Euphorbia albomarginata (Whitemargin Sandmat) and E. polycarpa (Smallseed Sandmat)

At most times of the year, there is a rather constant influx of E. albomarginata and E. polycarpa observations. Because of their similar appearance to each other at various parts of their respective ranges, many end up misidentified. Hopefully, this will help provide a reference for people.

The main way to separate the two is by looking at the stipules:

Euphorbia albomarginata (left); Euphorbia polycarpa (right). Photo credits: (left) dlbowls; (right) Jay Keller (used with permission).

Varieties of E. polycarpa

In the United States, essentially two varieties exist (not currently formally recognized due to complications in the phylogeny; for more details read Maya-Lastra's phylogeny paper and the Flora of North America treatment). The two varieties are separated by the peninsular ranges (including the Santa Rosa Mnts. and San Jacinto Mnts.) and transverse ranges (the San Gabriel and San Bernadino Mnts.) in California. The two varieties differ primarily in the size of the glandular appendages but may include other more subtle differences in stipules and habit. When comparing to E. albomarginata, the plants are rarely confused and easily distinguished east of the previously mentioned mountain ranges (E. polycarpa produces somewhat mounding plants with very small appendages). However, the plants west of these mountain ranges are more difficult to separate from E. albomarginata. The difference is that the apical stipules of E. polycarpa are divided (rarely united into a lanceolate segment) and relatively insignificant, the stems don't root at the nodes, the plants may be hairy, and the leaves lack a small, white membranous margin around the leaf blades. Another complication is that the stipules of E. polycarpa are often fused on the undersides of stems. A close look at the nodes may alleviate this complication. The nodes of E. polycarpa lack the prominent bumps of root initials which are usually present in E. albomarginata.

It is also worth noting that E. polycarpa may be completely hairless or densely hairy on both sides of the mountain ranges. This variation has been given varietal status in the past but shows no distributional trends and both forms may pop up in the same population.


Euphorbia polycarpa west of previously mentioned California mountains (left); Euphorbia polycarpa east of previously mentioned California mountains (right). Photo credits: (left) katgrom; (right) Jay Keller (used with permission).


Euphorbia polycarpa stipules (west of mountains). Photo credits: (left and right) Jay Keller (used with permission).


Euphorbia polycarpa, hairy form (east of mountains). Photo credits: Jay Keller (used with permission).


Euphorbia albomarginata (left); Euphorbia polycarpa (right). Photo credits: (left) dlbowls; (right) katgrom.


Euphorbia albomarginata leaf membranes. Photo credits: (left) lonnyholmes; (right) Alex Abair.


Left: iNaturalist observations of E. polycarpa. Right: iNaturalist observations of E. albomarginata.

Posted by nathantaylor nathantaylor, August 16, 2018 03:45

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