| Pediocactus simpsonii | 
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| Pediocactus simpsonii blooming Mount Falcon Park, Jefferson County, Colorado | 
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| Scientific classification | 
| Kingdom: | Plantae | 
 
| Clade: | Tracheophytes | 
 
| Clade: | Angiosperms | 
 
| Clade: | Eudicots | 
 
| Order: | Caryophyllales | 
 
| Family: | Cactaceae | 
 
| Subfamily: | Cactoideae | 
 
| Genus: | Pediocactus | 
| Species: | P. simpsonii | 
| Binomial name | 
| Pediocactus simpsonii 
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| Synonyms | 
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 Echinocactus simpsonii  Engelm. (1863)
 Echinocactus simpsonii f. minor  (Engelm.) Schelle (1907)
 Echinocactus simpsonii var. minor  Engelm. (1863)
 Echinocactus simpsonii var. robustior  J.M.Coult. (1896)
 Mammillaria purpusii  K.Schum. (1894)
 Mammillaria simpsonii  (Engelm.) M.E.Jones (1893)
 Mammillaria spaethiana  Späth (1895)
 Pediocactus hermannii  W.T.Marshall (1954)
 Pediocactus nigrispinus subsp. indranus  (Hochstätter) Hochstätter (2003)
 Pediocactus robustior  (J.M.Coult.) Arp (1972)
 Pediocactus simpsonii f. acklinii  Hochstätter (1997)
 Pediocactus simpsonii subsp. bensonii  Hochstätter (1995)
 Pediocactus simpsonii var. caespiticus  Backeb. (1961)
 Pediocactus simpsonii f. campestris  Hochstätter (1997)
 Pediocactus simpsonii f. dinosauriensis  Hochstätter (1997)
 Pediocactus simpsonii f. flaminggorgensis  Hochstätter (1997)
 Pediocactus simpsonii f. flinspachii  Hochstätter (1997)
 Pediocactus simpsonii var. hermannii  (W.T.Marshall) W.T.Marshall (1957)
 Pediocactus simpsonii subsp. idahoensis  Hochstätter (1997)
 Pediocactus simpsonii subsp. indranus  (Hochstätter) Hochstätter (1995)
 Pediocactus simpsonii var. indranus  Hochstätter (1990)
 Pediocactus simpsonii f. kuenzleri  Hochstätter (1997)
 Pediocactus simpsonii var. minor  (Engelm.) Cockerell (1918)
 Pediocactus simpsonii f. montanensis  Hochstätter (1997)
 Pediocactus simpsonii f. muehlii  Hochstätter (1997)
 Pediocactus simpsonii f. prairie  Hochstätter (1995)
 Pediocactus simpsonii subsp. robustior (J.M.Coult.)  Hochstätter (1995)
 Pediocactus simpsonii f. sandiamontanus  Hochstätter (1997)
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Pediocactus simpsonii, known by the common names mountain cactus, snowball cactus, and mountain ball cactus, is a relatively common cactus that has adapted to survive in cold and dry environments in high elevation areas of the western United States. It can be found at higher elevations than any other cactus in North America. While not a landscape dominating plant, it is a relatively common species and the most common member of the genus Pediocactus. Because of its beauty and adaptation to cold environments it is sometimes grown by gardeners in areas that have few other choices due to the limited number of cactuses with cold adaptations. Like many cactuses its populations are sometimes threatened by this desirability due to the theft or removal of plants from the wild by collectors.