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Agave parrasana
 
Dense, clumping rosettes to 24″; Best in full sun.
 
  Hardy to 15F 
  Full Sun to Part Sun
  Cactus Soil
  Low Water

This plant is not actually from Big Bend NP, but is a cultivar called Big Bend. The species is actually from Mexico and is called more commonly called the Cabbage Head Agave. Interesting!