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Psorothamnus arborescens

Psorothamnus arborescens (Torr. ex A. Gray) Barneby

Mojave Indigobush

Fabaceae (Pea family)


USDA Symbol: PSAR4

USDA Native Status: L48 (N)

"Stiffly irregularly branching, often thorny shrubs up to 3-10 dm tall, variably pubescent to almost glabrous, the foliage gray-tomentulose, silky-canescent, thinly pilosulous, or green and glabrate, the branchlets at nodes immediately preceding the terminal peduncles usually persistent as rigid lignified spines, the young parts (especially the raceme-axis) charged with subulate or prickle-shaped glands, the leaflets gland-pustulate beneath (the glands sometimes concealed by vesture)." (bibref: 1812).


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Plant Characteristics

Duration: Perennial
Habit: Shrub
Leaf Arrangement: Alternate
Size Notes: "3-10 dm tall." (bibref: 1812).
Leaf: "Leaf-spurs up to 1 mm long; stipules narrowly subulate to Unear-attenuate, 0.7-2 mm long, soon papery, brownish, fragile; intrapetlolular glands 0; post-petiolular glands 0 or minute; leaves {1) 1.3-3.5 (5.5) cm long, petiolate, with margined rachis and (3) 5-7, rarely up to 17 leaflets variable in outline according to varietas from ovate or lanceolate to linear, the terminal one often continuous with the rachis, the last pair often confluent, sometimes all decurrent on rachis, sometimes all sessile but jointed, the leaf rarely reduced to a simple blade." (bibref: 1812).
Flower: "Raceme sessile or subsessile (the lowest bracts sometimes foliose, or a branchlet intervening between the lowest and succeeding flowers), loosely but sometimes shortly 8-40-flowered, the flowers spreading-ascending, the axis (1) 1.5-12 cm long, becoming stiff and persisting after fall of the pods; bracts papery, narrowly subulate or linear, 2.5-4 mm long, deciduous; bracteoles similar but shorter, 1-2 mm long; calyx 5-9 mm long, externally glabrous to variably pubescent, the hypanthium 1.5-3 mm deep, the tube (measured to a lateral sinus) 3-4.5 mm long, the ribs becoming bluntly prominent, the firm intervals charged with small brownish glands, the 2 broad ventral intervals with 2 rows, the rest with 1 row of 3-5 glands, the teeth dissimilar, the ventral pair broadly lanceolate to triangular short-acuminate, 2-4.5 mm long (1 mm shorter to 0.5 mm longer than tube), (1.2) 1.4-2 mm wide near base, the rest lanceolate, half as wide, the dorsal one often shortest, the sinus behind the banner commonly much shallower than the rest; petals indigo-blue or violet-purple, glandless, deciduous; banner 6.3-10.5 mm long, its claw very short, its blade flabellate or flabellate-obovate, deeply retuse, at base either cuneately tapering or subauriculate; wings a trifle longer than banner, the claw 2.2-3.6 mm long, the broadly oblong or oblong-obovate, obtuse or subemarginate blade 5.4-8.2 mm long, 2.2-3.5 mm wide; keel usually a trifle longer than wings, rarely a little shorter, 8.2-11.2 mm long, the claws 2.6-3.7 mm, the obliquely obovate blades 5.9-8.2 mm long, 3.3-4.9 mm wide; androecium (5.5) 6.5-9.8 mm long, the filaments free through (3) 3.5-5.6 mm, the anthers (0.85) 0.9-1.3 mm long." (bibref: 1812).
Fruit: "Pod obliquely ellipsoid or ovoid-eUipsoid, the subcompressed body 8-10 mm long, (3.6) 4-6 mm diam, at least beyond the middle 2-carinate by the sutures, the convex valves charged with large, round or broadly elliptic blister-glands, between the glands either villosulous or glabrous, the ventral suture ciliolate; seed (4.7) 5-6 mm long." (bibref: 1812).

Bloom Information

Bloom Color: Blue , Purple , Violet
Bloom Time: Jan , Feb , Mar , Apr , May , Jun , Dec
Bloom Notes: "Petals indigo-blue or violet-purple." (bibref: 1812).


USA: AZ , CA , NV , UT
Native Distribution: "Southwestern Mohave Desert in western San Bernardino and adjoining Kern counties, California; northwestern Sonora; north-central Arizona.(P. a. var. arborescens and P. a. var. pubescens); also northern Colorado to northern Mohave Desert, Owens and Death valleys, and Mineral County, Nevada, local in Coachella and San Jacinto valleys and vicinity. Riverside and southern San Bernardino counties, California (P. a. var. minutifolius and P. a. var. simplicifolius)." (bibref: 1812).
Native Habitat: As given for the varieties. (bibref: 1812).

National Wetland Indicator Status

This information is derived from the U.S. Army Corps of Engineers National Wetland Plant List, Version 3.1 (Lichvar, R.W. 2013. The National Wetland Plant List: 2013 wetland ratings. Phytoneuron 2013-49: 1-241). Click here for map of regions.

Additional resources

USDA: Find Psorothamnus arborescens in USDA Plants
FNA: Find Psorothamnus arborescens in the Flora of North America (if available)
Google: Search Google for Psorothamnus arborescens


Record Modified: 2020-12-07
Research By: JAM

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