Yucca baccata ssp. thornberi (McKelvey) Hochstätter Succulenta. 80 (4):166-167, 2001.

Synonym:
Yucca thornberi McKelvey in Jour. Arnold. Arb. 16:268, t 138 (1935).

Type location:
Arizona, Pima Co., Foothills of Ricon Mts., slightly north of Ricon Creek, a tributary of Pantano Wash at 1200 m.

Distribution:
Extending in southeastern Arizona from the Ricon and Santa Catalina Mountains, north to the Pinal Mountains,  east to the San Simon Valley and the Chiricahua Mountains, and southeast to the region about Bisbee.

Short description of the species:
Yucca baccata ssp. thornberi is forming crowded thicket-like clumps from 200 to 550 cm  wide, such thicket has from 6 to 25 stems and rosettes of leaves. The stems are from 65 to 150 cm long, covered with old dead leaves, standing erect or erect-ascending. and is very rarely procumbent, the stems are branching at around 130 cm above ground. Leaves yellow-green (young leaves a little glaucous (slightly bluish-green)), straight or slightly twisting, 30 to 100 cm long (occasionally up to 130 cm long!), 1,2 to 4 cm wide, concavoconvex, with the leaf margin without fibers, but later the leaf margin is splitting up as fine long fibers (7,5 to 30 cm long!). Inflorescens from 100 to 130 cm tall, scape short and slender with around 25  branchlets, 13-14 cm tall, flowers starts well below the leaves, and is only exceeding those with 1/4 to 1/2 of the length. Flowers few compared to the large inflorescence, white or cream, pendulous (bell shaped), 7,5 to 12 cm long Fruit large 12 to 18 cm long, 3,2 to 4,5 cm. fleshy, indehiscent ; Seeds black, thick, and wingless.

Notes:
According to WEBBER is this species a hybrid of Yucca baccata and Yucca arizonica, In time when someone has made DNA studies of the genus Yucca, this thesis would be tested.

Hardiness:
I haven't tried Yucca baccata ssp. thornberi outside yet, but I have two plants one which is grown in a pot which is stored dry inside a unheated greenhouse in the winter, and one which is grown in a bed inside the unheated greenhouse. None of the plants has been tested in hard frost yet, but they have survived down to -17C at night inside the unheated greenhouse.

Related species in the Series Baccatae:
Yucca arizonica
Yucca baccata
Yucca baccata ssp. thornberi
Yucca baccata ssp. vespertina
Yucca confinis

Pictures:
 

Yucca baccata ssp. thornberi near the Rincon Mountains Arizona. Photo by Craig Howe ©2000	Yucca baccata ssp. thornberi Grown in Alexander Heim's garden in Greece. Copyright 2007 Alexander Heim.

My own plants:
#911A Yucca baccata ssp. thornberi, Arizona, Nogales, fh 1178.82, sown 1998. Grown in an unheated greenhouse,
#911B Yucca baccata ssp. thornberi, Arizona, Nogales,  fh 1178.82, sown 1998. Grown in pot that are stored unheated greenhouse during the winter, and are outside during the summer,

Reference:
MCKELVEY, in Jour. Arnold. Arb. 16:268, t 138 (1935).
MCKELVEY, Yucca of the southwestern USA 1:58-64 (1938)
WEBBER, Agric. Monograph U.S.D.A. 17:27-31 (1953)