Henderson in Field Guide to the Palms of the Americas (the bible for any enthusiast of S. A. palms) does not consider the trunk thickness a reliable characteristic by which to distinguish filifera from robusta. According to him, robusta has a more compact crown, petioles are brown (as opposed to green in filifera) and 'markedly thorny' (as opposed to 'not markedly thorny' in filifera) but most important of all, robusta has a tawny (pale brown) patch on the base of the lower leaf surface. This works for me!

The easiest and most obvious difference between them is at the petiole bases (where the leaf attaches to the trunk). W. robusta usually has purple/reddish petiole bases, while W. filifera has green.

W. robusta also has a thinner trunk, is taller, and is a significantly faster grower than W. filifera”

One of them has bigger thorns, but I can't remember which.

The petioles (leaf stalks) of W. robusta are shorter than those of W. filifera, and they have a red streak on their undersides.

FWIW, the leaves are usually not the same shade of green. W. robusta usually has a bright green leaf, while that of W. filifera is light, bluish shade of dull green.

Differences between Washingtonia filifera and Washingtonia robusta

	Washingtonia filifera	Washingtonia robusta
Petioles (leafstems) of young palms	are green and relatively unarmed (no thorns)	are brown and distinctly thorny
Basal sheath (bottom of the base of the leaf blade)	does NOT have a bright tawny-colored patch	has a bright tawny-colored patch
Crown (of mature specimans)	has leaves in a loose and open arrangement	is dense and compact
Leaflets	are pendulous and swinging (not stiff), and the cottony threads persist	are stiff and their cottony threads fall off with age
Trunk	is barrel shaped and the palm rarely exceeds 60' in height	is slender (slightly swollen at the base) and can get 100ft (30.5 m) tall

Washington Palms

Differences between Washingtonia filifera and Washingtonia robusta