Plant Species of the Borrego Desert: Polemoniaceae: Eriastrum diffusum, E. eremicum, and E. sapphirinum, woolly stars
We have three Eriastrum species in the Borrego Desert:
- E. eremicum is the most common one by far, found essentially everyplace, from our lowest elevations to well above 3000 feet. If you are in the Borrego Desert and recognize that you are seeing an Eriastrum species, if you just guess you are seeing E. eremicum, you'll be right at least 90% of the time, and probably closer to 99% of the time.
- E. sapphirinum is found only at our higher elevations on the westernmost edge of Anza Borrego Desert State Park, barely making it below the 3000 foot cutoff for our flora. It is widespread and common above 3000 feet on the desert side of the mountains, and is also common on the coastal side of the mountains.
- E. diffusum is uncommon, being found primarily in the desert transition zone of the San Felipe Valley, Earthquake / Shelter Valley, and Mason Valley, as well as in Collins Valley. It is also found in small numbers in other areas, such as near the town of Borrego Springs and along SR78. It has such a tiny flower that unless you have sharp eyes, you are unlikely to even notice it.
E. eremicum is also the easiest one to identify, from seedlings to dried-up plants, since it is the only one with a pinnately-lobed leaf that typically has five lobes. The other two species have entire (unlobed) leaves, or leaves with at most two short lobes at the base of the leaf.
The following picture shows the lobed leaves of E. eremicum compared to the two variants of the leaf for E. sapphirinum, which is lobed the same as the leaf of E. diffusum. (Note: this picture has been flipped left to right to keep the two species on the same side in all the pix below. This reverses the handedness, making a left hand into a right hand.)
The following picture shows the flowers of all three species to the same scale (the inset picture of E. diffusum comes from a different picture under different lighting conditions; see also larger version of the E. diffusum flower with scale):
The size and shapes of all three species are extremely variable, and thus are not very useful in discriminating the species. See pictures of different-sized plants of E. diffusum plants, tiny from side with pen for scale, tiny from top, middle-sized with white flower, and large. The other two species show the same dramatic variation in plant size and branching.
One characteristic that had confused Tom for years was that the Munz key gives Eriastrum eremicum as having the largest flowers, whereas when you look at the flowers in the field, it appears that E. sapphirinum has the largest flowers. The Munz key is:- corolla 15-16 mm long ... E. eremicum
- corolla smaller (8-10 mm long in description; gen 10-12 mm long in key) ... E. sapphirinum ssp. sapphirinum
Tom remembers staring at a plant of E. eremicum and thinking that there is just no way that those flowers could be larger than the ones of E. sapphirinum. You can get the same sense of disbelief by just glancing at the picture above and noting that the flower of E. sapphirinum is much larger and showier than the flower of E. eremicum, even though reading the Munz key you'd expect the opposite!
Everything suddenly became clear to us when we looked at the flowers outside of the calyx and put them side by side. E. eremicum has a much longer corolla tube, and hence has a greater corolla length, even though E. sapphirinum has a greater corolla width. Remember that a corolla size is always measured from the bottom of the corolla tube to the tips of the lobes unless it is explicitly stated that a measurement refers to a corolla width.
The Jepson Manual 2 key distinguishes E. eremicum from E. sapphirinum by whether the stamens reach the same height or not. The following picture shows that E. eremicum has one short stamen, and one long stamen. (The fifth stamen in the E. sapphirinum pictures has lost its anthers, so only its filament is left. It appears longer since the anthers extend down from the tip of the filament.)
Copyright © 2013 by Tom Chester and Kate Harper.
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Comments and feedback: Tom Chester
Updated 7 May 2013 (two typos corrected 6 April 2014)