Pediocactus simpsonii is an extremely hardy and very variable ball cactus of mountains and prairies of north America, the spines almost completely obscure the stems.

Description: Generally singular but occasionally forming small clusters. This plant is an exceedingly variable species. a bit of variation throughout its range and some taxonomists recognize several varieties. The segregation of the many forms, varieties, and subspecies on the basis of morphology, however, is difficult to support, given the continuous range of variation in stem size and flower color over its geographic range.
Stems: Depressed-ovoid to ovoid or globular stems that are 2.5-15(-25) tall × 2.5-15 cm in diameter.
Areoles: circular to oval, villous.
Tubercles: Spirally arranged, conicalabout 5 to 15 mm long.
Spines: Located on the tips of the tubercles smooth, relatively hard, distinguishable as radial and central.
Radials: 15 to 35 widely spreading, white, slender, which are 3-13 mm long.
Centrals: 4 to 11 widely spreading about 1 to 2 cm long reddish brown (rarely black) with basal half cream or yellow, rigid, straight or slightly curved. Sometimes the spines almost obscure the surface of the plant.
Flowers: 2 to 2,5 cm, bell shaped with scales and outer tepals minutely toothed and laciniate or entire and often undulate; ranging from magenta, pink, straw-yellow, to whitish outer tepals with greenish brown midstripes.. The buds and flowers originate in a small circle at the apex of the plant.
Blooming season: Open from mid April on into May. The flowers usually remain closed on cloudy days.
Fruits: Almost spherical to short cylindric 6-11 × 5-10 mm , green at first, turning tan, drying reddish brown and splitting along the side when mature.
Seeds: Gray to black, 2-3 × 1.5-2 mm, papillate but not rugose.
 

Scientific name:  Pediocactus simpsonii (Engelm.) Britt. & Rose
Published in N. L. Britton and A. Brown, Ill. Fl. N. U.S. ed. 2. 2: 570. 1913 (as simpsoni).

Origin: The species has the widest distribution of any Pediocactus, and is found at the highest elevation. throughout much of the mountain in west USA (Arizona., Colorado, Idaho, Montana., Nevada, New Mexico, South Dakota., Utah, Wyoming)

Habitat: Its preferred habitat is generally higher in elevation and cooler and moister than most other cacti. This is one of the most cold-hardy species and is found as high as 1400-3500 m. These plants can be found growing in a variety of, prairie grassland, rocky steppes, sagebrush, pinyon-juniper woodland, and montane coniferous forest habitats. This cactus is frequently concealed under larger vegetation and is frequently camouflaged under plant litter and grasses. During winter dormancy it may actually shrink below the ground surface. Interestingly, the flowers are generally pink on the eastern slope and yellowish on the western slope plants.

Conservation status: Listed in CITES appendix 2.

Common Names include: Mountain ball cactus,  Simpson’s footcactus, Simpon’s hedgehog cactus

Etymology: Named in honour to the army engineer James H. Simpson who led an expedition in Colorado

• Echinocactus simpsonii Engelmann,
  Trans. Acad. St. Louis 2:197, 1863. (as simpsoni)
• Mammillaria simpsonii (Engelmann) Jones,
  Zoe 3:302, 1893.
• Mammillaria purpusii (Engelmann) Schumann,
  Monatsschr.Kakteenk. 4:165, 1894.
• Pediocactus hermannii Marshall, Saguaroland
  Bull. 8:78, 1954.
• Pediocactus simpsonii (Engelmann) var.caespiticus Backeberg,
  Nat .Cact. & Succt. J. (GB), 14:63, 1959. nom. nud., Cactaceae, 5:2846 f. 2672 nom. inval.1961.
• Pediocactus simpsonii (Engelmann) var. hermannii Wiegand & Backeberg,
  Die Cactaceae 5:2846, 1961.
• Echinocactus simpsonii var. minor Engelmann;
• Echinocactus simpsonii var. robustior J. M. Coulter;
• Pediocactus simpsonii subsp. bensonii (Engelmann) Hochstätter;
• Pediocactus simpsonii subsp. idahoensis Hochstätter;
• Pediocactus simpsonii var. indraianus Hochstätter;
• Pediocactus simpsonii subsp. robustior (J. M. Coulter) Hochstätter;
• Pediocactusd simpsonii var. robustior (Engelmann) L. D. Benson

Cultivation: Rot prone and not the easiest cactusv  to grow on it's own roots in cultivation it need a well drained, porous soil is preferred. Partial shading, especially during the hottest part of the summer, is helpful. Water should be provided in late winter, early spring (the main growing season), and also in Autumn, when flower buds are produced and it starts growing again. Do not l water during the  winter seasons. Problems with this plants are most likely to result from improper watering. Too much water and the plant may rot; too little water may result in poor vigor or even death. They are good for outdoor cultivation in raised beds, rock gardens, balconies, window sills etc..

Frost Tolerance:  Winter hardy from -10° to -25° C (depending on origin)  

Photo of conspecific taxa, varieties, forms and cultivars of plants belonging to the Pediocactus simpsonii complex:

• Pediocactus simpsonii
• Pediocactus simpsonii (forma caespitosa)