloides
Euphorbia dulcis
Euphorbia virgata
Euphorbia esula subsp. tommasiniana

Subgenus Euphorbia
Euphorbia mauritanica*
Euphorbia milii*
Euphorbia polygona*
Euphorbia tirucalli*
Euphorbia trigona*

Subgenus Chamaesyce section Alectoroctonum:
Euphorbia antisyphillitica (native)
Euphorbia corollata (native)
Euphorbia fulgens*
Euphorbia graminea
Euphorbia marginata (native)
Euphorbia xanti*

Other:
Euphorbia cotinifolia*
Euphorbia cyathophora (native)
Euphorbia pulcherrima*
Euphorbia tithymaloides*

Sources:
gardenia.net
Missouri Botanical Garden
Dave's Garden