Isocoma acradenia, alkali goldenbush

I've never been a big fan of the varieties of Isocoma menziesii, and I'm not crazy about the varieties of Isocoma acradenia, either.

Aaron Schusteff details the problems in separating the varieties and the inconsistencies in the descriptions in the floras for the varieties of I. acradenia

The key to distinguish our two varieties in the Jepson Manual Second Edition is:

2. Leaves all entire or proximal leaves toothed or lobed ... var. acradenia
2' Leaves all toothed or pinnately lobed ... var. eremophila

I and colleagues often find one plant of one variety amidst hundreds of plants of the other variety, which is clearly nonsense. We've also found a dramatic variation in the leaves on a single plant, ranging from just about as lobed / toothed as you can get on new leaves, to essentially-entire older leaves. See the pictures from Borrego Palm Canyon below, which are of a single plant.

Nonetheless, we have been assigning varieties to the plants we see in the Borrego Desert, based on the margin of the leaves being predominantly entire or predominantly toothed. That doesn't mean we believe in the varieties; only that this allows us to study the distribution of the putative varieties to see if there is geographic coherence in them. The San Diego Plant Atlas distribution map, of specimens all determined by Jon Rebman, does appear to show some geographic separation in general. Some areas appear to have only var. eremophila. Although there may be a few areas with only var. acradenia, most of the locations with var. acradenia also have specimens of var. eremophila at exactly the same locations, or very nearby.

This rest of this page gives some pictures of the variation from entire leaves to very toothed leaves, for our reference in calling plants we see. Plants near the top of the following pictures would be called var. acradenia; plants near the bottom would be called var. eremophila. Click on the pictures for larger versions.


Grapevine Canyon, 15 November 2011, var. acradenia

Grapevine Canyon, 15 November 2011, var. acradenia

Grapevine Canyon, 15 November 2011, var. acradenia
(intentionally left blank)

Bighorn Canyon, 7 February 2014, var. acradenia

Bighorn Canyon, 7 February 2014, var. acradenia

Arroyo Salado, 19 December 2008, var. acradenia

Rockhouse Canyon off Coyote Creek, 15 January 2009, var. acradenia?

Ella Wash, 4 December 2008, var. eremophila

Ella Wash, 4 December 2008, var. eremophila

Borrego Palm Canyon, 15 February 2010, var. eremophila

Borrego Palm Canyon, 15 February 2010, var. eremophila

Borrego Palm Canyon, 15 February 2010, leaves of var. eremophila and var. acradenia (see leaf at bottom left edge) on the same plant!

Borrego Palm Canyon, 15 February 2010, leaves of var. eremophila and var. acradenia on the same plant!


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Copyright © 2014 by Tom Chester.
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Comments and feedback: Tom Chester
Updated 10 February 2014