Not a palm tree, but close.

Yucca rostrata.

Okay, not really. Nevertheless, Yucca rostrata 'Sapphire Skies' has a very tropical feel. Its upright habit and rigid leaves make it a striking specimen in the garden. 'Sapphire Skies' might be slightly hardier than the species. Its leaves are stiff, shorter than the species, and, according to Monrovia, "form a dense crown atop an elegant single trunk." Like most yuccas, Yucca rostrata is slow-growing, but it is likely one of the hardiest tree-like yucca species...and it is certainly one of the most beautiful. On the issue of hardiness, siting seems to be a significant contributor to success or failure. Depending on the source, Yucca rostrata 'Sapphire Skies' is hardy to USDA Zone 5b, while the species seems to be hardy to USDA Zone 6. Again, proper siting is key. Its soil should drain well and it should be in a location with full sun. In Colorado's dry climate, this would be an excellent candidate for the tropical garden.

Yucca rostrata 'Sapphire Skies'.
Courtesy of Monrovia.

Other hardy tree-like yuccas include Yucca elata, commonly called the Soaptree Yucca (hardy to USDA Zone 6a, though mine sailed through temperatures that dropped below -10 Fahrenheit this past winter), and the compact form of the famous Joshua Tree, Yucca brevifolia v. jaegeriana (reportedly hardy to USDA Zone 5). Even the standard form of Yucca brevifolia is hardy to USDA Zone 6a. Yucca torreyi (reportedly hardy to USDA Zone 5), Yucca faxoniana 'Giant Faxon' (from the northern population of the species, reportedly hardy to USDA Zone 5, though other sources indicate the species is only hardy to Zone 8; perhaps Yucca faxoniana is similar to Agave parryi, where different populations and subspecies can vary wildly in hardiness), and Yucca schottii, the Mountain Yucca (hardy to USDA Zone 6a or maybe Zone 5b) are a few others that might prove marginal in Colorado but would work great in the right microclimate. Many of these species appear far more at home in an arid desert landscape, but Yucca rostrata lends itself to either the tropical or the desert garden, as would a mature grove of Yucca schottii. With marginal yuccas, perhaps the only way to know if a species is hardy in your garden is to try growing it. It is definitely worth it - these are amazing plants!

Yucca schottii. Photograph by Georges Jansoone.
This reminds me of a tropical Pandanus species, minus the prop roots.