https://www.inaturalist.org/observations/32073682 by @larryhendrickson


Photo by Larry Hendrickson

[Edit 12/17/09: See http://tchester.org/plants/analysis/malacothamnus/enigmaticus.html for excellent photos and explanation of how to distinguish the 3 species of bushmallow found in SD County.]

Puzzling, ambiguous, inexplicable—an enigma. Such was the state of identification of one species of bushmallow found in San Diego County until Keir Morse (@keirmorse) and Tom Chester (@tchester) undertook a thorough analysis comparing the morphologies of 3 local taxa which resulted in their naming and describing a new species. In a paper published in Madroño in September, Morse and Chester described Enigmatic Bushmallow (Malacothamnus enigmaticus) resolving the confusion that had persisted in botanical circles for many years. The plant found only in San Diego County, previously identified as M. aboriginum in some floras and checklists, is now known as M. enigmaticus.

Enigmatic Bushmallow is a rare plant. Populations have been found in only two locations in our county, both in the desert transition: the vicinity of Culp Valley and the Laguna Mountain crest area. Like our county’s two other species of bushmallow (M. fasciculatus and M. densiflorus), Enigmatic Bushmallow is a fire follower, but fewer of its plants seem to persist long term after a fire.

Many-Flower Bushmallow (M. densiflorus) occurs in San Diego County in the foothills and west side of the mountains. Unlike our other two species, you can usually see its green stem clearly due to the absence of dense hairs. All three species may have 10 or more flowers per node. Please see Comment below by Keir Morse (this post was edited in response to his Comment).

Chaparral Bushmallow (M. fasciculatus var. fasciculatus) occurs mostly from the coast to the inland valleys in San Diego County and like M. enigmaticus the stem has dense hairs making it appear more yellow or whitish. To distinguish M. fasciculatus from M. enigmaticus examine the three slender bracts found just under the calyx (the “calyx bracts”). In M. enigmaticus the longest calyx bracts are longer (5.5 to 13 mm) than the longest calyx bracts of M. fasciculatus (2.5 to 6 mm). In addition, the bracts found at the base of the dense, compact clusters of flowers (the “stipular bracts”) of M. enigmaticus are generally wider (2 to 8 mm) than the widest stipular bracts of M. fasciculatus (0.5 to 2(-4) mm).

This was one identification puzzle that prior molecular analysis had failed to solve, but thanks to Morse and Chester we now know about this unique San Diego County endemic, Enigmatic Bushmallow.