Answer: The Mexican fan palm (Washingtonia robusta) and the California fan palm (Washingtonia filifera) are closely related and quite similar. They differ in subtle characteristics, and even palm experts have trouble telling them apart.
On the Washingtonia filifera (California fan palm), the petioles (leafstems) of young palms are green and relatively unarmed (no thorns); the basal sheath (bottom of the base of the leaf blade) does NOT have a bright tawny-colored patch; the leaflets are pendulous and swinging (not stiff), and the cottony threads persist.
In Washingtonia robusta (Mexican fan palm), the petioles are brown and distinctively thorny; the basal sheath has a bright, tawny-colored patch; the leaflets are stiff and the cottony threads fall off with age.
The photo on the site is Washingtonia robusta. Thanks for your thorough inspection of the photo - it made us do our homework!
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